Help Velocette tripping my breaker

[paddy]

Hi guys,

I'm hoping someone here can help me with this one as I'm
nearly bald from pulling my hair out.

I am working on a Trace Elliot Velocette. It arrived with a blown
fuse, which I replaced, and when fired up, it tripped the RCD on my
switchboard. Resistor R33 was charred and reading 15 K instead of
10 k. This resistor ties the power ground to the signal ground.
When I disconnect the HT secondary, the RCD does not trip and
the heater circuit works fine.
When I run the amp through my isolation transformer (which
has no ground connection) the amp works fine and the RCD does not
trip.
As soon as I try to run the amp plugged directly into the wall outlet,
the RCD trips. I'm not sure if you call them RCD's in the States, but
it stands for residual current device. They detect a difference in current
flow between live and neutral conductors (leakage to earth).
I'm sure that there must be quite a large voltage across R33 in order
to make it burn, but I can't for the life of me figure out where it could
come from.
Any help would be greatly appreciated.

Cheers,
Paddy

[Structo]

Yeah, they are called a GFI or ground Fault Interrupter.

Mainly they are for use outside and around possible wet conditions.
Bathrooms and kitchens or outside outlets.

I think the large inrush current draw of a tube amp is probably tripping it.

If it doesn't blow a fuse when connected to a normal 15 amp breaker then it is probably fine.

I'm not sure where C1 and C27 is?

Where is R33?

[passfan]

Yeah, they are called a GFI or ground Fault Interrupter.

Mainly they are for use outside and around possible wet conditions.
Bathrooms and kitchens or outside outlets.

I think the large inrush current draw of a tube amp is probably tripping it.

If it doesn't blow a fuse when connected to a normal 15 amp breaker then it is probably fine.

I'm not sure where C1 and C27 is?

Where is R33?

GFCI's do not trip on current draw, they trip on current differential between hot and neutral. The trip circuit in a GFCI is a resistive circuit placed across hot to neutral in such a way as to show the trip sensor a current imbalance thereby simulating a short of current going to ground. His GFCI is either malfunctioning, the primary winding has enough leakage to cause an imbalance or it's shorting under load. Not sure where in the curcuit the resistor that burned is but if it is in the PT primary couldn't it be drawing some current off the neutral to ground ? If that5 was the case then the amp is probably fine and would never work plugged into a GFCI.

[paddy]

Hi guys,

Thanks for the replies. I was working on the amp this morning and the
transformer has developed a permanent short (bulb limiter glowing
fully-secondaries disconnected) I can't tell where but I have ordered a
new one.
I meggered between all windings and the case but had no low resistance
readings. I am still puzzled as to why R33 burned.
I'll wait and see what happens when I fit the new tranny.

Thanks again,
Paddy

[ChrisM]

I'd be removing R33, C21 and C27.

[paddy]

Hi Chris,

I considered doing this but I'm worried about the safety implications.
There must be at least 50 Volts acroos that 10K resistor in order for
it to burn up.
One thing I have discovered is that there is only one cap across the
resistor (100nF) and it is open circuit. I don't know if this is part
of the problem but I will replace it before I fit the new tranny.

Cheers,
Paddy