Are you measuring nominal voltage levels as per the schematic - that includes the various cathode voltage levels, and either using a bias probe for the 6V6, or inserting cathode sense resistors. Imho this is the very first step to take. If those levels are nominal, then inserting a test tone across something like R186 (input of preamp stage) can be used to measure the signal gain as it progresses towards the output stage - but you need to be able to identify where to measure that is safe, and use a meter or better still a scope with 10:1 probe to avoid loading down the signal. There are various switch contacts along the way, and all such switch contacts and valve base terminals should preferably get a squirt of contact cleaner and some exercise to reduce the chance of poor contacts causing you grief.
Also, did you measure each of the resistor values? Many may have drifted quite high, and some are quite high nominal values for grid leak purposes, which indicates that tube rolling would also be wise to confirm you are using a suitable 12AU7 and 12AX7's. Any of these parts in old equipment could cause you grief. This is not a simple restoration to take if you can't adequately test and fault find through experience.
Also, did you measure each of the resistor values? Many may have drifted quite high, and some are quite high nominal values for grid leak purposes, which indicates that tube rolling would also be wise to confirm you are using a suitable 12AU7 and 12AX7's. Any of these parts in old equipment could cause you grief. This is not a simple restoration to take if you can't adequately test and fault find through experience.
Thanks for the tips. Well, now I have tried inputting a signal at the phono input but using an outboard preamplifier. It sounds great and I am measuring the full expected clean 12 watts off the speakers. So it seems the power amp stage is in good shape.
Oddly, the "organ" (polyphonic) section is putting out a much lower volume than the "solo" (monophonic) section. I may well have drifted resistors or caps or goodness knows what in that area - there are a lot of them to measure considering they have to be taken out of circuit.
I have tried some known good tubes in v20 (preamp) and v27 (phase inverter).
For the time being, I'm more interested in finding where I can insert the input of an instrument-level guitar input, and get preamp gain before going into the power amp. Is R186 the right place for that? I wouldn't have guessed that.
Service manual is here
and S6 schemo here
too big to attach
Oddly, the "organ" (polyphonic) section is putting out a much lower volume than the "solo" (monophonic) section. I may well have drifted resistors or caps or goodness knows what in that area - there are a lot of them to measure considering they have to be taken out of circuit.
I have tried some known good tubes in v20 (preamp) and v27 (phase inverter).
For the time being, I'm more interested in finding where I can insert the input of an instrument-level guitar input, and get preamp gain before going into the power amp. Is R186 the right place for that? I wouldn't have guessed that.
Service manual is here
and S6 schemo here
too big to attach
Disconnecting R267 removes any noise/signal from the pedal control circuitry, but V20B input needs a grid leak path, so R278 and R279 could be rewired to just act as a single fixed grid leak to ground using say 18k+39k=56k or up to 1Meg - that will also raise the sensitivity noticeably. Bypassing R260 would also increase gain of V20B stage.
Disconnecting C174 and inserting signal across R186 appears to be the easiest insert point. Whether that gives enough sensitivity for a guitar input is up to your guitar setup.
The output stage likely has a lot of treble roll-off - but without a frequency spectrum test set up that wouldn't be easy to appreciate.
If you needed more gain then disconnecting R191 and R192 would allow signal from the solo balancer section to be used. That would require some modifications and testing as you then have various gain circuitry sections to include/remove/modify, given those sections are designed for tone signals not broadband audio.
Is your output transformer AO-20936-2 ? That was measured by some as a 14k4 PP to 4 ohm (1D to 2D) ratio, whereas an 8k PP load (ie. 2 ohm speaker) may provide better output power capability with 6V6's.
Disconnecting C174 and inserting signal across R186 appears to be the easiest insert point. Whether that gives enough sensitivity for a guitar input is up to your guitar setup.
The output stage likely has a lot of treble roll-off - but without a frequency spectrum test set up that wouldn't be easy to appreciate.
