No announcement yet.

half a peavey classic 60/60!

  • Filter
  • Time
  • Show
Clear All
new posts

  • half a peavey classic 60/60!

    hi guys,
    i just got this off ebay n' only one channel works. (ch1=good, ch2=bad)
    i've already tried swapping power & pre/inv tubes to various slots.
    and i've disconnected and reconnected all of the connectors to the board.
    two power slots stay cool in an x pattern and do not register on my weber bias meter. the other two were around 35ma.

    also, i can hear a very faint output on the bad channel, and the input sensitivity control no effect on the level, even at zero i can still hear it.

    any ideas would be greatly appreciated.

    oh, can you guys help me identify which slots go to which channel, just incase we can't troubleshoot this, i can at least use ch1 and know what tubes to replace.

    stock photo:

  • #2
    In general, Peavey are very good about coughing up schematics for stuff that's out of production - sometimes it's even on their site free for the downloading (not this time, though).

    An email to one of the addresses here: will get you in touch with the right people.

    Worst case, expect to pay about 3USD - sometimes that's per page, sometimes for the whole deal - and maybe postage if international.

    Your x-pattern description sounds really odd, as in that's probably the worst possible way to lay out a stereo pair of power stages. The tubes that stay 'cool' probably aren't getting heater power, so a tubes-out check to verify that would be good. Then maybe resolder or retension those cool sockets and see if they come back.

    Those 4 big cement resistors right near the output tubes - are they around 1 ohm, and are they connected between the cathode pins and GND? That would make them built-in bias measurement points (voltage across resistor)/(value of resistor) = (bias current).

    While you're waiting for the schematic, go ahead and measure the resistance between the plate pins of the 4 output sockets to get an idea how they're paired, and see if someone has an open or something. It's just a tube amp, twice, so general debug processes should get you there.

    Last edited by Don Symes; 05-28-2006, 05:05 AM. Reason: A little more detail.


    • #3
      hey Don,
      thanks for the reply!
      i actualy have the schematic. i got it from a zip file filled with bias pot mod instructions. that's where i got that photo in first post.

      i cant remember where i got the zip file from, so i've re-hosted it here:

      i've only read schems for a bias mod on a marshall 2553, to try out different types of tubes for more clean headroom (ended up going back to el34s) & once to replace a diode on a peavey classic 50 head.

      i have no idea what to look for in this situation.

      maybe you guys can take a look.



      • #4
        oh wow! i've never scrolled down to the 2nd page of the pdf.
        it has the circuit board layout. that's gotta help!



        • #5
          Originally posted by mocklah
          it has the circuit board layout. that's gotta help!
          Just a whole bunch.

          Those cement resistors are for the screens, so no biasing help there.

          I also don't see a plausible way for the board to break in such a way that you'd get that X pattern of non-heat, so I'm sticking with the broken solder joints or loose sockets.

          Once that's straight, I'd probably check out the OTs and their connections fairly carefully. Then, those diodes - CR1, CR2, CR3, CR4, CR20, CR21, CR22 and CR23 are supposed to protect the output tubes from loss of bias or no-load conditions, and one or more of them could be bad.


          • #6
            hey thanks for all the help.
            steve was helping me over on the old forum too. he lead me too pin 4 on the power tube sockets...n' sure enough...low to no voltage on the bad channels.

            so this project is on hold till i get my resistors from mouser. then if that don'd fix it, i'll follow the rest of the advice you guys gave me.

            here's a repost for anyone else runs into the same problems:
            Steve Dallman says:

            You can call PV and they will e-mail you the schematic. You need to test all voltages. Do you have the correct heater voltage, plate and screen voltages? If the tubes stay cool, you could have lost the screen resistors which would turn off the tubes, or the heater voltage may be absent. Is the bias correct?

            Just swapping out tubes without testing the signal path and voltages won't get you far. There are tried and true procedures for going through a tube amp.

            The schematics will show you what tubes do what and will have the voltage info you need.

            Ooops...I just found the schematic and the layout and voltages aren't listed, but here's some info.

            Looking at your picture, the small sockets are (left to right) V1, V7 & V4.

            Each channel has half a tube for an input gain stage and then goes to the phase inverter which feeds the power tubes.

            The four power tubes are V3, V2 and the ones below them are V6 and V5.

            Half of V7 is the preamp stage for Channel 1 and the other half is the preamp stage for Channel 2.

            V1 is the phase inverter for Channel 1. V4 is the phase inverter for Channel 2.

            V2 and V3 are the power tubes for Channel 1. V5 and V6 are the power tubes for Channel 2.

            Check the plate voltages on the preamp tubes. They should be in the 200-300vdc range. These would bre pins 1 and 6. Check the heater voltages. Pins 4 and 5 of the preamp and phase inverter tubes are wired together. The voltage between pins 4&5 and 9 should be close to 6.3vAC.

