Peavey Classic 20 Low, Distorted volume

[DPM309]

I am new to this forum but have been building and repairing amps for several years. I have a Peavey Classic 20 on my bench that is experiencing a very low, distorted sound. The only thread I found on this site regarding the Classic 20 had to do with filament heater problems. All the tubes check out (both with tube tester and swapping known good tubes) and I also plugged it into another speaker. The heaters are working on all of the tubes. The client changed out the master volume recently and the problem came up a few weeks after that. My next step is to check the tube voltages but this will be a challenge since I will have to pull the board, etc. Does anyone have a tube voltage chart for this? Also, anything else I should be looking at? I will check all of the soldered connections and jumpers while I have it apart. Any advice would be appreciated.

Thanks,

Dan

[DPM309]

I pulled the board and went over it with a fine tooth comb and did not find any bad solder connections or broken jumpers. I checked the plate and screen voltages with the tubes removed and everything checked out. I got 380 V on the plates on the preamp tubes and 390V on the plates and screens on the power tubes. I constructed a shorted winding tester as per another thread on this forum and the OT does not have any shorts. I re-soldered the volume control that the client changed out recently but have not put the amp back together to test it. I am running out of options on this amp and would appreciated any advice.

Thanks,

Dan

[Enzo]

Wait, I just read someone with a Classic 20 with a OT primary winding measuring 6 ohms. That wasn't you?

[DPM309]

Enzo, no that wasn't me. I have a good Hammond 125E that I want to use to see if the OT is bad. It is a push-pull transformer that is rated at 15 watts. My question is what secondary leads should I use for a 16 ohm speaker? There are 6 secondary leads and it depends on the Primary Impedance for which pair to use. How do I calculate or measure the primary impedance? I know I ran across this before on this or another amp forum but cannot locate it. Would appreciate any advice. Here is the link to spec sheet: Hammond Mfg. - 125 Hook-Up Data

Thanks,

Dan

[Jazz P Bass]

Did you measure the bias voltage on BOTH tubes?
The transformer data from Hammond does not indicate 16 ohm hookup.
Seeing as 15 ohms is close enough, what the hey.
I would try for 10K primary for 2 push pull EL84's.
That means use lead 1 & 5.
Link: Hammond Mfg. - 125 Hook-Up Data

[Enzo]

Here is the deal. If all you want to do is sub a different OT in there to see if that was what the problem was, then don't even worry about impedance. I have an old Fender Bassman 50 watt OT on the bench that I use as a sub in ANY amp I might suspect the OT in. We are not looking for subtle nuance in the sound, all we want is a go/no-go on the old tranny.

COnsider most any tube amp you have. Likely it wants a 4 or 8 or 16 ohm speaker load. Now what really happens if you connect a "wrong" load from that list? perhaps at high levels the power transfer is diminished, but so what? I can do the test I need at low to moderate levels anyway. perhaps the distortion might be a little higher. Again, I am clipping the sub transformer in there because I have a gross problem, not a subtle problem. An extra percent or two of distortion is not going to mask the gross defect of the old part. My 4 ohm bench cab will work just fine on a 16 ohm output, at least to make sure the amp functions. Will I crank the amp to the sky that way? No, but the testing works fine.

Now then the turns ratio of the transformer might not be just what the tubes are expecting, but again, it won't be so far off that you can;t tell if changing the transformer solves the problem or not. And that is the point, I use clip wires to sub in a differnt tranny to find out if a new tranny is what I need. If my sub happens to be exactly what I would install, GREAT! But if not, it still will tell me if that was the problem or not.

Once you have determined that you need a new tranny, then we can worry about the best way to install it.

[Amp Kat]

Check the voltage across the Cathode resistor and see what you get. Should be around 20 volts. Divide that by the Cathode resistor value to get the ma's then multiply by plate voltage to get the wattage. Jumper wires were the last cause of low volume on the one I did. Check them out good.

[J M Fahey]

I got 380 V on the plates on the preamp tubes

This sounds very unusual to me.
You read that on the actual plates, or it's just the general PS voltage that feeds the preamp?
It's not the same.

[DPM309]

The 380V was from the power supply. It is almost impossible to measure voltages with the tubes in this amp since you would have to take the folded chassis out of the amp, prop it up, and make sure the OT, PT and grounds are all connected. I did however rule out the OT. I substituted the Hammond 125E and still got the low, distorted sound. I am in the process of putting the amp on my scope and using a signal generator to see where the signal is getting messed up. I can access most of the signal by connecting to the various jumpers that hold the 3 pieces of the board together. I traced out the signal path and referenced it to the jumpers which are accessible with the PCB installed in the chassis. I will report my findings later today.

Dan

[DPM309]

Here are my voltages with tubes in and powered up. I was able to get these from the jumper wires connected to the pins:

Tube 1 2 3 4 5 6 7 8 9
V1 12AX7 170 - 1.2 - - 166 - 1.2 5.6
V2 12AX7 188 - 1.1 - - 243 - 1.1 5.6
V3 EL84 - - 9.6 AC AC - 317 - 317
V4 EL84 - - 9.6 AC AC - 317 - 317
Plate: 328, Screen: 319, B++: 304, and B+: 288

I tried scoping the signal path and it appears that the signal distorts, degrades, after each stage in the pre-amp. The signal coming out of the power section looks even worse than the last pre-amp stage. I am new at using a scope so please bear with me. So far, I have ruled out the tubes, OT, and the power supply. This leaves the pre and power amp sections. I will check the cathode bypass caps and resistors as well as go over every jumper on the PCB again. Do the voltages look in line? Let me know if anyone has any other ideas?

Thanks,

Dan

[MWJB]

Pin 8 of V2B should be much higher than 1.1vdc.

[Enzo]

I agree, there appears to be something funny about your phase inverter.

[DPM309]

I thought that was the case. On my home brew amps, I usually get about 70+/- volts. I also used an amp stethoscope and found that the amp distorts between the plate of V1B (Pin 6) and the input grid to V2A (Pin 2). I seemed to narrow the culprits down to the coupling caps (C3 and C2). Everything after this point is distorted. This seems to be where the signal started getting distorted using the scope. I will look at the phase inverter closer to determine the low voltage on Pin 8.

Thanks,

Dan

[DPM309]

I replaced C2 and C3 and while I had the board out, I changed out all of the filter and bypass caps. This did not solve the problem. I also replaced R13 which was giving me erratic readings. I double checked all of the wire jumpers and everything checked out OK. This has to be the most frustrating amp I have ever worked on. The owner has previously changed out the volume control with no success. While using the listening amp, I noticed that it loses a lot of volume after the tone stack and master volume. Rotating the MV control did not change the sound at all. Could I be looking at a bad MV pot or something else in the tone stack? The readings on all of the resistors and caps in the tone stack seem in line with the schematic. Here are the voltages after I re-measured them:

Tube 1 2 3 4 5 6 7 8 9
V1 12AX7 170 - 1.2 - - 166 - 1.2 5.6
V2 12AX7 188 - 1.1 - - 243 18 60 5.6
V3 EL84 - - 9.6 AC AC - 317 - 317
V4 EL84 - - 9.6 AC AC - 317 - 317
Plate: 328, Screen: 319, B++: 304, and B+: 288

Anyone have any other ideas?

Thanks,

Dan

[DPM309]

Just want to let you all know that the problem turned out to be a bad MV pot. Swapped it with a good 1M pot and the amp came back to life. Thanks for all of your suggestions and comments.

Dan