AB763 Super Reverb Clone Blowing a cap

[72Blazer]

This is repeatable! 

The amp keeps blowing the 50uF/50v mod caps in channel 2.  I mean it blows the cap the cap to smithereens.  Sounds like a firecracker when it happens.  I know this is the 4th of July but geeze.  This amp was recently working.  The amp won’t stay stable long enough to check voltages now. 

I did get good sound out of channel one before she blew. 

Channel 2 obviously was not working well out at all. 

Any clues what to check?

[stratomaster]

Shorted V2b.  That cap should not be seeing more than a few volts.

Leave the cap out and measure the voltage at the cathode eyelet.

Then remove V2 and measure again.

[72Blazer]

Thanks Stratomaster, but I am unsure where you want me to measure.  Both caps are out of the circuit already as well.  I had already removed them.

Vr
J.

[stratomaster]

Thanks Stratomaster, but I am unsure where you want me to measure.  Both caps are out of the circuit already as well.  I had already removed them.

Vr
J.

On your diagram there's a flag labeled Z. Measure the voltage at the eyelet to the left of that note.

[72Blazer]

23v

[stratomaster]

23v

I'd say your v2 tube is toast or there's an error in the wiring of the socket. 

That point is a cathode connection that sets the bias of your triode.  It should be less than 3 volts. 

[72Blazer]

Thanks.  Reverifying all that.  I'll report back.

[AlNewman]

Is it blowing both caps?  are you sure your polarity is correct?  Positive lead towards the tube socket.

The cathodes are Pins 3 and 8 of the tube.  Also check all resistor values and solder connections around tube.

[72Blazer]

Yes polarity is/was correct. 

I measure 4.8v at pin 8 of V2 and 0.8v at pin 3.  (The voltage I posted above was no kidding at the "Z" points.) 

Just the cap with the 2.2K resistor strapped is blowing

And oh btw, I am on a lightbulb current limiter right now/for those measurements.

[stratomaster]

Yes polarity is/was correct. 

I measure 4.8v at pin 8 of V2 and 0.8v at pin 3.  (The voltage I posted above was no kidding at the "Z" points.) 

Just the cap with the 2.2K resistor strapped is blowing

And oh btw, I am on a lightbulb current limiter right now/for those measurements.

Z should be ~ 200+v. You circled the shared cathode cap along with the reverb driver cathode cap that you now say is the one blowing.

I'm not able to follow you anymore.

Please mark exactly what is happening and include photos if you can.

Go through this thread and do what it says: https://el34world.com/Forum/index.php?topic=19361.0

We'll be able to help you better then.

[sluckey]

Z should be ~ 200+v.

Look again. Point Z should be zero volts.

[stratomaster]

Z should be ~ 200+v.

Look again. Point Z should be zero volts.

That's what I get for jumbling the vintage wiring with the reissue schematic in my head on the fly. Thanks for setting me straight.

[AlNewman]

This is pretty confusing.  On my AB763 v3 is the reverb driver, it's a 12AT7, and both the cathodes should read the same because they are tied together.
That is the tube with the 2.2k resistor.

What are your resistances on your reverb transformer?  I have a replacement one here, the primary reads around 1k, the secondary reads around 1R.

[tdvt]

Just the cap with the 2.2K resistor strapped is blowing

This is pretty confusing.

Agreed,

And you are correct, if wired as drawn, the 2.2K/25uF combo are the reverb driver (V3A/V3B) shared cathode components.

Should only affect channel 2 by way of the reverb circuit, I would think.


Edit:
That said, if wired as stock, V3 would be the center of attention & are you otherwise able to play the amp with V3 pulled?

[72Blazer]

V3 was bad indeed.  Replaced and finally got all the DC off this fiber board.  All is well again.  Thanks all.

Vr
J.