AA764 Champ weirdness

[Randall]

Champ comes in, and it makes a loud dive bomber noise as soon as the tubes warm up. Someone else tried to fix it before, and I found a 2.7 ohm resistor where the 2700 feedback resistor is supposed to be. Changed that, and the noise is gone. Can someone explain how a near short in that spot would make such a loud and obnoxious noise?

The noise is gone, but the sound is week and distorted. I found the OT had been changed, and the original came with the amp with the owner wanting the original back in if the OT wasn't the problem, so I put it back in. Still same low output and distortion. The cap can has been changed as well, with the first node doubled up to 40uF, but otherwise looks good. However, the B+ on this is 511v, with 304v on the preamp plates. The secondaty of the PT looks good enough with 385vac on each red wire, that's about 20% high.

So why am I getting such a high B+? The can is only rated at 475v. And chasing the sine thru the amp, it looks OK right up to the plate, but drops to next to nothing at the speaker. Both OT's pass the neon test, but only when the battery is on the speaker side, does that matter? And what the heck is going on with the low and distorted output? What kind of trouble am I in here?

I have swapped all tubes, and tried another speaker, with the same results.

https://cdn.shopify.com/s/files/1/23...97210715640943

[Enzo]

What AC voltages at the PT for B+? Are you using a real 5Y3?

[Helmholtz]

Champ comes in, and it makes a loud dive bomber noise as soon as the tubes warm up. Someone else tried to fix it before, and I found a 2.7 ohm resistor where the 2700 feedback resistor is supposed to be. Changed that, and the noise is gone. Can someone explain how a near short in that spot would make such a loud and obnoxious noise?

The low value resistor causes extreme NFB. Excessive NFB is likely to cause ultrasonic oscillation.

However, the B+ on this is 511v, with 304v on the preamp plates. The secondaty of the PT looks good enough with 385vac on each red wire, that's about 20% high.

The high DC/AC ratio may indicate that the 6V6 isn't properly conducting. What is its cathode voltage?

And my standard question: What is the heater voltage?

[Randall]

"What AC voltages at the PT for B+? Are you using a real 5Y3?"

Not sure what you are asking, as stated I have 385vac to ground on both PT secondary red wires. Primary side is 124vac.

It has a coke bottle American made 5Y3, so yes. It acts the same with a different one as well.

"The high DC/AC ratio may indicate that the 6V6 isn't properly conducting. What is the cathode voltage?"

Cathode voltage is 65V, which I forgot to mention, also quite high.

[Helmholtz]

Could it be that the amp has silicon rectifier diodes bridging the 5Y3?

[Randall]

"Could it be that the amp has silicon rectifier diodes bridging the 5Y3?"

Negative.

[g1]

Did you measure the cathode resistor? If it is opening, you would measure higher than normal cathode voltage and plate voltage.

[Randall]

I did measure the cathode resistor, and it was strange because it measured open in circuit, but close enough out of circuit, or so I thought. I put the 6V6 on a bias probe and it was almost not conducting at all, like 0.5 mA. Thanks helmholtz for getting me thinking in that direction. I clipped in a 470 ohm and a new cap, and things went back to normal, That's when I noticed what I took out was a 470K ohm instead of a 470 ohm. I put in a 470 ohm 2 watt and a fresh cap, and now it draws around 42mA and the B+ is 412v. I can live with that.

So both issues here were caused by someone not knowing there resistor color codes too well. Both had been previously replaced.

[Enzo]

You DID say 385vAC, I just missed it.

But that explains really high voltage.

65v across a 470 ohm resistor gives me about 138ma tube current. Schematic wants about 40ma. I get about 250 ohms for the OT primary, at 40ma and a 10v drop, does it measure around that? I would think 138ma tube current would drag the B+ way down at the plate of the tube.

ANy chance the 0.02uf feeding the grid of the 6V6 is leaking DC? Is there zero DC on pin 5 of the tube?

Have we checked the speaker jack? An internal short could be killing sound. Try having teh OT secondary wires unterminated - sitting in mid air. Clip wire a speaker right to them. Still problem? or now OK?

Oh ignore me now, you all solved it while I was typing that.

[Randall]

Thank you Enzo, see above. We simul-posted.

[Randall]

So in retrospect, if the shorted feedback resistor caused the screech, but the way too big 6V6 cathode resistor caused it not to conduct, how was it the screech got through? And it was very loud.

[Helmholtz]

So in retrospect, if the shorted feedback resistor caused the screech, but the way too big 6V6 cathode resistor caused it not to conduct, how was it the screech got through? And it was very loud.

Don't forget the cathode cap. If fully charged it won't conduct DC and the tube will be in quasi cutoff. But at turn-on the discharged cap looks a low value "resistor" to the tube. With increasing cap charge, tube current will decrease and the screech frequency should change.