Hello everyone. I was given a 60s Farfisa full tube guitar-organ amp from a good friend and after a long restoration and repair project got this beautiful amp working great again, except for reverb. ECL86 (triode+pentode) was missing from chassis and driver transformer was dead (primary windings open reading). Did my best to save it, unwind as much as i could, but unfortunately the break seemed to be deeper in the core. After some hundreds of turns i quit.
It uses an ECL86 (triode pentode) ---> driver transformer ---> rvrb tank ---> AC151 for recovery stage.
Schematic:
http://b.baldach.free.fr/farfisaFR40/FarfisaFR40_Schematic.jpg
I finally found an ECL86 from an old kind man here, told me it was working back then. Slightly used. He hadn't test it though.
Now about the transformer, found some guys with same issue that used Hammond 1750AX (7500Ω-8Ω) and worked perfectly.
I was suggested any trafo about P:5-10Κ and S:4-8Ω would work there.
So i found an OT from an old defective tube radio i had laying around for parts. Radio uses one EL84 (ECL86 is half EL84, exactly my case). Radio's OT is P:7000Ω S:4Ω and about 3-5W, so it would work perfect as replacement.
OT 7K/4Ω and ECL86 were installed.
Everything was ready, fired it up, push the reverb footswitch, tapped the springs with my finger and could hear the big splash at least.
Then, while i was grabbing my guitar i heard some loud weird soft scratchy/heartbeat noises, like not good contact or smthng, signal was coming and leaving.
I tried a new rvrb tank with similar specs (4AB3C1B) , as the original one had one transducer's magnet broken (one spring only defective, not both) just to see if that caused the issue.
To my surprise, when i powered on and tubes get warmed up, i got this oscillation high pitch kind of noise you can hear in my vid below. This same thing happens since then. Seems like it starts from around 1650hz and gets decreased to lower frequency range.
- Noise appears only with the ECL86 installed on chassis. Pulled it off and the issue disappears indeed.
- With the tank disconnected, same issue again.
So it's either the tube itself or the transformer and not after that, like in recovery stage, AC151 etc.
- Osc noise is there with all pot volumes down.
Don't have another ECL86 to test and they are pricey, so wouldn't like to get one just to see the issue remains.
I uploaded a quick video so you can hear the noise.
*WARNING: Be careful with volume. Headphones aren't recommended. :icon_lol:
https://we.tl/t-317qJZTEsV
What do you think guys? Would it be wise to disconnect trafo again (as it was before, when original was dead) in order to isolate the problem to ECL86 only, or it could be smthng else?
Oscillation problem solved. I had forgot to ground the trafo's mount to chassis, so the secondary gnd winding lead (the one that goes to rvrb input gnd sleeve RCA) was ungrounded.
It was also feedback network issue, swapped primary windings and osc completely gone.
Now the only thing remained is to get signal mixed with reverb. At this moment i get only the effect splash sound when tapping the springs and the dry signal.
Input driver troubleshooting just started.
Any opinions are welcomed.
I have an idea (needs checking) the ECL86 is an improved ECL82? The 82 is still easy to find. Pin out may be different.
If the input of the spring is fully isolated, you could try driving with a small solid-state amp with driver tube taken out ( because no load). Spring sounds but no signal implies nothing from the driver tube?
Quote from: anotherjim on May 07, 2021, 04:26:14 AM
I have an idea (needs checking) the ECL86 is an improved ECL82? The 82 is still easy to find. Pin out may be different.
If the input of the spring is fully isolated, you could try driving with a small solid-state amp with driver tube taken out ( because no load). Spring sounds but no signal implies nothing from the driver tube?
ECL86 is more or less half ECC83 and EL84 in the same glass.
Any ideas of solid state amp circuit i could use there?
(https://i.imgur.com/anbLxbh.jpg)
ECL82 is the same kind of tube. E=6v heat, C=triode, L=pentode. It was pretty much standard for ceramic cartridge record players. I think the ECL86 has slightly higher output power from the L (but less than an EL84) and higher voltage gain from the C. But if you're prepared to consider the reverb as a sideshow, I'd have thought the 82 was worth considering as a substitute.
As long as the reverb tank input is low impedance, any amplifier should be able to push enough audio into the reverb. I only suggest it for testing purposes and it looks like it's got a phono plug connection on the tank?
QuoteInput driver troubleshooting just started.
Any opinions are welcomed.
If the thing oscillated because of the transformer connection issues then you would assume the reverb driver amp is working. In fact I'd say both tube stages seem to be doing something. Can you trace the signal feeding the reverb driver amp? Maybe the signal isn't making it to the driver amp.
I just tried an ECL82, started to get blue sparks inside the tube and i immediately turned it off. So it's not the tube itself. Hopefully, ECL82 didn't get damaged. Neither blown fuse or smthng bad.
So whatever tracing i will do, it will be with no ECL86/82 installed. Don't want to shot them. :-[
I think i had blue sparks with the ECL86 too, when first installed it. But since then no, maybe i managed already to damage a part of it.
