Koch Power Transformer

[Simpson]

Hello!

I have repair a Koch Jupiter 45 a month ago. The amp came with the transformer HT tap with only 10VAC ( it should be 250VAC ).

This amp has two 12ax7 on the pre amp section and two mosfets on the power section.

I have send an email requesting a quote for the transformer and since they didnt bother answer me i send it to a shop for a rewinding.

After one month playing with it, the same thing as happen again. this time 50VAC on the HT so i send it again for repair.

My question is;
Do you think the circuit has a fault that is pulling a large amont of current of the transformer?
Or it was just a bad winding?

Thanks

[alexradium]

i guess there is a fault in the power section and there is not an adequate protection via fuse or other.
When powering an amp you have to apply voltage gradually and monitor current.

[Simpson]

The voltage was applied gradually and everything was fine during a month playing.

[Simpson]

two mosfets on the power section

I mean, on the output section

[Simpson]

I really need some help on this.

I send the transformer once again for a rewinding ( same shop ).

I put a smaller fuse on the HT tap, connected the amp to a load while monitoring the output and the voltage on the B+.
I let the amp about 1 hour on the load, everything looked ok, so i was about to turn off the amp, immediately after i pull the jack with the signal the B+ drop to 70v.

It happen again,

What may be causing this?

[g1]

Can you post a schematic?
Are the mosfets running off the same HT?

[Simpson]

I can't find the schematic.
The mosfets are running on other transformer tap.

[g1]

Does that (other) tap remain working when the HT tap goes bad?

[Simpson]

Yes.
Only the HT stops working.

[g1]

Maybe bad diodes or filter caps in HT supply?

[Simpson]

Before connecting the transformer i replaced the diodes ( just to be sure, they were ok ), check solder joints and the resistors around the tubes.

The capacitors seem ok to me. I thought replacing it but i didnt do it.

While testing, i replaced the fuse ( 160ma to a 100ma ). I guess if the circuit was pulling to much current, the fuse will blow??

[g1]

The fuse should be designed to blow to protect the winding, but I'm not sure of the value. You would not need a 100 or 160mA winding to run a preamp tube or 2. So it's possible the winding would be over-stressed before the fuse blew.
I would replace those HT filter caps anyway.
But I also think you should try a phone call to them directly in Netherlands. They should be able to tell you whether there have been any problems with those transformers. http://www.koch-amps.com/koch-amps-contact.html

[Simpson]

I will try to contact them and replace the caps.
Hope they will answer me

[Simpson]

I have talk to them ( koch ) on the phone. They said its very strange. Its not a usual thing like this to happen

[g1]

Maybe the guy who did the rewind could tell you the current capacity of that winding?