Peavey Classic 20 Distorted sound

[dpm309]

I have a Peavey Classic 20 on my bench that is experiencing a very low, distorted sound.  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. This amp has a 3 piece PCB connected with a series of jumpers and folded into a "C" shape.  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.  I have attached the schematic and layout for reference.
 
Thanks,
 
Dan

[phsyconoodler]

exact voltages aren't need for diagnostics.Check both the screen and plate voltages and inspect the effects loop jacks for corrosion.
   You can check voltages right at the tube sockets with no tubes installed.
Look for about 350v on the plates and close to that on the screens.If that doesn't get you closer,then the chassis will need to come out for more closer investigation.

  J20 and J21 are hard to find on that schematic.Silly way to build amps IMHO.

[dpm309]

Yes, this is not a good way to make an amp.  It is extremely hard to work on.  Checked the voltages with the tubes removed and I get about 380V on the plates and screens.  I am also getting 380V on the plates on the preamp tubes.  This amp does not have an effects loop.  I pulled the board and went over it with a fine tooth comb and could not find any bad solder connections etc.  Could I be looking at the OT?  I ran some preliminary tests and found nothing out of whack.

Dan

[dpm309]

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:  http://www.hammondmfg.com/125.htm

Thanks,

Dan

[sluckey]

P/P EL84s are typically rated for 8000 ohms. So look in the 15 ohm column for the 8000 ohm primary. Follow that row to the left for the secondary connections (2 & 6 in this case).

[phsyconoodler]

Do you have a signal generator and a scope?If so,put a signal in and trace it with the scope along the signal path.If not,maybe a trip to Hoffman's library of information to build yourself a listening amp.

[dpm309]

Well, the OT was not the culprit.  I hooked up the Hammond and got the same, distorted low volume.  I was able to get the voltages with the tubes in and powered up.  Here they are:

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

The only thing that looks strange is that the plate and screen voltages on the power tubes are the same.  Should the plate voltage be closer to 328V?  My next step is to put a scope on it and trace the signal path as per phsyconoodler's suggestion to see where the signal could be messed up.  I also built the listening amp but have not used it yet.  I do not have a dummy load but am planning on constructing one.  I have an old Heil talk box - could I use this as the dummy load?  I will post the results of the scoping later today.


Thanks,

Dan

[dpm309]

I used the listening amp and found that the signal distorts between the plate of V1B and the input grid to V2A.  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 have replacement caps on hand and will try swapping these out to see if it does the trick. 

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

[plexi50]

You have a ground that is has come dissconected some where. What do your input jacks read on a DVM? 

[dpm309]

What type of readings am I looking, Voltage or Resistance or?  From what I can tell, this amp has 2 chassis grounding points, 1 at the input jack, and the other at the external speaker jack.  The grounds are connected by a 47ohm resistor located under the volume/gain control.  The external jack speaker jack is connected by a wire ribbon.  I found the ground connection on the ribbon to the amp to be loose and resoldered it but it did not solve the problem.  Would the master volume control have anything to do with this?  I measured resistance across the 1M pot and I get 33K between pins 1 and 3 and from 33K to 860K between 1 and 2.  I have a good 1M pot and I get close to 1M across 1 and 3. 

Thanks,

Dan

[plexi50]

Plug a cord in the high impedance input jack. You should read 1 meg or a little less. Do the same to the low impedance jack. You should read 134 or therabouts.  Then you can confirm or know if the input jacks are reading right from the jacks themselves and to the tubes. The master must have a grounded terminal. The terminal all the way to the left facing the rear of the pot.Clip DVM neg to chassis and confirm all grounds for your cathode resistors and caps.

Also know that EL84 tubes fry bacon and eggs when working. They get very hot. Check all solder tube socket pins to the board. Also i once had a Blues Junior that had a EL84 socket pin broke in between the board and the socket itself and intermitantly making contact. Aslo retention the EL84 tube sockets good. These tubes are almost always a little loose from being removed and replaced over time. Some customers i have known didnt know how to remove tubes and they would pull them to one side and yank them out. This makes the pins pretty loose so make sure there good and tight

[dpm309]

The readings checked out on the jacks.  Also rechecked all of the ground connections.  I then swapped the MV with a know good 1MA pot and the amp came back to life!  I couldn't believe that a bad pot could cause that much trouble.  Anyway, the amp sounds great now.  Want to thank everyone for your comments and suggestions.

Dan

[Rev D]

 I agree with plexi on checking the tube socket connection to the board. I've had my old blues jr distort and had it redplate, and it was always a connection of either the ribbon cable where it soldered on or the tube socket itself. Last time i worked on it before I sold it it was the tube socket. Reflowed things and it was fine. He's right they run really hot and that would be one of the first places I'd look (only because I've seen this before and its the easiest thing to check, I'm no amp tech, but I play one one TV...)

Regards,

Don