- Before the ECL82, i had tried the ECL86 once again, to check tube's voltages. It was OK, I had 340V on pin6 (schem calls for 320V), BUT when that loud pop appeared on speakers and was like "smthng bad is happening for a moment" voltage suddenly decreased to 190V for a second and immediately came back again to 340V. 😯
Could be smthng wrong about pins3,5 that are connected with B+? (arrow symbol in schem corresponds to B+)
- I get splash sound either with Reverb footswitch enabled or disabled. Don't know if this is normal and explained from schem, as i cannot find the switch "Pedal R" (like Pedal T for tremolo) in schemo. If it's this one at S1 - P1 points in schem, then it's not normal that i get splash even if R switch is pushed off.
Could you please make this area on schem clear to me? As i understand it, by closing the switch the rvrb out signal after Rvrb vol pot gets shorted to ground and stops there before getting to S1-P1/PI stage, right? Don't know if this has to do with signal not going to driver amp though.
(https://i.postimg.cc/TLT47ZNF/IMG-20210507-090657.jpg) (https://postimg.cc/TLT47ZNF)
EDIT Add: I get continuity between S1-P1 point to GND (20Ω) , whatever i do with rvrb footswitch. Either on or off position. 😯
> I get continuity between S1-P1 point to GND
That's wrong. Remove stuff until it un-shorts. May be a stray bare wire. May be the switch has sprung its guts.
QuoteI get splash sound either with Reverb footswitch enabled or disabled. Don't know if this is normal and explained from schem, as i cannot find the switch "Pedal R" (like Pedal T for tremolo) in schemo. If it's this one at S1 - P1 points in schem, then it's not normal that i get splash even if R switch is pushed off.
Could you please make this area on schem clear to me? As i understand it, by closing the switch the rvrb out signal after Rvrb vol pot gets shorted to ground and stops there before getting to S1-P1/PI stage, right? Don't know if this has to do with signal not going to driver amp though.
A brief explanation of the signal flow is this:
- The two preamps mixed via the resistors before the 3xcap network at the top/centre (2x4n7 + 47n + 1M etc).
(The caps remove thumps from the tremolo.)
- The output of the 3xcap network feeds down to S8. That's essentially the dry signal which couples across from S8 to P8.
- The dry signal then feeds the dry input of the power amp via the 100k and feeds the reverb drive circuit directly.
- The output of the reverb comes off the transistor stage and feeds the 10k pot,
the switch and then goes to S1, then P1.
The switch kills the reverb by shorting the output of the 10k pot to ground. [That's why there's continuity to ground.]
- The reverb mixes back with dry signal via the 100k+250k pot just before the input of the power amp (22nF + V5).
The dry signal at this junction is the 100k dry input I mentioned two points back.
As for the splash. The thing to notice is the signals on P8 have a path to ground and that's the ground path
for the grid of the reverb driver tube input V6. When the reverb switch is grounded it changes the impedance to ground.
The change in impedance is *very* small it's less than 100k + 100k + 250k pot vs 100k + 100k + 250k pot + 10k.
In order to have a significant splash I'd be suspicious there is DC on the P6 node. How it gets there I don't know.
The small leakage from V8 would seem unlikely to cause it. I guess it would be wise to measure the DC voltage
there and see what happens to with the reverb switch on and off.
--------------------------------------------------------------------------------------
EDIT:
Something that occurred to me later is if there is a poor connection on S6/P6 you could be getting
DC across the grounds between the two halves of the circuit. You could measure the voltage between
the grounds. Having power ground and signal ground passing over one pin is really asking for trouble in
the long term.
QuoteI just tried an ECL82, started to get blue sparks inside the tube
I did say check if the pin out was different. It is and seriously so! Never try substitution without checking first!
Really, I mentioned the ECL82 because if they are easier to get and cheaper, then the chore of rewiring the socket could be worth it.
Sorted it out, i think case solved.
- First, by chopstick tapping i realised i had a bad solder at 200uf EL34 cathode cap. :icon_razz:
(https://i.postimg.cc/0MBZYy35/IMG-20210509-001658.jpg) (https://postimg.cc/0MBZYy35)
That was causing the 50hz hum i had before. Amp is buzz free now, almost dead silent.
Here, amongst others, i had spotted back then.
https://www.diystompboxes.com/smfforum/index.php?topic=126741.0
(https://i.postimg.cc/21sj3QT2/IMG-20210508-234823.jpg) (https://postimg.cc/21sj3QT2)
- Second, about reverb issue and S1-P1 shorting to ground whatever position rvrb switch was, after troubleshooting and removing stuff, connections were fine.
Turned out the problem probably (i say so cause I'm not 100% sure it was that, as it wasn't a "found that wire shorting and remove it" case, you will see what i mean) was at the rvrb pedal footswitch itself. By disconnecting the switch from circuit and continuity test it for functionality, i realised that in one position it was open as it should, but in the other position stayed "open" 2000ohms or smthng and was taking enough time until resistance was gradually dropping to 20, 10 and finally 5ohms beeped, as it should.
Read somewhere this can happen specially in old footswitches, i think they call it sticky symptom or smthng.
Used my trusty Teslanol Oszillin T6 contact cleaner w/lube and problem solved.
