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Need help identifying this amp

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  • #16
    thats funny enzo. its 330 in the morning and needed a laugh thx

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    • #17
      Special thanks to those who contributed on this thread. I know it is old thread but it came in handy today as I was trying to understand the evolution of the early 70's Peavey Classic Amps.

      A guy in town is selling a Classic B - the second amp in this series. The amp cab appears to be in very rough shape and there are no usable speakers. This one has four inputs (as Enzo mentions in #2 above, Bright, Normal, Series, Parallel). If nothing else, the chassis and electronics could be useful for testing 6L6 tubes!! So that is my question to you ...

      It looks like the Power Amp is fed from the two 10K resistors as seen on the bottom right side of the Preamp schematic. (Sorry, there are no resistor numbers on the schematic). If I was to disconnect that point from the 25uf cap that feeds the two 64278 transistors and install a 1/4" jack there, that would serve as a "Line In" type connection. Agree?

      The other things I would do?

      - Add 1 Ω resistors to the GLG Cathodes and add banana plugs to the chassis - two for measuring the bias current, one for a ground point, and one for the plate voltage.
      - Move/Change the 50K bias resistor to the back of the chassis.

      BTW, I also build a 4 socket bias probe that measures both Plate Voltage and Bias Current but the mods I describe would make it much easier!!
      Attached Files
      Last edited by TomCarlos; 11-03-2019, 08:09 PM.

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      • #18
        That schematic is not downloading. Here is another to post here...
        Attached Files
        When the going gets weird... The weird turn pro!

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        • #19
          Glad I found this thread. I was able to get one of these classic A & B's off CL recently. Mine is in terrible shape compared to the one in the picture. Someone had shoehorned a different power transformer into the chassis and fried the can capacitor. I've replaced the capacitor but I can't find a transformer # EH 449. I'd like to put the proper transformer in it before I try to power this thing on again.

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          • #20
            Jimmy - The EH 449 transformer (the exact Peavey part) will be difficult to find. You will see them now and then on eBay - also using the identifier 682427. If you were to look for a Hammond replacement, you'll be spending way more than what you probably paid for the amp. In these cases, you might need a "donor" amp - or your amp becomes a donor to fix a different amp. Be patient, keep looking.

            In the mean time, tell us what type of transformer is currently in the amp. Maybe we can see if this is something you can live with.

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            • #21
              At least it is an utterly conventional power transformer for a 50 watt amp. about 360vAC HV winding, 6v winding for two 6L6 heaters. ROughly 55v bias winding. Note, bias winding is separate, not a tap on the HV.
              Education is what you're left with after you have forgotten what you have learned.

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              • #22
                I see the amp isn't worth much. If I would have noticed the botched transformer install I may have passed on it but I like learning new stuff so now I'm into it.
                Try to understand how the bias winding works. It's not center tapped and one leg goes to ground? I was able to get my head around the other windings I just don't understand bias circuits yet.

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                • #23
                  Big difference in price for the transformer with the separate winding for the bias winding. I'll keep my eyes out for another solution. The Hammond one is way over the top. I might try to use a cheapo import from eBay. Just to get it turned in. https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.com%2Fulk%2Fitm%2F20263111 4687
                  Last edited by 76jimmy; 11-11-2019, 11:14 PM. Reason: Link

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                  • #24
                    What transformer is in there now and are you sure that's what fried the cap? Any part numbers on the transformer you have? Maybe it could be made to work?
                    "Yeah, well, you know, that's just, like, your opinion, man."

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                    • #25
                      Yes it has two transformers wired up.
                      83-611-000 / 130012 = wired to the HV and another set of wires used for fills

                      17-7248 / F-46X = being used for bias

                      I started to try and figure out what was what and got the FI'ts and decided to just try and find the proper replacement.

                      I don't have the scratch paper in front of me but none of my meter readings were even close to what I thought should be there. I'm new to talks forum here trying to figure out how to post IMG.

                      Thanks all for any input.

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                      • #26
                        The bias supply is about as simple as supplies get. A half wave rectifier. Your bias supply is a voltage referenced to ground. In the case of bias it is negative voltage. The AC of the winding is rectified, and filtered by caps.
                        Education is what you're left with after you have forgotten what you have learned.

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                        • #27
                          Only way I can make sense of things is to apply them to physical real world objects. Now I understand it as a throttle return spring. Had to do some reading last night.

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                          • #28
                            Originally posted by 76jimmy View Post
                            17-7248 / F-46X = being used for bias
                            \
                            Can't find anything about the HV transformer, but if the F-46X being used for bias is the same as this one (https://www.newark.com/triad-magneti...4va/dp/03F1003) I don't see how that one will work for biasing, it won't have enough voltage as it is a 24VAC secondary instead of something like 55VAC as Enzo said above. Seems like you couldn't get sufficient negative voltage to keep the tubes from running way too hot. If someone changed the factory bias circuit to have a voltage doubler then maybe?

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                            • #29
                              The HV and heater trans is from a RCA TCS-127 Console TV. The trans used for the bias is definitely insufficient. It's all coming out. Going to give the cheapo import a try. Thanks all for your help.

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                              • #30
                                Got it working. Used a Basler Tube Amp Power Transformer 120 VAC 6.3V 6.5A 300-350V 28 VCT BE32901001 from eBay. Had to wire an artificial center tap to the 6.3v winding but it seemed to do the trick.
                                Everything works but the tremolo. That's for another night. Thanks everyone for your help.

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