Is My Diagnosis Correct?

[Thisismyname]

This is an AB763 Bandmaster I built for a friend. The B+ fuse blew. Pilot stayed on.
Using a limiter, I determined there were no shorts anywhere. Tubes in and out, no shorts.
When the power tubes are in, I'm hearing an intermittent crackling, whoosing, and thumping. So I check the bias using a plug-in probe. I notice a jump of 5-10mA in plate current whenever I hear these noises. One tube jumps higher than the other.

This amp experienced this exact same problem last year. Same noises, same plate current jumps.

Tapping on the top of the amp will cause these noises too. These power tubes were always a little microphonic. One of them has a little clinking sound when I flip it over in my hand, as if something inside is loose. Still connected, but just loose.

I'm thinking maybe this loose connection vibrates around while playing and eventually shorts something bad enough to blow the B+.

I'm thinking it's just a bad power tube and they should be replaced.

Is it this simple, or could there be some other cause inside the amp?

Is it just a coincidence that this same problem happened last year?

Thank you for any and all help!

[tubeswell]

Tube swap?


Bias voltage at output tube grid pins?

[shooter]

This amp experienced this exact same problem last year.

was there a solution last year, or the can just got kicked down the road?


With the PA tubes out and monitoring the grid pins for bias voltage, when you slap the amp, bounce it, does the bias voltage stay solid or fritz out?

[Thisismyname]

Tube swap?


Bias voltage at output tube grid pins?

Different tubes, no noises. Steady bias.

PT grid bias -47Vdc at 18 watts dissipation.

Tapping the amp does not change the bias.

[Thisismyname]

This amp experienced this exact same problem last year.

was there a solution last year, or the can just got kicked down the road?


With the PA tubes out and monitoring the grid pins for bias voltage, when you slap the amp, bounce it, does the bias voltage stay solid or fritz out?

IIRC, last year, I found out that the cathode bias/fixed bias switch was slowly shorting out, which biased the tubes at 0V.

I actually just removed that switch and replaced the tubes. Everything was fine.

I forgot that that problem was the failed switch.

This amp also blew a power transformer once within 2 weeks of me finishing it. It was then I installed the B+ fuse. I never knew why the PT failed. This amp has been a headache ever since I built it.

Back to today, with different tubes install as well as with no tubes, bias voltage remains steady when tapping the amp.

[Thisismyname]

So it does seem that it's just a bad pair of power tubes and not anything else, right?

[Thisismyname]

This amp experienced this exact same problem last year.

was there a solution last year, or the can just got kicked down the road?


With the PA tubes out and monitoring the grid pins for bias voltage, when you slap the amp, bounce it, does the bias voltage stay solid or fritz out?

Tubes out, grid pin bias voltage is steady when tapping. Different tubes installed, no noises, no bias shift when tapping.

I'm thinking it's just bad power tubes, but since I'm not a tech, there's a lot I don't know.
Is there a possibility that something inside the amp caused these tubes to fail?

The amp ran fine for 5 minutes with different tubes installed, so either it's a simple case of bad tubes, or there is something in the amp that could slowly kill the power tubes.

I'm probably overly worried about this, but this is the only amp I've built that has ever had problems and it's had more than a few of those.

What do you think?