            Then on to the power tubes. Or you can start with the power tubes.

            You should have high voltage on the plates. Pin 3 on each power tube should have 400-500vdc on them. All voltage measurements are from pin to ground, except for the heater voltages).

            On pins 4 should be the screen voltage, which should be a few volts less than the plate voltage. I suspect the screen resistors may be gone on the non working channel. These will be R3 and R25 on Channel 1 and R4 and R26 on Channel 2. These are fairly large 5 watt power resistors. If you wish, you could change these out, and use 1k5W resistors which will help prevent screen failures in certain current production tubes. If the screen fails, the tube will light up but will stay cold and not work.

            Check for the bias voltage on pin 5 of each power tube. It should be around negative 40-50vdc, more or less.

            There are more tests you could do, like measuring the cathode voltages of the 12AX7's.

            In testing an amp, IF all the voltages are where they should be, I'll put a signal into the amp, making sure I have a speaker or dummy load on the speaker outs. Then I use a tool I made. I have a probe that connects to the center wire of a shielded cable with a .1uf/630v cap. The other end of the shielded wire has a 1/4" plug soldered AND has a long wire soldered to the ground of the plug with an alligator clip on the other end. I plug this cable into a small practice amp, put the alligator clip on the chassis or other ground point in the test amp, and then use the probe to follow the signal through the signal path of the amp. The cap prevents high voltage from hitting the practice amp. I usually use a radio as the signal source, but sometimes use an sine wave generator.

            Make sure the probe/cap assembly is solid and well insulated. I also added a 1meg, audio taper pot between the cap and the center wire of the shielded cable, so I can adjust the signal level going to the practice amp as the signal should get louder and louder as it goes through the amp.

            There is always the possiblity the output transformer is blown. With the power off and the caps drained, take out all the power tubes and check the resistance between pin 3 of each power amp pair. Your resistance should be the same measured on each channel. Plug a 1/4" plug into the speaker out, and measure the resistance from hot to ground. It should be the same for each channel. If you get an open reading on one of the channels, either in the primary (pins 3) or the secondary (speaker jack) then your transformer is shot. If the transformer was shorted, I'd expect the fuse to blow.

            There are several diodes in the amp. I'd expect the power supply caps to be OK because the one channel works.

            There are two diodes in parallel with each 220k bias resistor. There is also a 100pF cap in parallel also. check these diodes. CR20 and CR21 in Channel 1 and CR22 and CR23 in channel 2.

            The B+ voltage on Channel 2 is at J15. The voltage goes through each half of the output transformer and to the tubes through J12 and J20 of Channel 2.

            In Channel 1, the B+ is at J13, and the voltages to the plates are on J11 and J19.

            Before you do ANY of this. Make sure you know how to work safely around tube amp voltages. If you don't know how to check the caps when the power is off and how to drain them DON'T try ANY of this. Fortunately in this amp there are two series pairs of filter caps bypassed with resistors. The resistors should drain the filter caps fairly quickly.

            There are voltages and currents in a tube amp that can kill you.

            The second thing to remember is there are voltages and currents in a tube amp that can kill you.

            Warnings to take measurments with one hand in your pocket aren't out of line.

            This should help get you through the amp. Fortunately, tube amps are easier to troubleshoot than SS least for me they are. Compared to some SS power amps, a tube amp is as complicated as a flashlight.

            A bad plate resistor, screen resistor, cathode resistor, etc could cause the channel not to work. An open cap or resistor in the signal path could as well. A bad input pot (sensitivity) could also cause the channel not to work.

            Have fun. Play safe.


            • #7

              hi guys,
              the resistors came in friday and i worked on it saturday. i replaced all the screen resistors, put it back together and fired it up.
              all the tubes were now equally hot. but the bad channel still didn't work.
              so i started poking around the input section with my meter and i found that the input pot was i bypassed it.
              now every thing is fine!

              i'm sure glad i didn't find that open pot first...'cause i may have never realized that i was only running two out of four tubes!

              thanks for your help.



              • #8
                You would have still found the problem, as only one channel would have worked as it should. The channel with only one working power tube would have been quieter, except for hum, which would have increased, plus the power would have been reduced.


                • #9
                  it ended up being one screen resistor per channel that was even the "good" side had a resistor out, but it was working (not to it's potential, i'm sure).
                  i have no idea if the "bad" channel would have produced sound if i would have found the open pot first.
                  i'm not about to pull the resistor to find out .


                  • #10
                    Do you know if these amps could handle KT77s or KT88s? Good going on the repair. I replaced those screen Rs in a newer fender recently. What's a safe wattage then- 5 Watt?