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70's Silverface Fender Champ AA763 overhaul - voltage issues

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  • #16
    OK I'm backing up a bit.
    Tubes out, I checked the power transformer. 6.3 VAC to the heaters. 5.5 VAC to the pilot. And roughly 383 VAC from the center tap to each plate on the rectifier tube socket (no tube installed).

    Still waiting on the 5Y3GT tube in the mail.

    In the meantime, I put the GZ34 rectifier in and measured the DC at the first filter cap (40uF) on the board and I'm getting 515 VDC! I'm guessing the GZ34 is NOT supposed to work with this circuit?


    • #17

      Go to this page and look at the chart in Post #3 by Bob Latino. For reference, you might want to copy the chart into MS Word and print.

      gz34 vs 5y3 vs 5u4 - The Dynaco Tube Audio Forum

      You can also do a Google search on "gz34 vs 5y3" and you'll find several threads on the topic.


      • #18

        By the way.... I am taking notes here... Can you please look at the Power Transformer and tell me what part number is stamped on it? A picture would also be nice. This is one of the differences between the Blackface vs Silverface. It would also be a good check to make sure no one swapped out the PT somewhere along the line.


        • #19
          CSA 827

          Thank you.


          • #20
            Just like the schematic says - it should be the L010020, for a Champ or Princeton
            Transformer made by - Woodward Schumacher (606)
            23rd week of 1974
            CSA 827, Might be Canadian Standards Agency #?

            At least you verified its the right one!


            • #21
              "23rd week of 1974"

              I don't think this is correct. 1977 probably.
              It's weird, because it WAS working fine.....


              • #22
                Originally posted by mlannoo View Post
                OK I'm backing up a bit.
                Tubes out, I checked the power transformer. 6.3 VAC to the heaters. 5.5 VAC to the pilot. And roughly 383 VAC from the center tap to each plate on the rectifier tube socket (no tube installed).

                Still waiting on the 5Y3GT tube in the mail.

                In the meantime, I put the GZ34 rectifier in and measured the DC at the first filter cap (40uF) on the board and I'm getting 515 VDC! I'm guessing the GZ34 is NOT supposed to work with this circuit?
                With no tubes but the rectifier in the amp, there is little or no conduction, so supply voltages will be high. Did you check the screen voltage on the output tube?
                "Yeah, well, you know, that's just, like, your opinion, man."


                • #23
                  Sorry for the break I went to NAMM!

                  Back at my bench. New JJ 5Y3S rectifier installed. Powered up with a current limiter first - no issues. Removed if from the current limiter and I'm getting roughly 370 volts AC across the rectifer from the power transformer. Then at the very first DC measure point I'm getting barely anything - like mV's. WTF. The likelyhood of a brand new rectifier tube being bad is possible but not likely. What would cause this? The 1K and 10K resistors on the power rail are functioning and within spec.

                  Also this is odd .. I have a brand new CE Manufacturing multi-section capacitor can (40/20/20) and my little PEAK atlas ESR capacitor tester is telling me that the middle section (20uF) is "leaky - in circuit". Was this perhaps due to an error on my end installing it (i sure hope not because it's pretty straight forward and i've done many before!) or is it likely that one of these cap sections was bad from the factory? Would that be why i'm getting extremely low DC voltage readings right off the rectifier?


                  • #24
                    Where do you have your black meter probe when checking that DC? Maybe a grounding issue?
                    Try another 5Y3 if you have one.
                    "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey


                    • #25
                      I've made it a habit to do some additional forming on CE Distribution can capacitors before installing them, which I do with an ancient Heathkit Condenser Checker that I've modified slightly to give more control over the output voltage. This process also tests the filter capacitor sections for leakage. I've found some of their cans not as formed as I would like.

                      You really shouldn't be running a power supply with the rectifier tube installed and no current demand without a Variac and monitoring of B+ voltage, even with current limiting -- because there's not much current to limit. It's possible that you hit the can capacitor with voltages so high that you damaged it. My Condenser Checker is inherently current-limited so that it would be very hard to do that. If you bring voltages up slowly, the capacitor can adapt; if you slam it with a high voltage without giving the oxide layer time to form, you can cause a short.

                      I would not use a GZ34 in a Champ because you get significantly higher B+ voltages with a GZ34. While a JJ 6V6 (which is a kind of 6V6/6L6 hybrid) may handle it, I have seen Champ output transformers fail under these conditions.

                      I also think 40uF is on the high side for a 5Y3 first filter stage.


                      • #26
                        Just an update for those who have been following this ... or those who may come across this topic in a similar boat ...

