Carlsbro 60 watt twin channel

[kepeb]

i think is sorted.
i was through measuring all the voltages on the el34 pins and from the OT then i did the input, and the mains power in was wired backwards.
i switched it around, turned on with the tubes back in and no hot plating!

awesome,
but what was the deal here? is it just working by coincidence? i thought the way around wouldn't make a difference as long as the fuse was on the live?

could the problem have been something else that may reoccur or does this make sense?
i think the output was wired like that for about a years worth of gigging!!! why did it not blow up?

anyway, looks like another happy ending if that was the issue

[Chuck H]

i think is sorted.
i was through measuring all the voltages on the el34 pins and from the OT then i did the input, and the mains power in was wired backwards. i switched it around, turned on with the tubes back in and no hot plating!

Do you mean the 120V black and white wires for the PT primary? Because that shouldn't make any difference at all in the amps operation. It can cause other problems though. Even dangerous.

Or do you mean the plate and screen wires were reversed on that socket? This would make more sense to me and it would explain the red plating (or red screening as it were).

but what was the deal here? is it just working by coincidence? i thought the way around wouldn't make a difference as long as the fuse was on the live?

could the problem have been something else that may reoccur or does this make sense?
i think the output was wired like that for about a years worth of gigging!!! why did it not blow up?

Well, if it was the actual PT primary "mains" that were wired backwards you didn't change anything about the amps operation that could effect only one tube socket. It's possible that you temporarily fixed the problem just tinkering around. You can probably expect a relapse of the issue. But you mention "the output was wired like that for about a years worth of gigging" Was it the power in, or the output that was wired backwards? If it was the plate and screen supply wired backwards on that socket then I would guess that the amp had been running with a tube in there that had an exceptionally tough screen grid until it started to fail. Then the new tube, not having a tough screen grid, as most don't, started failing right away.

So what did you actually change???

Chuck

[kepeb]

it was the mains in.
from the wall.

i swapped it under advisement about 3-4 years ago and just moved the wires into the same position from the old bulgin plug socket to an IEC socket. the old ones were only rated 125v and we have 240 mains

i must have done it wrong i guess.

still the amp was running for a long tome with both tubes.

so you think the problem will likely re-occur in the future?

[kepeb]

yep. its definitely all fine now.

this has made me question what i thought i knew about ac power.
i'm not sure whether i should swap them back just to see for sure if that was it or not.

surley, even if it was backwards up to the transformer, the diode rectification after would render that irrelevant because its all dc???

any thoughts?