I finally have working reverb unit guys! :)
(https://i.postimg.cc/Wh2cW9YB/IMG-20210320-204626.jpg) (https://postimg.cc/Wh2cW9YB)
(https://i.postimg.cc/KRrBFYNK/IMG-20210320-222210.jpg) (https://postimg.cc/KRrBFYNK)
(https://i.postimg.cc/2VfhPW4R/IMG-20210321-011920.jpg) (https://postimg.cc/2VfhPW4R)
(https://i.postimg.cc/sMTWQ0bs/IMG-20210507-100507.jpg) (https://postimg.cc/sMTWQ0bs)
(https://i.postimg.cc/Nyr0sJcN/IMG-20210320-194800.jpg) (https://postimg.cc/Nyr0sJcN)
EDIT: The only thing that worries me a bit now is that sometimes while playing with rvrb on, i get some random splashes once in a while. That could be due to vibrations, bad rca plugs, interference etc. Voltage spikes/Driver issue?. Maybe the ECL86 that isn't NOS, the trafo replacement that i use now or smthng with this
Quote from: Rob Strand on May 08, 2021, 02:30:18 AMHaving power ground and signal ground passing over one pin is really asking for trouble in
the long term.
?
Remember this thread?
Today i got sparks in ECL86 tube again. :-[
Quote from: PRR on May 08, 2021, 12:55:38 AM
> I get continuity between S1-P1 point to GND
That's wrong. Remove stuff until it un-shorts. May be a stray bare wire. May be the switch has sprung its guts.
@PRR : Maybe it's not wrong. If you look at the scheme the upper leg of 10K lin Reverb pot (S1-P1 area) gets connected with gnd. So with rvrb pot all way closed to zero its center leg gets normally connected with the upper leg.
So, when pot is closed S1-P1 gets connected to gnd.
I had the pot at zero that's why i got continuity.
(https://i.postimg.cc/WD908jVt/IMG-20210530-123933.jpg) (https://postimg.cc/WD908jVt)
All those days things weren't exactly 100% normal as i always had cracks and splashes from tank only when i had rvrb foottswitch on.
As i have said above, when ECL86 removed, amp runs perfectly.
Smthng about rvrb circuit is wrong.
What other things should i check guys?
QuoteAll those days things weren't exactly 100% normal as i always had cracks and splashes from tank only when i had rvrb foottswitch on.
As i have said above, when ECL86 removed, amp runs perfectly.
Smthng about rvrb circuit is wrong.
What other things should i check guys?
To me it looks like there's a problem with the ground connection S6/P6. That causes DC to be injected into V8 pin 1.
I suspect the thing that makes it worse is the DC coupling into V8 pin 1. You could probably reduce the chances of bad symptoms by putting an AC coupling cap (say 22n) between the node with V8 pin + 10M and the line to P8 (ie put a cap in place of that short vertical wire on the schematic). If the resistor on pin 1 is 10M you might have to drop it to 1M as the grid leakage through 10M might affect V8 biasing.
Thank you Rob. So you think it has to do with DC coupling there.
Will try the mod and report back. You mean like that right?
(https://i.postimg.cc/w3WX9mjt/received-165045122247339.jpg) (https://postimg.cc/w3WX9mjt)
Quote from: sarakisof on May 30, 2021, 06:42:01 PM
Thank you Rob. So you think it has to do with DC coupling there.
Will try the mod and report back. You mean like that right?
(https://i.postimg.cc/w3WX9mjt/received-165045122247339.jpg) (https://postimg.cc/w3WX9mjt)
Yes, exactly like that.
Just to add a bit of explanation: The first stage of V8 has quite a bit of gain. The DC path to ground for V8 is on the "other side" of S6/P6. If there is a DC drop across S6/P6 the DC coupling of V8 will amplify the DC drop by a large amount and possibly saturate V8. It is a set-up asking for trouble, and even more so on old equipment with old connectors. AC coupling V8 will prevent the excessive shifts. I'm not 100% sure why/how the sparks are occurring.
Got it. This could explain random loud crackly solaahes, but what about sparks?
QuoteI'm not 100% sure why/how the sparks are occurring.
What could be the reason for this according to schem? Should i check ECL86 nearby components, what test measurements should i do with ECL86 removed?
QuoteWhat could be the reason for this according to schem? Should i check ECL86 nearby components, what test measurements should i do with ECL86 removed?
The only theory I have at the moment is suppose the DC at the input of V8 is jittering off and on erratically due to some bad connection. The first stage of V8 will the turn off and on quite hard. The off and on signal drives the second stage of V8 hard off and on. The load on the second stage is a transformer. If it's getting turned hard off and on then perhaps there a strong back emf and that's causing arcing. For example if you get a battery and old it on an iron core transformer or inductor then let disconnect you get a very large arc. I had great fun doing that as a kid. It's like the ignition spark on a car coil (only smaller).
In this case the arcs would be short term and intermittent more than a continuous arc.
The ECL86 datasheet shows maximum voltages of 550V with a 320V rail it wouldn't be hard for the plate and grid 2 to get to twice 320V = 640V.
Can you see exactly where where is occurring? I don't quite understand exactly what's happening when the fault occurs.
-------------
EDIT: Something that popped in my head was if the ground at the ECL86 is disconnecting maybe there is flashover to the heaters. Bit of a long shot.
Do you remember this old thread? Since then i was living with ECL86 arcing and crackling drip reverb noises. The other day reverb stopped working.
Rob had mentioned this:
QuoteI suspect the thing that makes it worse is the DC coupling into V8 pin 1. You could probably reduce the chances of bad symptoms by putting an AC coupling cap (say 22n) between the node with V8 pin + 10M and the line to P8 (ie put a cap in place of that short vertical wire on the schematic). If the resistor on pin 1 is 10M you might have to drop it to 1M as the grid leakage through 10M might affect V8 biasing.
