Spring Reverb For Farfisa Organ

[Rob Strand]

Resistance measurements

The resistors look OK then.

So all as it should be. Only prob is that i get that zero voltage at V2 pin3. But should this be the problem for reverb? I mean the organ works perfect, Rob i think it is smthng in the reverb tank itself and its carts.

The V2 pin 3 voltage cannot be affected by the cartridge.  While I don't know if the cartridge is OK there is *definitely* something wrong around  V2 (pins 1, 2, 3).    The audio probe should get audio out on pin 1.  With out that the Reverb circuit can't work.

But, about the second issue, at V2 pin3 zero voltage, now i got 0.9 V. ...

So maybe the V2 socket has a connection problem.
While you have 0.9V there you should redo the audio probe test to see if you now have a signal at V2 pin 1.

Rob i swapped the tubes and thats the result: For the first weird voltage issue at V1 pin6 95V the situation was about the same. Now it got at about 110V (instead of 120V before) and then reduces and gets stable at 95V (instead of 90V).

OK so the V1 and V2 tubes are probably OK.

The other thing that doesn't make sense is the circuit for V2 (pin 1,2,3) is identical to the circuit on V1 (pins 6, 7, so that means V2 pin 1 should be at 95V but it's not.  It doesn't make sense unless there is a problem somewhere!    We know the tube isn't making the plate voltage different so it mush be the circuit.   Since the organ is working and the reverb is not.  I can only assume the problem is around V2 (pin 1,2,3) and the fact the audio probe does not get signal on V2 pin 1 more or less confirms that.  One caveat though.  If C506 has a problem then that could be the cause of the different voltages on V1 (pin 6, 7, compare to V2 (pin 1,2,3).

So there's some things that don't make sense and that usually means there's a problem!

[sarakisof]

Quote
But, about the second issue, at V2 pin3 zero voltage, now i got 0.9 V. ...
So maybe the V2 socket has a connection problem.

You might got confused. I swapped just the tubes, the sockets are the same. And by using V1 in place of V2 i got 0.9V, so this maybe shows that V2 is not good ...
Anyway i used the "good" V1 in its original place/socket and replaced "bad" V2 with a new 12AX7. Again, i had 0.9V at pin3 and guess what ... i got signal at pin1, point "7" and of course at send cartridge !!

Tried the reverb with a sustained note pushed continuisly with tape mask (i had to hold carts with my hands as they are both dissasembled from mounting), reverb switch on, but no reverb effect produced, only dry signal again    tried swapping wires-combinations-carts etc. same results. Will try tomorrow with two piezo disks (no carts at all) to see what happens.

If C506 has a problem then that could be the cause of the different voltages on V1 (pin 6, 7, compare to V2 (pin 1,2,3).

are you sure it is C506? you mean this one at V1 pin8 ?

[Rob Strand]

You might got confused.

Yes, now I'm really confused.  I guess when you say V1  I don't know if you mean the *tube* that *was* in  the socket for V1, or the V1 socket itself regardless of the tube in there.  It makes more sense to refer to V1 as the socket as this is the position in the circuit, not the tube.

Tried the reverb with a sustained note pushed continuisly with tape mask (i had to hold carts with my hands as they are both dissasembled from mounting), reverb switch on, but no reverb effect produced, only dry signal again    tried swapping wires-combinations-carts etc. same results. Will try tomorrow with two piezo disks (no carts at all) to see what happens.

If you want to bypass all the problems with the reverb circuit.  One very simple test is to get an audio source like a radio,  wire the speaker output to the send cartridge via a 1k resistor, set the output to a low level,  then see if you can hear the sound come through the reverb circuit.  No need to press any keys on the organ.  No need to debug the reverb circuit.

[sarakisof]

Yes, not I'm really confused.  I guess when you say V1  I don't know if you mean the *tube* that *was* in  the socket for V1, or the V1 socket itself regardless of the tube in there.  It makes more sense to refer to V1 as the socket as this is position in the circuit, not the tube.

yeap you are right it makes more sense like that but i was meaning "V" as the tube 

One very simple test is to get an audio source like a radio,  wire the speaker output to the send cartridge via a 1k resistor, set the output to a low level,  then see if you can hear the sound come through the reverb circuit.  No need to press any keys on the organ.  No need to debug the reverb circuit.

yeap i was thinking of doing that using a laptop headphone output at very low volume, will try later with many different experiments. I just received those two cheapo chinese "ceramic" carts from ebay (used in those garbage Crosleys tt's etc.) so i will have to play with many options: original farfisa's carts, piezo disks, cheapo carts and swapping send with receive terminals to see what happens.

Also i received today another order i had made, a cheap multimeter that can measure capacitance (not esr-in circuit) so i can test some suspicious caps in there after decharge and desoldering. It also can measure AC at low range 200mV-2V-20V etc so i can also do that key pushing test that you proposed for 0.1vac or so.

If C506 has a problem then that could be the cause of the different voltages on V1 (pin 6, 7, compare to V2 (pin 1,2,3).

Are you sure it is the C506?