If you needed more gain then disconnecting R191 and R192 would allow signal from the solo balancer section to be used. That would require some modifications and testing as you then have various gain circuitry sections to include/remove/modify, given those sections are designed for tone signals not broadband audio.
Is your output transformer AO-20936-2 ? That was measured by some as a 14k4 PP to 4 ohm (1D to 2D) ratio, whereas an 8k PP load (ie. 2 ohm speaker) may provide better output power capability with 6V6's.
Okay thanks.
I tried inserting the guitar at R186, and unfortunately the gain is about the same as when using the phono in - very low. Maybe since guitar amps usually use 12ax7a, this circuit isn't doing much gain anywhere. I did try a 12ax7 in that spot but it was still quite low. Or maybe something is out of spec here, considering the organ output is also low. The voltages on v20 look bang on.
The output transformer is AO-26656. It has two 8ohm speakers in parallel. Where is stands is that the power amp section is plenty strong enough, I am just looking for preamp gain.
You're right there is a lot of unwanted treble roll-off - I can clearly tell with the guitar - but I'm not sure how late in the circuit, because phono in with preamp sounds pretty flat. The speakers themselves are also very dark.
I tried inserting the guitar at R186, and unfortunately the gain is about the same as when using the phono in - very low. Maybe since guitar amps usually use 12ax7a, this circuit isn't doing much gain anywhere. I did try a 12ax7 in that spot but it was still quite low. Or maybe something is out of spec here, considering the organ output is also low. The voltages on v20 look bang on.
The output transformer is AO-26656. It has two 8ohm speakers in parallel. Where is stands is that the power amp section is plenty strong enough, I am just looking for preamp gain.
You're right there is a lot of unwanted treble roll-off - I can clearly tell with the guitar - but I'm not sure how late in the circuit, because phono in with preamp sounds pretty flat. The speakers themselves are also very dark.
The AO-26656 output transformer reflects a 4 Ω secondary load (2 x 8 Ω 10" speakers in parallel) into a primary load of a textbook like 10 kΩ plate to plate impedance. Note the CT'ed secondary winding, one half of which is wound beneath the primary winding, the other half of it is placed on top .
Best regards!
Best regards!
If you need a bit more gain for the guitar you could lash up something with a J309/J310 JFET or another 12ax7, in the unit and feed the R186 or the phono input from the drain through a coupling cap.
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That's not a bad idea. Actually with a solid state preamp and the tube power amp, it sounds quite a bit like one of the MusicMan hybrid amps that Leo Fender made.
Since most guitar amps have 280v on the plate of a 12ax7 I guess this organ is not really designed to make that kind of gain at any stage - I'm just learning about this stuff.
Still feel like the volume of the organ itself is very low in the preamp stage so something is off.
Since most guitar amps have 280v on the plate of a 12ax7 I guess this organ is not really designed to make that kind of gain at any stage - I'm just learning about this stuff.
Still feel like the volume of the organ itself is very low in the preamp stage so something is off.
Update; someone on the organ forum tipped me off and the whole problem turned out to be the knee expression control lever. Who knew? Never would have figured that out. Jumpered it and now working full blast.
@baudouin0 you are absolutely right that there is a strong lowpass filter somewhere in the circuit. When I track that down, I think that's the last obstacle to using it as a nice little guitar amp.
Many thanks for the helpful comments.
@baudouin0 you are absolutely right that there is a strong lowpass filter somewhere in the circuit. When I track that down, I think that's the last obstacle to using it as a nice little guitar amp.
Many thanks for the helpful comments.
The lowpass filter may be the feedback in the power amp. Try disconnecting C237 to try. If you need to keep C237 for the sound, maybe reduce the coupling cap from your guitar preamp to form a high pass.
Thank you so much. That looks like it. Like a guitar tone pot I guess, the cap to ground. I just took it out for now.
