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Thread: 70's Silverface Fender Champ AA763 overhaul - voltage issues

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    70's Silverface Fender Champ AA764 overhaul - voltage issues

    Hello all ...
    I'm overhauling a 70's silverface champ and having an issue after what should have been a very easy job. I replaced the AC power cord, the multi-section cap can for filtering (20/20/40 uF), three electrolytic caps on the board (25uF/25v, 2uF/50v, 25uF/25v), and new glass including (JJ GZ34S, JJ 12AX7A, JJ 6V6S). I know that the AA764 came with a 5Y3GT rectifier but this amp had a GZ34 in it and worked prior to me opening it up.

    I used a current limiter for the initial power up with no problems. The second time I ran it without the limiter. Played guitar through it and it was horribly distorted and it was very quiet for the volume being on 10.

    My DC voltage reading right off the filter cap can was 475 v on the top and near 515v on the bottom. That seems really high any thoughts on what to do/not do next?

    Thanks in advance.
    Matthew

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    Last edited by mlannoo; 01-04-2020 at 03:15 AM. Reason: mistake made on circuit number

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    Supporting Member The Dude's Avatar
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    1) Did the amp work before you overhauled it? If not, the problem could be unrelated.
    2) I'd stick with the 5Y3 in that amp. It has a higher voltage drop.
    3) What do you mean, "475 v on the top and near 515v on the bottom"? One side of the filter caps should be grounded.
    4) What are the voltages on the output tube pins? Are you missing screen voltage maybe? Higher than normal voltage shouldn't cause those symptoms and could be in part because the tube is barely conducting.

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    Supporting Member TomCarlos's Avatar
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    For those of us following at home :-)

    Is this the Champ AA764? I do not see a schematic for a AA763.

    Thx.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by The Dude View Post
    1) Did the amp work before you overhauled it? If not, the problem could be unrelated.
    2) I'd stick with the 5Y3 in that amp. It has a higher voltage drop.
    3) What do you mean, "475 v on the top and near 515v on the bottom"? One side of the filter caps should be grounded.
    4) What are the voltages on the output tube pins? Are you missing screen voltage maybe? Higher than normal voltage shouldn't cause those symptoms and could be in part because the tube is barely conducting.
    All this ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

    Especially 3) in bold!!! If you have HV on both the + and - ends of any capacitor something is dangerously wrong. So be careful.

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    Quote Originally Posted by TomCarlos View Post
    For those of us following at home :-)

    Is this the Champ AA764? I do not see a schematic for a AA763.

    Thx.
    YES! My mistake. I'm referring to the AA764 circuit. Let me see if i can edit the initial post. Thanks!

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    Supporting Member TomCarlos's Avatar
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    Ok... I'm with you.

    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.

    Guys, would it be a good idea to remove the 12ax7 and 6v6 tubes, use a dummy load and check the voltages before inserting tubes? Maybe hang a resistor off the 330v node? How much current should this amp pull? I don't think this amp would reach 70ma would it? I see that spec on a Hammond Transformer (for the Champ). If that were true, using a 5K Ω @ 20 watt would be safe just for testing?

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    Quote Originally Posted by The Dude View Post
    1) Did the amp work before you overhauled it? If not, the problem could be unrelated.
    2) I'd stick with the 5Y3 in that amp. It has a higher voltage drop.
    3) What do you mean, "475 v on the top and near 515v on the bottom"? One side of the filter caps should be grounded.
    4) What are the voltages on the output tube pins? Are you missing screen voltage maybe? Higher than normal voltage shouldn't cause those symptoms and could be in part because the tube is barely conducting.
    1. yes it worked before the overhaul. it was noisy and needed some love but it did work.
    2. will try the 5Y3 when it arrives in the mail.
    3. the multi-section capacitor can has 3 values. 20, 20, 40. I was measuring the DC voltage on the top section (20uF) with my DMM (black lead on chassis and red lead on cap can top terminal). The bottom one is the 40uF terminal and it measured around 515 vdc. Maybe I'm not understanding how to measure the cap cans - you mentioned one side of the filter caps should be grounded - can you elaborate please?

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    Is that with the tubes in or out?

    Also: the Silverface Champ does not follow the AA764 schematic. It follows the CBS Fender Champ schematic. The B+ on the SF Champ is 420VDC. With modern wall voltage it could go as far as 430 or so. Mine usually ran about 435.

    Get the other stuff cleared up first, keep a GOOD quality 5Y3 (NOS are not too horribly expensive & can last forever), & work from the correct schematic. Look for "CBS Champ."

    Justin

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    Quote Originally Posted by TomCarlos View Post
    Ok... I'm with you.

    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.

    Guys, would it be a good idea to remove the 12ax7 and 6v6 tubes, use a dummy load and check the voltages before inserting tubes? Maybe hang a resistor off the 330v node? How much current should this amp pull? I don't think this amp would reach 70ma would it? I see that spec on a Hammond Transformer (for the Champ). If that were true, using a 5K Ω @ 20 watt would be safe just for testing?

    I noticed the filter cap values on the schematic indicating 20/20/20 @450vdc but the original Mallory cap can I removed said 40/20/20 @450. I'm guessing a minor change in the circuit?

    The 1K and 10K resistors are within spec (surprisingly close too).

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    Aaaaah CBS Champ ... OK will do thanks.

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    Gotcha. The can is three separate caps. The + terminals are designated with symbols on the side of the can, usually. They share a common ground, which is usually the can itself. I suspect a combination of problems. One, the rectifier tube you are using has a lower voltage drop. Two, modern line voltage is probably higher than when that amp was built. Three, I don't think your output tube is conducting as it should be, which you could tell by measuring the cathode voltage on the output tube. Again, possibly you are missing screen voltage? Measure it also. If output tube current is low, B+ will go higher. I would run the amp on a variac monitoring B+ until you get it sorted. You have 515V on caps rated at 450V, and you could destroy them/it. If you don't have a variac, at least leave it on the limiter.

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    Last edited by The Dude; 01-04-2020 at 05:23 AM.
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    The can cap being 40/20/20 is no biggie. Fender frequently changed on the fly.

    Justin

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    Supporting Member TomCarlos's Avatar
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    Ok... I think I found the CBS Champ schematic...
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

Name:	ChampSilverface.gif 
Views:	17 
Size:	121.7 KB 
ID:	56515  

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    Bent Member Chuck H's Avatar
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    I've run more than one brand/era 5y3 in both higer voltage single ended and lower voltage small bottle push/pull amps with a 47uf cap and NEVER had a failure. Not saying it can't happen, but the tube was spec'd at a time when electrolytic caps were commonly rated at +80%/-20%!!! The tube manufacturers surely knew this and they don't make money on returns. I think you're fine (better actually) using the 40uf section as the main filter.

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    Last edited by Chuck H; 01-04-2020 at 05:47 AM.
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    I agree with Chuck, but when faced with a 40/20/20 to replace a 20/20/20, if the rectifier tube concerns you at all, then make a 20 the input cap, and save the 40 for the screens node or preamp node.

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    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?

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    mlannoo...

    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.

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    Supporting Member TomCarlos's Avatar
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    mlannoo...

    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.

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    L010020
    CSA 827
    EIA-606-723

    Thank you.

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    Supporting Member TomCarlos's Avatar
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    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!

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    "23rd week of 1974"

    I don't think this is correct. 1977 probably.

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    It's weird, because it WAS working fine.....

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    Supporting Member The Dude's Avatar
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    Quote 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?

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    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?

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    Where do you have your black meter probe when checking that DC? Maybe a grounding issue?
    Try another 5Y3 if you have one.

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    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.

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