What does this scope signal look like?

[gui_tarzan]

Refer to post #4 - Did you check the effects loop jacks to see if the one shorting jack is open?

[beasleybodyshop]

Refer to post #4 - Did you check the effects loop jacks to see if the one shorting jack is open?

Yeah, i just finished checking that, thanks. Still wonky. Mouser cant place my order fast enough and im only 30 minutes away from them!

On another note, i see they are not accepting Paypal any more. that was a short experiment.

[stephenl]

How is the OT secondary ground connected? Is it through a non-isolated speaker jack to the chassis?

[beasleybodyshop]

How is the OT secondary ground connected? Is it through a non-isolated speaker jack to the chassis?

its a grounded speaker jack. wierd thing is two of the fillter caps were actually grounded to the speaker output ground (?) Even looks to be a factory made connection.

[stephenl]

I mentioned it because I picked up a Yamaha T50 for cheap at a pawn shop - it hummed horribly. The speaker jack was loose and the connection to the aluminum chassis was corroded. I replaced the jacks with isolated types and grounded the the OT sec. at the PA screen supply ground node....problem fixed. Might be worth looking at.

[stephenl]

Did you find the issue yet?

[beasleybodyshop]

UPDATE

I have replaced the big 220uf filter caps for the OT CT. no change in noise. i double checked the effects loop - whenever i break the signal, the hum stops. I bypassed the effects loop entirely by soldering a connection from C21 to Pin 8 of the PI. noise is still there.

If i pull V5, its still noisy. If I pull the PI, hum goes away.

I double check my voltages on V5. everything is within 10 percent EXCEPT pin 2 (one of the grids) which is getting 100 volts. According to the schematic posted earlier its supposed to get something like .3v, so i check all signal caps for DC leakage around that part of the circuit. none are leaking.

Im guessing this tube is being used as a cathode follower - signal output is through pin 3 of V5. im getting 196V here. Huh? How? Im looking at the schematic and i see NO WHERE where high voltage DC could even get to pin 3. But schematic shows that 190v is where its supposed to be.

I still have no idea how im getting 100 volts on the grid. i trace the pin through r36 and r38 to ground, both those resistors check out ok. im getting almost 200 volts on the bottom side of R36, and no voltage on the other side of C14 or C15.

How is pin 2 and 3 getting voltage??? And is the schematic wrong?? I am so stumped.

[xtian]

On the schematic, those voltages inside rectangles are AC or signal voltages, not DC voltages. Also, don't expect only a volt or two on the cathode of a cathode follower--not the same bias setup.

i double checked the effects loop - whenever i break the signal, the hum stops.

What? How can you get the hum to stop? Breaking the signal where?

[beasleybodyshop]

On the schematic, those voltages inside rectangles are AC or signal voltages, not DC voltages. Also, don't expect only a volt or two on the cathode of a cathode follower--not the same bias setup.

i double checked the effects loop - whenever i break the signal, the hum stops.

What? How can you get the hum to stop? Breaking the signal where?

whenever i disconnect C21 (signal cap connecting V5 to bridging jacks) from pin 8 of the PI the hum goes away. I completely disconnected the bridging jacks from the situation (to make sure they werent the culprit)

It even makes a humming noise when there is no tube in V5 - i can disconnect the wire from C21 to pin 8 and hum goes away.

[Reeltarded]

Leave that wire disconnected and close the amp up!!

Good deal!!

[xtian]

Let's follow up on the idea that the noise is coming in somewhere before the PI. Use an alligator jumper instead of disconnecting stuff. For example, ground pin 8 of the 6AN8A using your jumper. That should get us the same result as removing C21, meaning the noise should stop. Confirmed?

Now, look for the next logical place toward the input to try. Somewhere there is zero DC. For example, point H looks well isolated from DC because of the caps around it. Ground that point. Does your noise stop?

Point N looks like the next logical place. If noise stops here, it's the reverb circuit.

Etc.

[Reeltarded]

It's causing an echo now.

Hear that?

lol

[beasleybodyshop]

If I ground out things and the noise stops, what exactly does that indicate? And I do apologize in advance if the answer is obvious and I'm not getting it.

[xtian]

We're trying to divide and conquer (to eliminate parts of the circuit that are working properly).

We already know the noise enters the circuit before the PI, right? Each time you ground the signal and the amp goes quiet, we know the portion of the amp AFTER the grounded point is clean.

Example, if you can ground the grid (pin 2) of V1 and the noise vanishes, we know the problem is in the first triode! But that's probably not the case, right?

[beasleybodyshop]

We're trying to divide and conquer (to eliminate parts of the circuit that are working properly).

We already know the noise enters the circuit before the PI, right? Each time you ground the signal and the amp goes quiet, we know the portion of the amp AFTER the grounded point is clean.

Example, if you can ground the grid (pin 2) of V1 and the noise vanishes, we know the problem is in the first triode! But that's probably not the case, right?

Thanks for the wisdom Xtian. I've been workin on this since 9am this morning. I'm going to decompress and start on it again in a bit when my sanity has recovered.