I saw C235/R289 - that's a highpass RC filter at 70hz to protect the speakers I suppose?
Oddly, it's still quite dark sounding. I guess the speakers are also pretty dark. Is C236 or C240 on the phase inverter the same kind of thing?
I don't suppose I can do anything like a "bright cap" because it's already straight in with no volume or tone pot...
I saw C235/R289 - that's a highpass RC filter at 70hz to protect the speakers I suppose?
Oddly, it's still quite dark sounding. I guess the speakers are also pretty dark. Is C236 or C240 on the phase inverter the same kind of thing?
I don't suppose I can do anything like a "bright cap" because it's already straight in with no volume or tone pot...
Do you have any tools or way to take a frequency response of the amp? Many use a soundcard and software. Manual adjustment of an oscillator requires a wide bandwidth measurement voltmeter, such as a scope.
Did you disconnect the 'soft expression' circuitry by lifting R283 and C230 ? That may slightly load the output of V20B stage, but also modify the grid bias of V27 pin 7, so perhaps need to parallel R289 with 1Meg if disconnecting R283/C230.
The V27 PI stage may also be somewhat unbalanced, but may be of no concern. That unbalance may be a bit tricky to identify as you would need to apply a meter to each of the 6V6 input grids to maintain balance, but that then doesn't account for imbalance of the output stage. A better way would be to use a spectrum analyser on the output speaker signal and adjust R293/R294 (in the old days a hi-fi amp would use a trimpot in the 240Vdc feed to those resistors).
Did you disconnect the 'soft expression' circuitry by lifting R283 and C230 ? That may slightly load the output of V20B stage, but also modify the grid bias of V27 pin 7, so perhaps need to parallel R289 with 1Meg if disconnecting R283/C230.
The V27 PI stage may also be somewhat unbalanced, but may be of no concern. That unbalance may be a bit tricky to identify as you would need to apply a meter to each of the 6V6 input grids to maintain balance, but that then doesn't account for imbalance of the output stage. A better way would be to use a spectrum analyser on the output speaker signal and adjust R293/R294 (in the old days a hi-fi amp would use a trimpot in the 240Vdc feed to those resistors).
thanks for the input. The expression circuit is that swinging bar that serves as a loud/soft control. It was broken on mine, so at present I have jumpered around that whole circuit to get the organ to work at full volume.
As for the high cut, I can just tell with my ears; when I use a bass amp's line out at a preamp and boost highs 6-9db it sound flat and great. I'm still trying to figure out which of the caps in the prior post I can lift without messing anything up; C234, C236, or C240
Also working on a separate problem; some leaking sound from the solo section of the circuit like a stuck key. Pulling tubes makes it go away but the tubes are not the problem but I know roughly where in the circuit the problem is located.
This thing is a bear to work on. Every single hand-wired junction has the wire twisted through so it won't come out without a fight.
As for the high cut, I can just tell with my ears; when I use a bass amp's line out at a preamp and boost highs 6-9db it sound flat and great. I'm still trying to figure out which of the caps in the prior post I can lift without messing anything up; C234, C236, or C240
Also working on a separate problem; some leaking sound from the solo section of the circuit like a stuck key. Pulling tubes makes it go away but the tubes are not the problem but I know roughly where in the circuit the problem is located.
This thing is a bear to work on. Every single hand-wired junction has the wire twisted through so it won't come out without a fight.
Do you mean a link across the 'Loud' contact of that switch, which is what I assumed you had done.
I would suggest you remove HT from all the sections you aren't using, such as all the oscillators and dividers, or alternatively the valves - as you have tried. Careful that supply rails may rise. I'd suggest you are asking for hassles by not doing that, as signal leakage may not just be through the B+ rails.
If you snip a few resistors and they have drifted high, then its likely most of them will be the same. Trying to unravel old resistors using a soldering iron and long-nose pliers is likely to stress them which is then a one-way street to the bin if you want to continue to use that portion of circuitry.