Farfisa Tube Amp EL34 Problem

[sarakisof]

Hello folks, i was recently given a nice old Farfisa FR-40 tube organ/guitar amp. ECC83s, EF86s pre, ECL86 for spring rvrb, EZ81 rectifier and two power EL34s.
I have done a full recap in Power section, put a fresh EZ81 rectifier, cleaned contacts, switches etc. It came with no ECL86 (tube for rvrb), have ordered one, so untill it arrives it runs without the ECL86- no rvrb indeed.

The issue is that i get about 130mA plate current on one EL34, the V8. This is about 50W (!!). V7 is fine. V8 anode = 392 V (!!) , while normal V7 is about 360V. In multimeter, V8 measures about the same as V7 at some point but then starts to increase, instead of decreasing (after the Volt.peak) like V7 does ...
Didn't make any measurements before restoration but i guess the issue was there already.
A symptom -don't know if it has to do with EL34- is that it has some 50hZ hum.
I cannot understand how still plays fine and how i was jamming about 4 hrs yesterday after recapping, since i decided to go on with fix biasing and got this bad measurement. 

Schematic:    http://b.baldach.free.fr/farfisaFR40/FarfisaFR40_Schematic.jpg

---At 240 Voltage Selector---

B+(after 1.5K ceramic): 344 VDC
AC Heaters: 6.1 VAC
P3 (-25V): -23.1 VDC
EL34'S Grid 5: -22.3 VDC
CT Volts: 372 VDC

V7 Tube:
Plate Voltage= 360 VDC
Drop Volts= 12 VDC
OT Res.= 198 Ω
Plate Current I7= 60mA
Plate Diss =  21.6 WATTS

V8
Plate Volt= 390 VDC !!
Drop Volt=  18 VDC
OT RES.= 138 Ω
I8= 130mA  !
Plate Diss = 50 W ! 



* Swapped the tubes, even tried two others and the problem remains in V8 socket only, so I guess the issue is in the circuit and not in the tubes themselves.

Where i'm starting from guys?

[sarakisof]

Hope it is just bad maths or that my multimeter got crazy

[sarakisof]

Now V8 plate voltage measures 380V that means 71mA. and about 26W. Cannot understand really 
It cannot stay stable actually. Gets about 378V to 382V

[anotherjim]

How are the 1k screen resistors? I've never seen a faulty one myself but I'm told it can happen. Also, some like higher power resistors than might be there not. 5W and should be ceramic coated?

[PRR]

The critical thing is the -22.5V on signal grid pin 5.

EDIT --- OK, I now see you quote both together. Are they really the same?

How can you have 344V from the supply and 360V after transformer drop? Free power?

It may be oscillating. Some meters will read big AC on DC asymmetrically and appear to be "more voltage". Ground V5 pin 7 (cathodyne grid) and see is EL34 DC voltages change.

[danfrank]

Here read this post from a couple of years ago:

https://el34world.com/Forum/index.php?topic=24833.0

Looks like he had the same problem as you. The bias supply in these amps stinks and is really half assed. They suggest redoing the bias supply which isn't too big a problem in that amp.
Other than that, what PRR said... Check 47n coupling caps to make sure they aren't leaky and make sure the 220k resistors are good.
Good luck.

[sarakisof]

The critical thing is the -22.5V on signal grid pin 5.
EDIT --- OK, I now see you quote both together. Are they really the same?

Yes they are both exactly the same! Isn't a 22-25V range good for pin5 grids?

How can you have 344V from the supply and 360V after transformer drop? Free power?

I don't get it. I have 344V after the 1.5K ceramic res. (B+). On the other leg EZ81 pin3  it is 372V (CT Volts). 360V and 380V are on EL34 anodes/OT windings to gnd.

I just got new measurements again a few minutes ago, because voltage raises a bit at night in my area, just to verify.

