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I rebuilt an old peavey amp VTM60 which sounds only ok but is sort-of loud still. The OT runs totally cold though. So I'm wondering of there is something wrong. It is rather smallish almost 6v6 sized but meant for 6L6 which it now has. Any ideas of what I can zero in on? If I measure the primary I need only disconnect the centre B+ in, right? It still has 100Ω screen grid resistors. Going loop in isn't that super loud and ballsy either.
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You will have to decide what you want from it and measure what you have. I am totally confused by your questions. Why suspect a transformer in an amp that you say is getting loud? What signal are you sending into the effects loop with which you are expecting it to be ballsy and super loud.
What power are you measuring output now? Use a dummy load and use a sine wave signal and measure the RMS voltage across the load when the gain is set to the onset of clipping, square that value and divide that by the impedance of the load to get the power output.
Did you ever hear the amp working where it DID sound ballsy? -
Power transformers get warm or even hot normally. Output transformers don't work anywhere near as hard and so don't heat up much.
COncentrate on finding what signal levels there are, rather than transformers.
Transformers do occasionally fail, but they are always the last thing on the list.Education is what you're left with after you have forgotten what you have learned.Comment
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The amp works but seems choked or stifled. I compared the normal operation with its Marshall type preamp with inserting (into effects return) my pedalboard instead. I only measured power by ear and it has volume but its a bit off. I supposed I could put my meter, set to AC Volts, and measure that. I've got BIG 8 and 32Ω resistors. Heard this amp model years back and they rock. The PT get only warm and the OT is stone cold. It has 100Ω screen resistors. I'll check the phase inverter over again and upgrade the screen grid resistors with 1K or higher for that touch sensitivity while there.
spasiba.Comment
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I would strongly advise against changing anything until you repair the amp. Changing the status of the amp even in minor ways will make diagnosing the problem more difficult. If you want to modify it, do so after it is working normally.
There are so many variables that make comparing amps difficult in pin-pointing the cause of a problem since there are different eq responses, different gain structures and non-standard effect look send and receive levels. Do you have access to an oscilloscope? If so, use that to monitor the output when putting in a continuous sine wave at 1khz. With tone controls mod scale, it should look like an undistorted sine wave. Compare the waveform of the input signal to the output signal. That would tell you whether the power amp section is the cause of the sound you do not like. It could be the preamp section just as easily. It might even be memory playing a trick on you since we do not remember sounds very long but we remember our impressions of the sound for a long time and normally confuse the two. That is the reason swapping tube types and judging sound to something in the past is usually very inaccurate.
If a transformer warms up a lot, that is more of an indication of a problem than if running cool. Regarding size, the amount of iron needed is inversely proportional to the frequency of the power being transferred though it. Guitar amps do not need the bottom end that bass or hi-fi amps need to reproduce so can be smaller for the same power.Comment
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The VTM is the only amp PV really hit the mark on. I don't understand why there isn't some variation of it still on the market. I liked it so much I borrowed the preamp circuit, tweaked it a little, and now its become the circuit I put in all my amps. Add an active mid boost and look out....
Anyway.....the OP never said how the dip switches were set. Try setting it with all switches off except for the 2 gain switches (first 2). If it sounds lifeless then.....you have a problem somewhere.The farmer takes a wife, the barber takes a pole....Comment
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Sorry a bit too late for the modifying but that is all to the preamp. The phase inverter is stock. All the filter caps in the amp were totally shot, so those got replaced and beefed up a bit. I'll try and check it Weds. The dip switches are set to how I like it but I can zero them. The compressor diodes don't do anything noticeable.Comment
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Finally swapped in another Peavey transformer and now the volume is back and LOUD. Awesome! So weak output can be a compromised OT.
Soon had to swap in 1K screen resistors as I had KT77s.
V2 also got an ECC823 (reverse 12DW7). Lowered R10 (100K) as that sends out to effects. Also swapped in a small trim pot for send level.
12AT7 for the PI.
Will crank it up over the next few days...Comment
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