Actually there is such a mod already from factory there, although missing from schematic. They have put a Philips coloured ceramic 150pF at ECL86's pin1 (P8), as AC coupling cap.
Today, i realized that while recapping, i had replaced everything except for one tiny electrolytic. It was the 1uf/6V connected to the cathode resistor of ECL86's pin2. Actually they had put an old Ero 5uF/6V (damn Farfisa, common schem-chassis non-correspondings 😡). This cap measured 32pF instead of 5uf so i replaced it.
I still have no reverb, but could that 1uf/5uf pin2 cathode cap caused all those bad things and probably got ECL86 shot?
(https://i.postimg.cc/JGnL3BsY/IMG-20210711-104713.jpg) (https://postimg.cc/JGnL3BsY)
With orange arrow the 5uf cathode cap that measured 32pf.
With green arrow the AC coupling 150pF cap that actually exists, although missing from scheme.
(https://i.postimg.cc/V0bH4MCn/IMG-20210711-105943.jpg) (https://postimg.cc/V0bH4MCn)
Edit: I will try to find a NOS ECL86 (previous was used) and change ECL86's tube socket with a new one. :icon_cry:
Quote from: sarakisof on July 11, 2021, 03:47:50 AM....could that 1uf/5uf pin2 cathode cap caused all those bad things and probably got ECL86 shot?...
No; but it sure could cause high pitch squeal. (100% NFB around power amp.)
As far as the original problem goes it's still not clear what was causing the arcing. It could be DC issues, heater breakdown, or something promoted by oscillation.
I agree with PRR. When the failed cap is open there is more feedback and that can definitely increase the risk of oscillations.
It's not clear if the cap fault caused the arcing.
If you changed the output transformer then that can also cause oscillations to occur. The specifics of the transformer does have an impact on amplifier stability.
If you look at this schematic you will see a series resistor + cap at the first stage and a cap across the output transformer. Instead of a cap across the transformer as resistor in series with a cap is usually better. You might be able to use the input network alone or the output network alone.
https://i.imgur.com/i9nGemc.jpg
The thing is determining these components isn't a simple task. It's more of a redesign. And you would only expect that such a redesign is required if the transformer change has affected the amplifier stability. We don't know if that's true in your case. Also we don't know if it caused arcing.
As far as getting the unit up and running. Measure the DC voltages on all the tubes. You should also check the DC resistances of the transformer - it could have shorted, not likely but possible. The cathode voltage on the output tube will tell you if that tube passing current, you could check the current looks OK as well.
Who knows, maybe you new transformer had a fault from day one and was breaking down and now it has failed.
PRR i see, cathode bypass cap can cause that.
Arcing comes alongside with pop for a second usually during power up warming, but sometimes also later.
ECL86 voltages:
Anode pin6 320V. (exactly as in scheme)
Anode pin9 133V.
By signal tracing (1khz to input then diy tube tracing cable to xtrnal cheap amp) i got signal through hole path until rvrb trafo out (tank in). Then i got pop/arcing again and immediately switched off. It's scary 🤣
So i guess ecl86 is fine.
I don't get splash sound by tapping springs. Tank is ok, so no recovery stage.
So my next step is to check recovery stage. Could bad AC151 germanium cause all this?
Edit: I know see Rob's reply. I'm reading it now.
Tube voltages are fine, didn't measured everything but when i have done back then were all fine. Remeasured B+ and anodes at most tubes again, they follow schemo's volts.
Trafo's res: 580ohms input, 3ohms output(tank in). It's ok, it was the first thing i checked when i got pop arcing.
EDIT: Trafo seems fine but i could order a new Hammond 1750AX just to be sure.
https://www.tube-town.de/ttforum/index.php/topic,24358.msg247927.html#msg247927
QuoteThe cathode voltage on the output tube will tell you if that tube passing current, you could check the current looks OK as well.
You mean ECL86 cathode voltage? I have a UNI-T 139C multimeter with a μA/mA scale. How do i measure cathode current there and what I'm looking for?
EDIT: About EL34s dissipation they are running at about 72-80% for V7 and 80-88% for V8.
https://www.diystompboxes.com/smfforum/index.php?topic=126741.msg1213275#msg1213275
QuoteYou mean ECL86 cathode voltage? I have a UNI-T 139C multimeter with a μA/mA scale. How do i measure cathode current there and what I'm looking for?
Measuring current like that is very inconvenient because the meter needs to go in series with the tube. What I was suggesting is by measuring the cathode voltage you can imply current is flowing simple because the cathode voltage is non-zero. You can also calculate the current as I = Vcathode/Rcathode; and for accuracy you could measure value of Rcathode.
QuoteTube voltages are fine, didn't measured everything but when i have done back then were all fine. Remeasured B+ and anodes at most tubes again, they follow schemo's volts.
Trafo's res: 580ohms input, 3ohms output(tank in). It's ok, it was the first thing i checked when i got pop arcing.
Q
It's looking like the tube part of the circuit is OK. (except for the arcing)
QuoteBy signal tracing (1khz to input then diy tube tracing cable to xtrnal cheap amp) i got signal through hole path until rvrb trafo out (tank in). Then i got pop/arcing again and immediately switched off. It's scary 🤣
So i guess ecl86 is fine.
