6G6B Bassman, airbrake, blown OT, tears

[rp]

Get a long straw up your nose and sniff around very slowly in and around every other part of the amp.

That's how I always clean up the insides of an old amp. I do inside the cab too and all around the speakers.

[mr_hankey]

(...)

[Ronald]

..

[CHIP]

Check the 500pf PI entry cap. If not stout enough it could have blown.

[mr_hankey]

(...)

[Firestorm]

Screen resistors or screen resistor leads?

[rdjones]

I suspect that the attenuator schematic has the input and outputs swapped.
ANY load or attenuator device needs to place at least some load on the secondary to prevent damage. (the 8-25 Ohms)
With the on-off switch in the open position there is no load to protect the output section.

Flyback voltage can burn open an output transformer but a common risk is arcing either inside the transformer or at the tube bases.

Carefully check the tube bases around the pins and the tube sockets for evidence of arcing.
A sustained arc could have caused the smoke, if it was across the pins it would damage the tube's base and possibly the socket.
Any remaining carbon trace from an arc can cause a recurrence in the same path.

rd

[jelle]

I would suggest to have a tech look it over. it is not clear what happened but something was damaged and actually burned.

Good luck!

jelle

[mr_hankey]

(...)

[rdjones]

If I reverse the leads and measure the input resistance with a speaker at the other end, it measures from 3.3ohm (lowest) to 8.1ohm (highest attenuation). Would that be safe to use?

Depends on the output impedance, but generally yes you should be safe with that range.
I don't think dipping down to 3.3 Ohms is any great danger, certainly way better than on open circuit on the OT secondary.

Thanks, I'll check the sockets next time.

On an amp with a grounded 6.3V centertap the shortest path to ground from the HT is often between pins 2 and 3 on the output tubes, either on the bottom of the tube base or the top or bottom of the socket.
Look for a black 'carbonized' trace.

regards,
rd

[vibratoking]

As discussed earlier, that attenuator circuit is rubbish. I would not use it.

[Richie]

Agree with firestorm, screen grid resistors. They can but out a good bit and still not look burnt. Need to measure those, Check the cap can inside. Since you say its ok with another set of tubes, i doubt there is an arc on a tube socket. Sounds like a tube failed,causing some problems. Not a fan of any load boxes.

[mr_hankey]

(...)

[rdjones]

I've now measured the screen and grid resistors, and they're all fine. The tube sockets have no marks on them.

I took a further look at the power tubes, and it appears one of them shorted from the plate to the heater. That particular tube also smells like the smoke.

As I was hoping that the tube was the cause of the smoke, I sawed the base off. There are some very clear black marks between the plate and heater pins. It smells strongly of the smoke. I'll put a photo up later. Since this shows the source of both of the symptoms (smoke and tube failure), I don't have any reason to suspect OT damage.

Now, a less important question: was the tube failure spontaneous, or caused by the attenuator?

The arc path was caused by flyback voltage due to insufficient or no load on the OT.
Yep, that "attenuator" put your amp at risk.
In the strictest sense the tube itself didn't fail, it received damage from a condition generated in another part of the circuit.

reddog

[Toppscore]

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