JCM 2000 - Breaking my heart

[Tom Phillips]

thanks - ...one of the 21v bias taps read continuity to certain points between the laminations on the PT...

It is feasible to take off the end bells and look for the problem. However, since you recently procured this transformer I think it would be best to ask for a warranty replacement.
Glad the root cause of the problem has been identified.
Tom

[anthillrich]

wait, wasnt the original PT "melted" ?
so....the new PT is the problem?
doesnt that sound a little strange?

[epis]

wait, wasnt the original PT "melted" ?
so....the new PT is the problem?
doesnt that sound a little strange?

Yes, especially with bias winding short to lamination (ground), he wouldn't be able to set bias current at all

[J M Fahey]

Well, it depends.
If the hot end were shorted to ground, yes, it would be impossible to set bias.
If the short lies close to the other end, no big problem.
I wind my own transformers and one potential problem I must be aware of is that the last couple laminations, which fit very tightly, may if skewed even a little cut through the bobbin and touch or scratch insulation and even cut some wires.
EDIT: what's important here is that there should be *no* continuity at all from any winding to the transformer core.
NONE.
Obviously, measured with the trafo floating.

[bakerlite]

Thank you folks for taking your time to help me out!

So an update of sorts.

The amp (with its new main board - new PT and new Tubes) Sounds fantastic, really really good, and what small hum was there before is gone -

I still 'feel' that the transformer gets too hot for my liking compared to any amp I have ever used.

voltages check out good, as I said there is no hum and she sounds very very good,

But I just don't know would I trust it to last with the transformer getting that hot. - its not red hot like you wouldn't burn your hand of it but its very very warm! doesn't seem quite as hot as before but to be honest i cant see the wood for the trees at this stage.

Taking it from the beginning like a new amp with a new problem where would you be looking if a PT ran hot?

[bakerlite]

Thank you folks for taking your time to help me out!

So an update of sorts.

The amp (with its new main board - new PT and new Tubes) Sounds fantastic, really really good, and what small hum was there before is gone -

I still 'feel' that the transformer gets too hot for my liking compared to any amp I have ever used.

voltages check out good, as I said there is no hum and she sounds very very good,

But I just don't know would I trust it to last with the transformer getting that hot. - its not red hot like you wouldn't burn your hand of it but its very very warm! doesn't seem quite as hot as before but to be honest i cant see the wood for the trees at this stage.

Taking it from the beginning like a new amp with a new problem where would you be looking if a PT ran hot?

[bakerlite]

well it seems you were right to be suspect at the end there-
PT mounting bolts are getting very hot still.
i've checked the iec board it seems ok - its relatively simple pcb.
maybe something i've missed on t