B+ = 345V again (schem calls for 330V)
V5 PI pin6 = 190V (scheme calls for 185V)

EZ81 pin3 (CT Volt) = 378V
V7 PlateV/Anode = 366V ,so about 64mA
V8 PlateV/Anode = 386V ,so about 58mA
Grids Pin5 =  -22.6V same in both tubes again (ok maybe -22.5V in one of them)

About the fact that i get 6.1VAC on EL34 heaters and  -22.5VDC on V1,V2 heaters / EL34 pin5 grids. This happens with 240V Mains Selector.
When i select 220V (in my area mains are about 230V but the amp came to me like that, in 220V selector position), i get about 6.58VAC  and  -24.5VDC on grids. Could this be better? The whole B+ volts are getting increased of course.

[sarakisof]

Here read this post from a couple of years ago:

https://el34world.com/Forum/index.php?topic=24833.0

Looks like he had the same problem as you. The bias supply in these amps stinks and is really half assed. They suggest redoing the bias supply which isn't too big a problem in that amp.
Other than that, what PRR said... Check 47n coupling caps to make sure they aren't leaky and make sure the 220k resistors are good.
Good luck.

Yes i have already read this. Do you think i could put a 100Ω cathode bias res at pins 8 to gnd like a guy there suggested? Could a fixed - cathode bias Frankenstein mod like this affect tone?

Plus, about the fact that i get 6.1VAC on EL34 heaters and  -22.5VDC on V1,V2 heaters / EL34 pin5 grids. This happens with 240V Mains Selector.
When i select 220V (in my area mains are about 230V but the amp came to me like that, in 220V selector position), i get about 6.58VAC  and  -24.5VDC on grids. Could this be better? The whole B+ volts are getting increased of course.

[danfrank]

I would use the 220v tap if that gives you 6.5vac for the filaments. What that other forum post was getting at was that the FR40 has a $hit bias supply, it doesn't do it's job well. It needs more voltage in order the give the EL34s proper bias.
The 100 ohm cathode resistors make the amp a hybrid cathode/fixed bias amp, it may sound different. You may even like the sound better with this mod. It seems like you want a definite answer from people here to fix your problem, and with nobody here able to see your amp, that's not gonna happen.
My first guess would have been leaky coupling caps, but if both output tubes are getting the same voltage at pin 5 and only V8 is red-plating, then that's probably not it.
If V8 is oscillating, then make sure g1 stopper is right at the tube and you may want to increase the value to 4.7K ohms. Make sure that g2 resistors are right at the tube also.
Make a better (more volts) bias supply for the amp.

I find it hard to believe that both EL34s have -22.6 volts at pin 5.
A picture would be so helpful.

[sarakisof]

I find it hard to believe that both EL34s have -22.6 volts at pin 5.
A picture would be so helpful.

I would love to go with 220V tap too, as i get 6.4V on heaters and -24.8V on both grids, but the bad thing is that mains voltage here raises up to 240V during the night.
220+10%=  242V max...
On the other side, i don't want to mod it to fixed-cathode buas amp by adding a 100Ω or so from pins8 to gnd, neither mod the circuit like this guy here https://el34world.com/Forum/index.php?topic=24833.0
It's straight forward, but i don't want to alter the originality of this old amp, call me romantic but that's it.
Is it so bad for the EL34s to run at 70-80% of max dissipation? Will be only for recordng moments, some hours only in a couple of months, won't run the amp all day. Have many other amps for practicing and so.
This amp was working like that for many years before landed my house. On the other hand it was used before 2004 systemically, when mains voltage was 210-220 in my country and it came here with 220V tap ... 

[antonis]

Σοφοκλή, get yourself a good multimeter...!!! 

[PRR]

> Is it so bad for the EL34s to run at 70-80% of max dissipation?

Two days ago you reported "Plate Diss = 50 W !     ", 200% of rating. This is bad.

[sarakisof]

> Is it so bad for the EL34s to run at 70-80% of max dissipation?

Two days ago you reported "Plate Diss = 50 W !     ", 200% of rating. This is bad.

Yes i know, as it seems turned out to be a bad measurement of that moment only, maybe multimeter fault or wrong math due to my tiredness that night.
The truth is that since then i am getting measurements many different times per day and the dissipation is about 18-20W for V7 and 20-22 for V8. That's about 72-80% FOR V7 and 80-88% for V8. I know it's pushing the limits for fixed bias, but it is so a no-no situation? No red plating though, just glowing more than in my Geloso cathode bias EL34's (or i may think so). Will swap them to see if they get "closer" in terms of dissip.