                        Got the amp fixed and biased - all good. Here's what had happened:

                        - New JJ 6V6S, JJ 12AX7 and a NOS Sylvania 5Y3 rectifer
                        - Replaced all the electrolytic caps on the board with Sprague Atom 25uF/25v caps. For the cathode bias cap I replaced it with a 25uF/50v.
                        - Replaced the filter caps with a CE Electronics multi-section cap can 40/20/20. Either one of the 20uF sections was bad from the manufacturer or "something" happened during surgery - I have no way of knowing. But when I swapped in another 20uF cap to test my theory, everything worked as it should. So I left the cap can installed and just bypassed the bad section by putting an axial lead F&T 20uF cap in its place inside the chassis.

                        - There was a bad solder joint on P3 of the 6V6 that was causing intermittent issues so that was fixed. (*this may have been why i was having issues early on!)
                        - Installed a 5w 1K ohm ceramic cathode resistor to set the bias.

                        Biasing this was a real pain. I ended up increasing the 10K resistor on the power rail to a 22K NOS carbon comp resistor (saw a youtube video on this topic and it was suggested so i figured I'd try it) and ...
                        1. 1K cathode bias resistor measured 990 ohms
                        2. voltage drop across bias resistor was 34.1 vdc
                        3. the plate voltage on the 6V6 pin 3 was 430 vdc

                        990/34.1=.034x430=14.8watts dissipation

                        Things seemed stable and the amp sounded ... well I guess sounded as good as a silverface champ can sound. I'm not a big fan.

                        Thanks to those who contributed to my question. I'm learning a lot.

                        Last edited by mlannoo; 01-25-2020, 05:06 PM. Reason: mistake made and info forgotten


                        • #27
                          .034x430=14.8watts dissipation
                          Your actual plate dissipation is lower as the tubes only see the voltage between plate and cathode (and because cathode current is about 6% higher than plate current because of additional screen current). I calculate 12.8W.
                          - Own Opinions Only -


                          • #28
                            yes - that puts me right in the suggested 12w range for a 6V6S. thanks!


                            • #29
                              Originally posted by TomCarlos View Post
                              From the schematic, the caps for the power supply should be 20/20/20 @ 450v. You mentioned 20/20/40? Double check that please. I would also check the 1K and 10K resistors to make sure they are in spec.
                              I am helping a friend overhaul his old Champ and find myself in the same predicament: the schematics show 20/20/20 but the can in the amp (looks original) is a 40/20/20 (the 40 uF cap being connected to the 1 K Ohm resistor and the output of the 5Y3)

                              I also have doubt with the 2 uF 25V cap as I would swear it was also a 25 uF 25V in the amp (don't have it in front of me)

                              also checked the 1 K resistor... was reading about 100 Ohm, it was burnt and cracked open when I took it off the circuit.
                              the 10K resistor was reading 16K, so I took it out as well.

                              all voltages are now below 20% deviation with the exception of:

                              PIN 1 12 AX7 +252 VDC (nominal +200V)
                              PIN 6 12 AX7 + 262 VDC (nominal +205V)
                              PIN 8 6V6 +25VDC for a nominal of 19 V (I've changed the 470 Ohm resistor here as the original was at 566 Ohm)

                              checked the 2x 100 KOhm resistors going to pins 1 and 6 of the 12 AX7 and they are within 5% tolerance.

                              no caps have been changed so far.

                              any suggestions for the next step?

                              Originally posted by Justin Thomas View Post
                              Also: the Silverface Champ does not follow the AA764 schematic. It follows the CBS Fender Champ schematic.
                              oh... my. I should have read further down the thread... voltages are OK for the CBS schematic

                              QUESTION: Fender Champs were still using TUBE RECTIFIERS in 1980? This is what we have dated the amp because of the speaker (465 8049, oxford wk 49 1980), OK maybe a Camp-kenstein with a non original speaker.
                              Transformer EIA 606-920 (wk02 1979 or 1969?) pots read R7934, R7944 so I am guessing it is 1979/80 but I did not know they were still not using solid state rectifiers back then.

                              AND... death cap is still in the Amp even though a three prong mains cable was installed. Is it a problem leaving the death cap in?
                              I am guessing it's useless if you plug into a wall outlet that is actually grounded. Please correct me if I am wrong.
                              What happens if the building has no grounding (I've been in practice studios where I've seen 2 prong mains on the walls so I am doubting even if I see a 3 prong the grounding is actually there)
                              The CBS schematics is showing a 3 prong and a death cap (?) in the circuit
                              Last edited by TelRay; 03-07-2020, 10:21 PM.


                              • #30
                                What's wrong with the amp?

                                Which version of the Champ? (Schematic?)

                                What is B+?

                                What is heater voltage?
                                - Own Opinions Only -


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