[Rob Strand]

Anyway i used the "good" V1 in its original place/socket and replaced "bad" V2 with a new 12AX7. Again, i had 0.9V at pin3 and guess what ... i got signal at pin1, point "7" and of course at send cartridge !!

OK, great.  One problem down.

Are you sure it is the C506?

Don't worry.    Changing the tube has pretty much fixed all the problems discussed in the last 10 or more posts.

Keep in mind, since the reverb drive was probably never working, some of the testing you did earlier on the reverb cartridges and spring might have showed bad results because of the bad tube.

So now we are back to the original problem of the spring reverb itself not working.   We know the both cartridges work to some degree but we were not sure if the lower sensitivity cartridge was OK or not.

[sarakisof]

Thats right  Rob could you tell me if it safe to work "free of dangerous thoughts" inside the tank? I mean i will do swapps, tests, changing carts and touching send/receiver connectors with the organ powered. At pin V2 pin1 i have high voltage, why i dont have high volts after the C512 cap (point "7") too? Is it due to C512? Cause i dont get high volts after it and so at send connectors... so is it safe? I am asking this cause till now i was extra careful working inside tank (apart from inside chassis indeed-ther i know where high volts are) and was handling carts only with electrician gloves etc...

And smthng bad: i still get 90-95 V. at pin V1 pin 6 even after changing V2 tube ...    maybe is normal? i mean according to Compact's schem it is not normal ( i should get about 150V like at the other pins (1 of V1 and 1,6 of V2) but probably is it normal for Foyer's schem that we cannot find ...
But i guess it is not normal as it is a classic tube amp circuit. Basically, V1 is the first tube signal gets into and first tube after Rectifier EZ81 tube. So maybe the problem has to do with EZ81? Do you think i have to replace all tubes first to see what happens? I didnt changed them after repair, cause the previous owner told me the organ owned by his father and had been played only a few hours back in the 70's ... I was the first one that opened it all screws and stuff were factory sealed/locked 

[sarakisof]

Roooob we have reverb mutha##zzz !!!  actually we had from yesterday (since changed tube and got signal at send cart) but i was pressing a key with masking tape continuously ignoring that i had to push keys constantly to realise reverb effect 
All thing was to achieve having signal at point "7" then things got clear !! Reverb effect is a bit short decayed and a bit "smaller"/smoothier, not that "big" but i guess i am used to those drippy surfy guitar splashes haha 

Rob, my trully respect, love  and huge appreciates to you my man, you are a real hero 

Now, only last thing i have to see is that 90 V. at V1 pin6, i will try another new 12AX7 later at nigt to see if there is any change.

Peace to all, this is a great forum guys, i learned so many things so far. One love.

[Rob Strand]

Roooob we have reverb mutha##zzz !!!

Hey, awesome!  I'm really glad you stuck with it until the end.

Now, only last thing i have to see is that 90 V. at V1 pin6, i will try another new 12AX7 later at nigt to see if there is any change.

It's worth tracking down why V1 pin 6 isn't the same as V2 pin 1.  If you change the tube and the voltage stays the same at the same *socket* that usually means the resistors are not the same.  However you measured the resistor and they looked OK.   

If you move the tube and the voltage moves with the tube you know it's the tube and if you move the tube and the voltage stays with the socket you know it's the other parts wired to the socket, eg. the resistors.

To tell you the truth I'm not overly concerned with the lower voltage, and maybe the higher ones are more of an indication the tube is old! 

By the way, what voltage are you seeing on V1 pin 6 now?   Actually you should measure all the voltages to know where you stand.

The voltages on the original schematic are probably OK.  *However* the thing to realize is your power rail is 218V not 235V, which is 17V lower.  So *based on the schematic voltages* I would automatically expect the lower voltages on your unit to be perhaps 10V to 15V lower.   So that would put the expected voltages at say 132V (and 187V).  Based on the circuit I would actually expect a little lower than that.

From your old measurements with the bad tube
V1 pin  1              155V                                 }
V1 pin 6                95V                                  }    All these should be the same
V2 pin 1                160V     (bad for sure)       }
V2 pin 6                150V

Compared to 132V or  95V is low by 37V, and 155V is high by 23V.   So they are sitting either side of the the expected value.   The 95V is more outside of the expected range but so are the others!

Rob could you tell me if it safe to work "free of dangerous thoughts" inside the tank? I mean i will do swapps, tests, changing carts and touching send/receiver connectors with the organ powered.

It's always a risk when working on tube amps.  Sure there are safer parts of the circuit, basically any points which don't measure a high voltage.   However,  everyone makes mistakes.  You might intend to probe a low voltage point but probe the wrong point which is 200V.  Also you can unintentionally knock your fingers or hands against high voltage points.   The safest work practice is to assume everything is bad and avoid touching stuff.   Double check what you are doing.    if you think you might slip and touch something else find a different way to do the measurement in a safer way.  I've done this stuff for a long time and I still work like that,  and I can still make mistakes and get zapped but luckily not very often.