I don't get splash sound by tapping springs. Tank is ok, so no recovery stage.
So my next step is to check recovery stage. Could bad AC151 germanium cause all this?
OK got it. Yes, could be the AC151. You can measure the DC voltages around the AC151.
It is possible the transformer is breaking down between primary and secondary and that is causing zaps. That would take out the AC151. Alternatively the arcing fault is on the tube side. For example a bad contact on the output tube socket. That would release the DC current through the transformer and cause a large voltage spike on the transformer - similar to a car ignition spark but less aggressive.
I doubt the AC151 can be taken out like that due to the reverb isolation.
You can see my previous post's "EDITs".
I see, i will check ac151 voltages and will be back.
This amp has driven me crazy. The other Italian tube amp (Geloso 227) was easy and had been flawlessly repaired bsck then.
This one is a bad guy 8)
Please disregard my the last part of my post - see my last comment.
QuoteYou can see my previous post's "EDITs".
Also I meant the output tube of the reverb circuit (not the main amp). There's two power amplifiers.
Ok but again, it could be a bad germanium from the beginning right? I don't have smthng to loose if i measure ac151 volts i guess? I'm saying this, cause it's a whole story to disassemble the other chassis (preamp stage S points) :icon_wink:
I think it's either the ecl86, tube socket, trafo or ac151. Wish i could know for sure so i could order just one thing. :icon_wink:
QuoteOk but again, it could be a bad germanium from the beginning right? I don't have smthng to loose if i measure ac151 volts i guess? I'm saying this, cause it's a whole story to disassemble the other chassis (preamp stage S points) :icon_wink:
Well at this point it would be best to pursue the AC151.
QuoteI think it's either the ecl86, tube socket, trafo or ac151. Wish i could know for sure so i could order just one thing. :icon_wink:
I honestly don't know. However before you had one problem, arcing, but now you have two problems, arcing and no recovery. So now you might need to get two things.
The chances of a bad transformer is very low. Still possible but low. Corroded tube sockets on old organ sounds quite possible. You might be able to clean the socket.
When you probe the secondary of the transformer and get an arc, does that happen all the time or even often? That would be quite odd and might be a hint for the cause of the arcing.
RG did a nice write-up on arcing,
http://www.geofex.com/ampdbug/popping.htm
At the end of the list. Cracked resistors is certainly another possibility.
Some crazy faults are caused by the smaller components.
ECL86 cathode voltages.
pin2: 0.12 V
pin7: 10.2 V
Is Triode Cathode normal?
Edit: seems fine.
QuoteECL86 cathode voltages.
pin2: 0.12 V
pin7: 10.2 V
Is this normal?
Edit: seems fine.
Yes seems OK.
I don't know if the voltage are exactly correct for that circuit but the voltages don't look unreasonable.
Stage 1: Ia1 = 0.12/150 = 800uA which would produce a 176V drop across the 220k plate resistor.
Stage 2: Ia2 = 10.2/560 = 18mA
QuoteWhen you probe the secondary of the transformer and get an arc, does that happen all the time or even often? That would be quite odd and might be a hint for the cause of the arcing
Sorry my fault. In that early post i wanted to say i got arcing when probed at the tank output.
I got news. Tried again to probe there and i get no signal. So the chain stops there. If i get signal at tank's input, should i suppose to also get signal at output transducer?
That means that my tank is defective/dead output transducer? But again this is a brand new appropriate MOD tank. And it worked... (Trafo thoughts....)
Receive transducer reads 220ohms so it works. Probed again, still no signal at out receiver transducer (tried with both rca connected and disconnected). Should the signal supposed to travel across springs and can be heard at the receiver?
QuoteSorry my fault. In that early post i wanted to say i got arcing when probed at the tank output.
From an electrical point of view it would be virtually impossible to invoke an arc by probing there. I suspect the arcing was a random event or perhaps the act of probing cause some vibration through the chassis and the real cause (bad connection?) created the arc.
QuoteI got news. Tried again to probe there and i get no signal. So the chain stops there. If i get signal at tank's input, should i suppose to also get signal at output transducer?
That means that my tank is defective/dead output transducer? But again this is a brand new appropriate MOD tank. And it worked... (Trafo thoughts....)
The bad AC151 might be shorting the signal. The signal at the output of a reverb tank is *very* small so maybe hard to hear. Unlikely to be the reverb tank. You can measure the DC resistance of the output coil of the tank, you might expect 200 ohms but it could be anything from 40 ohms to 1k ohm. Try measuring in circuit first and if the measurement doesn't make sense you might have to desolder one of the wires - if the tank has connectors then it's easy to pull out the cable.
QuoteReceive transducer reads 220ohms so it works. Probed again, still no signal at out receiver transducer (tried with both rca connected and disconnected). Should the signal supposed to travel across springs and can be heard at the receiver?
You beat me to it. Yes, right on the mark.
So perhaps the AC151.
You can check the AC151 by injecting a signal into the reverb recovery amp.
- disconnect the reverb tank output.
- inject a *low level* signal in the recovery amp input
- slowly increase the reverb level
you should hear a clean signal coming through.
Just did it. Unfortunately AC151 works. I say unfortunately because troubleshooting to be continued ..
