70's Silverface Fender Champ AA763 overhaul - voltage issues

[mlannoo]

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

[The Dude]

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.

[TomCarlos]

For those of us following at home :-)

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

Thx.

[Chuck H]

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.

[mlannoo]

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!

[TomCarlos]

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?

[mlannoo]

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?

[Justin Thomas]

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

[mlannoo]

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).

[mlannoo]

Aaaaah CBS Champ ... OK will do thanks.

[The Dude]

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.

[Justin Thomas]

The can cap being 40/20/20 is no biggie. Fender frequently changed on the fly.

Justin

[TomCarlos]

Ok... I think I found the CBS Champ schematic...

[Chuck H]

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.

[Enzo]

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.