[sarakisof]

Back again after some tests. Found another two 12AX7s (not new though just used some hrs) and put in sockets V1 and V3,  V2 with the new one like before.
I guess the results now make more sense compared with that "less voltage in mine compared to Compact" that you correctly noticed. The only maybe (maybe not?) "anomaly" now is the V1 pin1 150-155 V.

New results:

V1 pin  1              150-155 V
V1 pin 6                95-100 V                           
V2 pin 1                105-110 V   good!         
V2 pin 6                145-150 V

I put range +- 5 V because all of them make a peak then reduce and then reduce very slowly, almost stable. After 2min they get stable but i dont wait to measure, so probably those in a more "spherical" way probably be:

V1 pin  1              150 V
V1 pin 6               100 V                           
V2 pin 1               100 V           
V2 pin 6               150 V

So probably according to Compact's schem (and the scale correspondance reduced volts adaptation in mine's) the only "anomaly" is the V1 pin 1, maybe not  ... Should i test C514 out of circuit capacitance?

I also noticed smtng weird tat xplains the fact that reverb is a bit weak (and its not my thought, listened to normal farfisa's rvrb today) : Remember in the first posts here that i was tapping the springs and got big splash(but input/send was not working bla bla)?? Well that was a big splash, with volume enough and with a "high" freq but good tone, you know a "healthy" splash, almost like when i tap on my guitar surfy's tank. Now that splash (isnt actually even a "splash") is more hazy, stuffed, low passed, weak, what changed? Maybe i have connected backwards the cart's connectors will try later, any other thoughts? Now its more like a weak plate/echo effect, not a spring thing ...

[Rob Strand]

V1 pin  1              150 V
V1 pin 6               100 V                           
V2 pin 1               100 V           
V2 pin 6               150 V

So probably according to Compact's schem (and the scale correspondance reduced volts adaptation in mine's) the only "anomaly" is the V1 pin 1,

I have some good news!    Your voltages are fine.
What's is wrong is the schematic!

Look at the resistor values you measured previously"

V1
pin1: 100K
pin6: 220K
pin3: 2.2K
pin8: 2.2K

V2
pin1: 220K
pin6: 100K
pin3: 2.2K
pin8: 3.3K

Now look at V1 pin 1.  It has 100k there but the schematic has 220k.  With 100k the anode voltage will rise-up compared to the tubes with 220k.  So all your voltages are fine.

I double checked the expected voltages with my circuit simulator,  and with the corrected resistor values I got these expected voltages,

                            Your measurement           Rob's simulator
V1 pin  1              150 V                               143V
V1 pin 6               100 V                               102V
V2 pin 1               100 V                               102V
V2 pin 6               150 V                               156V

So all the voltages look quite believable.

also noticed smtng weird tat xplains the fact that reverb is a bit weak (and its not my thought, listened to normal farfisa's rvrb today) : Remember in the first posts here that i was tapping the springs and got big splash(but input/send was not working bla bla)?? Well that was a big splash, with volume enough and with a "high" freq but good tone, you know a "healthy" splash, almost like when i tap on my guitar surfy's tank.

Yep, understand you 100%.

Now that splash (isnt actually even a "splash") is more hazy, stuffed, low passed, weak, what changed? Maybe i have connected backwards the cart's connectors will try later, any other thoughts? Now its more like a weak plate/echo effect, not a spring thing ...

Hmmm,  given it more or less works I can only think of the wrong cartidge, or perhaps the springs aren't mounted right.   If possible it's probably worth swapping the cartridge to see if it has an effect or not, that would be a good start.

You could also swap the tubes.  I don't expect a change but it's very easy to do.

[sarakisof]

I have some good news!    Your voltages are fine.

i am very glad and honored

Now look at V1 pin 1.  It has 100k there but the schematic has 220k.  With 100k the anode voltage will rise-up compared to the tubes with 220k.  So all your voltages are fine.

yeap according to my situation's schem resistors and circuity i also had realised that. Get 100% what u say. Its like i learn, realise, notice, understand things and some hrs later you are back here confirming my thoughts, it is amazing 

Hmmm,  given it more or less works I can only think of the wrong cartidge, or perhaps the springs aren't mounted right.   If possible it's probably worth swapping the cartridge to see if it has an effect or not, that would be a good start.

yeap i will do that by next week as i will be on road for the weekend, so will be back -hope- with results. Actually i had an accident one day as a result of plenty test and xperiments done, that i didnt mentioned before (we had more severe things to deal with hehe) and was that one of the 3 receive springs (black receive cart) - actually its terminal ring that normally is soldered to the wire that ends at the black cart - got desoldered from that wire so i had to resolder back. but it seems i didnt soldered it exactly at its original position inside the circle ring and that has caused this third spring to sit a bit skew/oblique than the others that are in total parallel. I will try to resolder and fix this too, but i dont think it is the source of reverb weakness. My 2 cents would go to the "bad receive cart" thought ...

[Rob Strand]

Its like i learn, realise, notice, understand things and some hrs later you are back here confirming my thoughts, it is amazing

That's a good thing!

I will try to resolder and fix this too, but i dont think it is the source of reverb weakness. My 2 cents would go to the "bad receive cart" thought ...

See how it goes.    You can only work though one problem at a time until there's no more left.