QuoteJust did it. Unfortunately AC151 works. I say unfortunately because troubleshooting to be continued ..
Well in some ways it's good news.
So upto now
- reverb driver works, signal makes it to output of transformer
- reverb recovery works, signal passes through recovery amp
So the unknowns remaining are the connections to the reverb tank, both tank input and tank output.
Can you tap the reverb and hear a spash? I thought a few posts back you said that wasn't happening (?).
That would point to a connection issue between the reverb tank output and the input of the recovery amp (AC151 etc).
Before continuing to the steps you mentioned, i just wanted to make sure i indeed had signal at transformer (so driver works), because earlier that i got signal it was like indeterminate and with this amp you never know ::)
And guess what. This time i had no signal by probing at trafo's output. And smthng else. Tried to probe at trafo's input (so to find if it is the curlpit) for about some secs (no signal again by the way) and the 1.5K 10W power resistor (between the two 32u filter caps) got extremely hot. Was smelling and took about 10 minutes to be able to touch it!
I mean really really hot.
Repeated (for just 3secs) just to verify and it started to get extremely quickly hot again and i switched off.
Is this normal? Maybe it could be a hint to the problem.
Input transformer = ECL86 pin3 and pin6...
QuoteBefore continuing to the steps you mentioned, i just wanted to make sure i indeed had signal at transformer (so driver works), because earlier that i got signal it was like indeterminate and with this amp you never know ::)
And guess what. This time i had no signal by probing at trafo's output. And smthng else. Tried to probe at trafo's input (so to find if it is the curlpit) for about some secs (no signal again by the way) and the 1.5K 10W power resistor (between the two 32u filter caps) got extremely hot. Was smelling and took about 10 minutes to be able to touch it!
I mean really really hot.
Repeated (for just 3secs) just to verify and it started to get extremely quickly hot again and i switched off.
Is this normal? Maybe it could be a hint to the problem.
Good idea.
The lack of signal is top of the list of things to debug. A short across the reverb input or transformer output could cause that.
As for the hot resistor. Yes, it is suspicious but it could be normal. I can't read the expect DC voltage on the high voltage side of the 1.5k 5W resistor. The output side is 330V. If you measure the voltage across the 1.5k resistor you can work out the power dissipated as V^2/1.5k if it is close to 10W then it will get hot, like 200C.
However, more useful is to measure the 330V rail. If there is a short or fault the high current will pull the voltage on that rail down. It should be clear there is a fault. If the 330V rail got pulled down to 250V then that would definitely increase the power dissipated in the 1.5k resistor beyond normal.
As i mentioned i don't get signal at transformer input too.
QuoteTried to probe at trafo's input (so to find if it is the curlpit) for about some secs (no signal again by the way)
.
So the problem could be earlier (tube pins 3,6?).
ECL86 no output...
As for the hot resistor, i said it gets hot (maybe even hotter than 200C) only when i put my diy probe at trafo's input. In any other case, it runs cool normal.
That's why i was asking if that's normal. Maybe my diy probe tracer (cap resistor inside cable)?
When no probing there, either probing elsewhere or no probing at all just playing guitar, it is normal cool.
Repeated many times today probing at transformer input/ecl86 pin3,6 i get no signal. Only crackling sounds. So no ecl86 output. By chopstick tapping at ecl86 while probing there i can clearly hear "tap tap" from cheap probe-amp's speakers. Microphonic ecl86?
QuoteAs for the hot resistor, i said it gets hot (maybe even hotter than 200C) only when i put my diy probe at trafo's input. In any other case, it runs cool normal.
That's why i was asking if that's normal. Maybe my diy probe tracer (cap resistor inside cable)?
When no probing there, either probing elsewhere or no probing at all just playing guitar, it is normal cool.
If the problem *only* occurs when you connect the probe to the transformer input it sounds like your probe is loading down the circuit. Perhaps to the extent where it is pulling current from the HV supply then through the probe to ground. That would cause high current on the HV rail and shunt out the audio signal on the plate of the tube.
You could confirm that by permanently connecting a multimeter to the HV supply or plate and watch the
voltage drop when you connect the probe.
Things that would cause that issue with the probe are low impedance input or even a low voltage cap
breaking down and shorting.
QuoteRepeated many times today probing at transformer input/ecl86 pin3,6 i get no signal. Only crackling sounds. So no ecl86 output. By chopstick tapping at ecl86 while probing there i can clearly hear "tap tap" from cheap probe-amp's speakers. Microphonic ecl86?
You can get microphonics from taps in many ways. It's likely to be that.
Do the DC voltage check I mentioned above and review your probe circuit.
The possibilities don't stop there. Another possibility is the capacitance of the probe is loading down the plate and causing the circuit to oscillate. One way around that is to put a 10k resistor in series with the probe. You might lose a bit of level on the probe but the idea is to trade that against capacitive loading on the tube plate which could trigger oscillation.
QuoteDo the DC voltage check I mentioned above and review your probe circuit.
Just did it. By just touching the gnd /sleeve probe only to any of transformer's inputs it immediately shorts (0.4 volts) with scratching at speakers (short sound) and power resistor gets immediately ultra hot. This doesn't happen with the hot probe.
