Troubleshooting help needed

[Bob-I]

I have a Dumble style amp I built a year or so ago, which has worked well. 4x6V6 output, built in Dumbleator and reverb. The PT is Hammond 272HX, 200ma B+, 8A heaters. According to my calculations that should be ok for 4 6V6's and 5 12AX7's but just barely.

The OT is a Mojotone bassman replacement.

When I crank it up, after a few seconds it cuts out, then kicks back in. With a voltmeter on the B+ I see it drop from 402 to about 300. The bias current goes from 40ma per 6V6 pair, to about 70ma when cranked, then when it cuts out the bias goes way up, 120-140ma.

Removing any one 6V6 and it won't cut out.

Signs are that the PT is being starved, but why now, after a year of weekly gigging with it?

One more thing. I had a 6V6 go bad and blow the screen resistor. I replaced both and the problem still exists. Could this have done some damage.

Any assistance will be appreciated.

[dave g]

Sounds to me like it could be arcing in the PT. Like you said, you're running pretty close to the max current rating on that PT, and if you had a tube go bad enough to blow a screen, it certainly could have drawn enough current to cause internal damage to the PT.

[Bob-I]

Sounds to me like it could be arcing in the PT. Like you said, you're running pretty close to the max current rating on that PT, and if you had a tube go bad enough to blow a screen, it certainly could have drawn enough current to cause internal damage to the PT.

Damn, that's what I was thinking too. That tube could've easliy damaged the PT, it blew a 5 watt screen resistor.

Thx for the confirmation.

[novosibir]

It's IMO not the PT, but the OT!

Cutting out and kicking back in wouldn't happen so quick w/ an intermitting short in the PT due to the storage in the filter caps.

But the suddenly dropping B+ might be caused by an intermitting short between primary & secondary of the OT, since the secondary nearly is ground for the B+

When your 6V6 went bad and blow the screen resistor, the plate current also was remarkable higher as usual, maybe 3-5 times higher. The insulation between primary & secondary probably melted, as the primary became so hot?

Just my idea & experience w/ cutting out sound, while the B+ is dropping so remarkable.

Larry

[Bob-I]

Interesting.

Problem is I don't have either a PT or OT replacement. I guess I'll have to order some replacements and do the trial and error thing.

[groovtubin]

It's IMO not the PT, but the OT!

Cutting out and kicking back in wouldn't happen so quick w/ an intermitting short in the PT due to the storage in the filter caps.

But the suddenly dropping B+ might be caused by an intermitting short between primary & secondary of the OT, since the secondary nearly is ground for the B+

When your 6V6 went bad and blow the screen resistor, the plate current also was remarkable higher as usual, maybe 3-5 times higher. The insulation between primary & secondary probably melted, as the primary became so hot?

Just my idea & experience w/ cutting out sound, while the B+ is dropping so remarkable.

Larry

i saw the SAME thing when i worked on some hammond gear as a tech at Music and Electronics!!!! The OT ""ATE THE B+!! LOLOLOL!!

[Bob-I]

Well thx everyone, but this is gonna hafta wait. I just need to watch my funds right now and since I have 3 other amps to use I may just have to use a different amp and invest in childrens shoes.

[rooster]

Bob-I - I don't hang here too often but I still hope you listen to what I have to add to the discussion. First of all, if it is your PT or OT I would be stunned.

BTW, here's a simple test to run on any PT that you think is not up to the task of suppling enuff filament juice to your tubes: Plug in all the tubes and measure the tube AC voltage. Is it 6.3VAC or more? Then stop dwelling on this. An 8A filament supply is not enough for the tube compliment you stated? It should eat that for breakfast...

OK, addressing your main problem, stop suspecting the PT and OT. You probably have a problem in your bias supply. You should run at least two caps in the bias circuit - whatever the amp - and over-volt by approx 50VDC if possible, i.e., -55VDC required, 100VDC cap is OK, 150VDC is better. Anyway, I would go over the entire bias supply path, starting from point A, (the PT bias supply tap) - and replace whatever caps you have in it right now with something new and of the correct VDC rating - ending at point Z (the pwr tube pin that recieves the bias supply voltage). And yes, double check all grounds in the bias supply....

Like you say, it doesn't make sense that the trannys are failing you after a year of service. You are right, this doesn't make any sense.