My most important concern now, is how to get rid off that (not so slight but not loud for sure) 50/60hz hum. Even with volume turned off, with nothing connected. It's not from preamp section. Pulled both EL34s out, hum completelly stops. Pull only PI ECC83 out, hum decreases at almost 98% ...
Have checked for ground loops, gnd connections, cold solders evrthng seems fine. Swapped many tubes (PI, outputs) with good ones, but same result.
Tried plugging in another mains socket, another room, turned off WiFi, wireless phones etc nothing changes.
It is smthng from PI section and after. Could it be from the too hot bias or from the fact that the ECL86 is completely missing (have order it and waiting to arrive) and rvrb pan is out for repairing/ in-out rcas floating around unconnected?
What other things should i check, maybe one or two slightly cracked mustard caps?

EDIT: This amp also uses a humpot, it's the 300Ω pot in parallel with heaters in the scheme. Tried to turn it to see if that stops the hum but nothing again, actually "that pot's hum" was dead silence and when i was turning it started to appears like "behind and in parallel with the louder existing hum", so i turned back the lug to the position it was first (about 56Ω) and had only the annoying hum I'm trying to get rid off.

http://b.baldach.free.fr/farfisaFR40/FarfisaFR40_Schematic.jpg

[anotherjim]

The heater wiring layout seems random. It could be better twisted and closer to the panel.
It looks like the sockets are riveted to the chassis. Is anything relying on ground continuity via those rivets?
The sockets look like Phenolic - could they have suffered over the years and have poor insulation (especially under HT stress)?

[sarakisof]

Rivets yes indeed.

Is anything relying on ground continuity via those rivets?

Don't get you 100%.

Some more hints: I have been told from my friend- previous owner that it had been slightly "repaired"/seen by a hobbyist back in 90s and that "he had put a cap and a new tube".
The truth is he had replaced the old  ECC83 PI with a new  Marshall one, had REMOVED the rare ECL86 😂 (rvrb pan is probably unfixable-broken little magnet cylinder transducer - will try to make a surgery though, if not, i will simply replace the whole pan) AND he had put a 47uF/450V electro between P4 and gnd!😯 Like an extra B+ filter cap, he was probably trying to get rid off that hum too? Who knows :p (but the hum is a 50hz one, so no sense either).
The amp when arrived. . In the last pic you can see that 47/450 cap he had put in there. P4 pin of the multipin preamp-amp connector socket.











Another hint: Pay attention to that "Organo" input in the scheme. Inside amp, this is populated next to the 300Ω humdinger pot. I have noticed that this area is a bit sensitive to noises and suchlike. This is an input that bypasses the whole preamp so for the Farfisa Compact (it has already tube preamp circuit) and other organs of the era to get connected straight into the PI and power circuitry.
If you read the Compact's operation manual it mention that "if you own a Farfisa FR40 amp this is the proper way to connect your Compact, but if you want more gain, you can also connect the Compact to FR40's Accordion input" (FR40's preamp section, alongside with Guitar, Mic inputs).
BUT, when i plug in any signal into "Organo" input, i get tons of gain, it's pretty loud, like i have turned the pre inputs volume at 6-8, almost all the way. And when the jack gets inserted/extracted into/from there it does this loud pop scratchy sound, you know like when you insert the jack with the volume turned up. Are all those explained by the fact that the (supposed to be) master volume is fixed (250K in the scheme) so for this input is like it's always turned up? Most likely this has not to do with the hum but i wanted to share it as it doesn't make sense that this "organ" input in chassis gives more loudness than if you plug a guitar in to guitar pre input controlled by the vol pot. Maybe it's me and the fact that i haven't turn those input pots even greater than 3-4 so in this levels seems like they are quieter than the organ input..

And something last. Look at how those two cloth OT's secondary wires are floating in the air. As it seems the OT has 8Ω and 16Ω secondaries for other speaker impedances. Could they be a reason for the hum?

In the end of the day my thinking returns to the fact that the hum stopped when i pulled out the EL34s or the PI ecc83, so i have to get concentrated to this area. 

Some more pics from how it is now. (Plz don't judge how the filter caps are being placed and that i used radial's, just got those in handy and will stabilize them better).