My signal tracer is like this one, with some slight variations at values if i remember well. Haven't opened it yet as i had put double heat shrink back then, but it's a series resistor, cap with hot and resistor parallel hot to gnd probe.
(https://i.postimg.cc/NK6XW0LZ/unnamed.gif) (https://postimg.cc/NK6XW0LZ)
And makes sense as it shorts pin3 or pin6 of the tube :icon_rolleyes:
By probing hot at pin6 and gnd to gnd -thsts the correct way i guess :p - i get only deep buzz.
If ecl86 isn't dead after having shorted its grid and anode pentode i guess it's the HighLander :icon_smile:
Don't know what i was thinking, i was confused by secondaries which are actually signal-gnd. I must have shorted out the tube now right? And that could explain that i completely lost reverb signal --> no splash (in my first efforts troubleshooting arcing the other days).
QuoteAnd makes sense as it shorts pin3 or pin6 of the tube :icon_rolleyes:
By probing hot at pin6 and gnd to gnd -thsts the correct way i guess :p - i get only deep buzz.
If ecl86 isn't dead after having shorted its grid and anode pentode i guess it's the HighLander :icon_smile:
Yes it all makes sense. The black clip of the probe is connected to mains ground via the guitar amp ground connection.
Since the Farfisa also ground connects to mains ground when you connect the black clip to anything that isn't ground
it is the same as shorting it to ground. For the tube that's like shorting the plate or power supply to ground. The
power supply would be pulling a lot of current through the 1.5k resistor and it starts to cook.
I doubt there's any damage to the tube since the all the faults divert current from the tube.
The buzz from connecting one lead is hard to remove. The best you could do perhaps would be to connect the
black clip to the Farfisa ground. Maybe through a 1uF cap or 10 ohm resistor.
QuoteThe buzz from connecting one lead is hard to remove. The best you could do perhaps would be to connect the
black clip to the Farfisa ground. Maybe through a 1uF cap or 10 ohm resistor.
I connect black probe to Farfisa's gnd and red to pin6 (ang get buzz).
To make it simple, if i would tell you "prob at trafo's inputs", this is the only way to go?
Update: OK i have signal at transformer's input!
So we are here again:
QuoteSo upto now
- reverb driver works, signal makes it to output of transformer
- reverb recovery works, signal passes through recovery amp
So the unknowns remaining are the connections to the reverb tank, both tank input and tank output.
Can you tap the reverb and hear a spash? I thought a few posts back you said that wasn't happening (?).
That would point to a connection issue between the reverb tank output and the input of the recovery amp (AC151 etc).
I don't hear splash. I will check tank's output
QuoteTo make it simple, if i would tell you "prob at trafo's inputs", this is the only way to go?
The last few posts were a result of the probe causing shorts giving a fault impression of lost signal.
You mentioned this a few post back.
Quote
When no probing there, either probing elsewhere or no probing at all just playing guitar, it is normal cool.
If we discard the problems caused by the probe it seems to work?
So now I'm confused about what isn't working.
Yes "cooked resistor" and "no signal by probing at trafo issues solved.
Just read at my last "Update" post.I get signal at trafo's input, trafo's out, tank's input and gets lost at tank's out.
But i get signal when isolating the tank's out from recovery amp and putting signal to rca recovery amp.
It's like signal gets lost through springs somewhere between tank's input and output transducers. Tank also works. :icon_cry:
Also, by probing at tank's output i can hear splash through probe amp's speaker.
QuoteJust read at my last "Update" post.I get signal at trafo's input, trafo's out, tank's input and gets lost at tank's out.
But i get signal when isolating the tank's out from recovery amp and putting signal to rca recovery amp.
It's like signal gets lost through springs somewhere between tank's input and output transducers. Tank also works. :icon_cry:
Also, by probing at tank's output i can hear splash through probe amp's speaker.
Something isn't making sense.
I guess the confusing thing is the test signal seems to go through the recovery amp but the splash of the reverb tank isn't going through the recovery amp.
What could be happening is the test signal is very strong and is making it's way through a damaged recovery amp.
The real signal from reverb tank is very small and if the recovery amp isn't *really* working then it might not amplify
the splash enough to be heard.
Measuring the DC voltages around the recovery amp would be useful (including the supply voltage on the low voltage electrolytic cap it's 100uF/25V or 200uF/25V).
Reverb problem solved.
QuoteThat would point to a connection issue between the reverb tank output and the input of the recovery amp (AC151 etc).
You were absolutely right Rob. The tiny hot signal lead of the rca cable from tank's output to recovery amp had cut off right at the solder joint (in the amp area) with the years of use.
The thing that was fooling me was the fact that it was still making intermediate contact in some cases and i realised that, by moving around the cable before opening the amp (it is placed in another external chassis on the cabinet). Signal was coming in and out.
So when i tested by probing at the rca's it happened that timeme to make contact, fooling me that recovery amp was fine.
So reverb runs fine now. And the weird -but good- thing is that i have it running about half an hour playing some tones ("Splish Splash i was takin a bath" 🤣) and the ecl86 hasn't started arcing yet.
Who knows how much will this last. 🤣
QuoteReverb problem solved.
Good news.
QuoteThe thing that was fooling me was the fact that it was still making intermediate contact in some cases and i realised that, by moving around the cable before opening the amp (it is placed in another external chassis on the cabinet). Signal was coming in and out.