[PRR]

> dissipation is about 18-20W for V7 and 20-22 for V8.

OK. Probably designed that way.

[amptramp]

Except for the tube heaters powered from the bias circuit, it looks like all the other tube heaters are referenced to ground by the humbucking pot.  If it is not one of the ones that automatically connects the wiper to ground, lift that connection and use a couple of resistors to tie the centre of the humbucking pot to a higher voltage.  This is a tip that comes from the hi-fi world where hum is a really bad thing.  If you look at a tube, you have the heater protruding above and below the cathode.  Although it doesn't have the oxide coating that provides optimum electron emission, it is still hot enough to emit electrons and this emission is modulated by the heater AC voltage and the heater is usually at a more negative voltage than the cathode or grid and certainly lower than the plate.  Your phase inverter cathode is probably a hundred volts above the heater, so when electrons come off the heater, they hit the cathode, grid and plate, all of which are way more positive than the heater.

If your electrolytic filter capacitors are old, they have probably lost capacitance and this is another common reason for hum to gradually increase.  It is quite common to replace all electrolytics when restoring an amplifier.  Bad electrolytics cause a buzz at double the power line frequency and other harmonics since they operate on the positive and negative line cycles equally.  If your hum is an actual power line frequency hum at 50 or 60 Hz, then coupling from the heater wiring to the input is more likely.

[sarakisof]

Ron, plz read my posts above and see the pics. All electrolytics have been replaced since i got the amp in my hands. And the hum is a 50/60hZ thing.
Me too here, i likely think it's smthng around hum pot.

If it is not one of the ones that automatically connects the wiper to ground, lift that connection and use a couple of resistors to tie the centre of the humbucking pot to a higher voltage. This is a tip that comes from the hi-fi world where hum is a really bad thing.

Could you please explain this better or give a link to read about?

[amptramp]

I will try to explain heater emission on my own as I have never seen a simple statement anywhere.

A long time ago, directly heated cathodes were tungsten, the same metal as the heaters in indirectly heated tubes.  It was found that adding alkaline earth carbonates to the cathode allowed an increase in emission and that is why cathodes in indirectly heated tubes look like they have a white powder on them.  This is a mixture of calcium, strontium, thorium and barium carbonates and they reduce the work function, that is, the energy required to emit electrons.  But even though the heater has no such coating, it can still eject a cloud of electrons and what happens to this electron cloud depends on the voltage between the heater and all the electrodes in the vicinity.  The emission to worry about is that which takes place above the upper mica spacer and below the lower mica spacer.

This cloud will be right next to the cathode but the cathode is usually a low-impedance point that can absorb a few microamps without any noticeable change in voltage.  The grid is a different story - it often has enough resistance to ground that it is biased by cathode emission, which is why you often see triode audio stages with the cathode grounded and a resistor from 6.8 to 10 megohms from grid to ground.  This bias, sometimes called contact potential, is enough to keep a 12AX7 or its double diode-triode equivalent, the 6AV6 grid at -1 to -1.5 volts, often enough for the tube to remain in its linear range without drawing grid current or having the grid go positive with respect to the cathode.  There may be emission to the plate as well since it typically sits about 100 volts above the cathode, but it usually has a resistance on the order of 100 Kohms and the lesser emission to the plate counteracts emission to the grid.  (Lesser because the plate is farther away and shielded by the grid electrode wires.)

Now imagine what happens to the cloud of electrons surrounding the heater.  As the heater goes negative with respect to other tube electrodes, the emission increases.  As the heater goes positive, emission decreases.  There is nothing saying that one side of the heater has the same emission as the other side, so there will be a large part of the emission that takes place unbalanced and therefore at the power line fundamental frequency.  In particular, the phase inverter may have the grid at about 100 volts positive with respect to the heater, causing a lot of emission.

The cure for this problem is to either run the heaters off DC as is done in this case with the input tubes being run from the bias supply.  The other possibility is to disconnect the heater connection to ground and put it at a positive voltage so the grid of the tube will be extremely negative with respect to the heater voltage.  That way, there will be less of a tendency for electrons emitted from the heater impinging on the grid and modulating the grid bias at power line frequencies.