So when i tested by probing at the rca's it happened that timeme to make contact, fooling me that recovery amp was fine.
Intermittent contacts are never easy. It's not uncommon to move or push something and it starts working but the fault turns out to somewhere else.
QuoteSo reverb runs fine now. And the weird -but good- thing is that i have it running about half an hour playing some tones ("Splish Splash i was takin a bath" 🤣) and the ecl86 hasn't started arcing yet.
Who knows how much will this last. 🤣
Yes, well good luck just same. The reason for the arcing still isn't clear to me.
Yeap definitely the arcing still isn't clear. It came back just a while ago when i was making my final voltage measurements. By the way, once again, all voltages are perfect.
At the third power up (when it comes for HV i use to attach the alligators of multimeter before power up then power off, attach alligators to the next measurement point and so on, just for safety reasons. I mostly work in live circuit for heaters and other low volts points) and during tubes warming it made the common immediate "pop" and small arcing.
No arcing after that, but i decided to keep amp working without playing smthng. Just looking and try to hear strange noises.
I got two kind of noises, one "drippy splash pop" when rvrb was on.
And another "cracking/poping" like noise, with rvrb off and even volume turned off.
This is the drippy noise: https://we.tl/t-Vnk3xD4t91
And this is the crackly poping: https://we.tl/t-RYi4rLAbIX
QuoteI got two kind of noises, one "drippy splash pop" when rvrb was on.
And another "cracking/poping" like noise, with rvrb off and even volume turned off.
This is the drippy noise: https://we.tl/t-Vnk3xD4t91
And this is the crackly poping: https://we.tl/t-RYi4rLAbIX
They both sound like a bad connection somewhere. Could be a the tube socket or the connector between the two sections. Could even be a failing component bad cap, cracked resistor.
They two cases might have a similar origin if it i coming from the connector or the preamp section.
In the first case the pop could be occurring before the reverb and the reverb is adding reverb to the pop sounds. Perhaps the second sound is the same as the first but without reverb. I'm sure you could determine if that's
the case better than I can. You need to adjust the reverb level while the pops are happening and see if the reverb
to can change one sound into the other.
QuoteAt the third power up (when it comes for HV i use to attach the alligators of multimeter before power up then power off, attach alligators to the next measurement point and so on, just for safety reasons. I mostly work in live circuit for heaters and other low volts points) and during tubes warming it made the common immediate "pop" and small arcing.
If the arcing is more reliable at power-on it might give a hint. The heaters are cold so the tubes current
will be low. The plate voltages will be high due to low current. Also the caps are charging up and perhaps
some take a while. It would be interesting to measure the ECL86 tube DC voltages on power up to see if any
'glitch' occurs when you get the arc.
Maybe a standby switch will help? That way (if you remember to do it) the heaters can get hot before you apply the HT.
Did you replace the ECL86 socket? If it's a phenolic socket, they can develop hidden insulation faults in the material. I guess ceramic ones can do it too if the surface glaze has been damaged.
If it is socket insulation, then as Rob says, the HT is offload and higher than normal until the heaters are hot which would be when insulation breakdown is most likely to start.
I had to be out for the weekend, but before leaving, i played with it about 2-3 hours testing its behaviour and waiting for those pops/splash pops to appear. For all those hours i got no splash pops at all, only the dry pops (like in the second video uploaded) but way far quieter.
If i didn't know, i would have not even realized it.
It's the crackling kind of fire burning dry cobs, but very quietly.
QuoteThey two cases might have a similar origin if it i coming from the connector or the preamp section.
That's my thought too. But this last time of 3 hour use i got no drippy pop at all.
So i wanted to do that
QuoteYou need to adjust the reverb level while the pops are happening and see if the reverb
to can change one sound into the other
but i couldn't.
After 3 hours i powered off and on (property) again about three times but still no splash or even normal loudness pops to test. No arcing too.
QuoteMaybe a standby switch will help? That way (if you remember to do it) the heaters can get hot before you apply the HT.
Yes maybe. I realised that by normal use, appropriate and not rapid turn on and offs i get no arcing at all. I start thinking about it was from that, alongside with the lack of stand by. I just let the tubes cool about 3-5' and then i turn on again.
QuoteDid you replace the ECL86 socket?
Not yet.
So for now, just a slight crackling pop that you can only hear when not playing (and if you get close to speakers).
If loud pops and splashes are back i will try to determine if they come from the same source. I guess if they do and the drippy sound is the first with reverb (very likely), my troubleshooting efforts will be concentrated on preamp, PI area, but not in ECL86's circuit and socket.
That's a good test. Weird the reverb drips have gone clearly that shows the two faults are different.
QuoteSo for now, just a slight crackling pop that you can only hear when not playing (and if you get close to speakers).
If loud pops and splashes are back i will try to determine if they come from the same source. I guess if they do and the drippy sound is the first with reverb (very likely), my troubleshooting efforts will be concentrated on preamp, PI area, but not in ECL86's circuit and socket.
The only way I can see a way of narrowing it down is to start at the output monitor various points in the circuit with an audio probe. As you work back from the output towards the input you might find a point where the pops stop.
You could even find it's occurring on every stage. That would point to a power supply issue.
Without narrowing it down you would have to swap out far too many parts. You could end-up replacing half the unit only to find it's a cracked resistor or dirty connection.