Koch Studiotone 20 Combo schematic

[DrTurbo]

Hi everyone,

I dont suppose anyone has a copy of the schematic for a Koch Studiotone 20 amp by any chance?

The version I have here is mk 1.3 schematic but doesnt ring true with the amp itself.

The problem is it came to me after being powered up for the first time in a month or so and let a puff of smoke out, and was switched off again.

The only trace I can see is what seems to have been hot is the cathode resistor for the output tubes. On my schematic (attached), the tubes have separate cathode resistors, but on the pcb the two cathodes are linked together, and use this one resistor, that reads several megohms so is dead I guess..

This resistor is labelled as R46, on the schematic R46 is the dummy load resistor, R42 & 43 exist on the pcb but aren't the cathode resistors just small 1/2 watt jobs..

But not sure as yet what caused it or if one of the output tubes caused it, but it ran fine before sitting for a month.

But if anyone has the correct diagram or knowledge of it it would be appreciated. I've attached the one I have that doesnt seem to be correct..

Thanks in advance.. koch-studiotone-schematicspdf.pdf

[Helmholtz]

A shared cathode resistor should be 150R or maybe 180R. Can you read any resistor code colors?
Power rating should be at least 3W, better 5W.

Question is what caused it to blow. One of your power tubes might have an internal short.

(Generally 2 separate cathode resistors and bypass caps is a better solution.)

[DrTurbo]

Hi,

Thanks for the reply..

Its a bit diffilcult as its discoloured, but measures about 8 or 9 meg!, But its a good 3 or 5 watts in rating I'd say from its size.

It does supprise me it having a shared cathode as the amp seems to be of a very good quality, very nicely built.

Thats the only real issue I can see, I'll remove the power tubes and power it up on the Variac, I dont have a tube tester unfirtunately, but I'll try and comare them with each other, maybe a shock from transportation damaged one of them.

Looks like you're spot on as it says 150E - 3W underneath it..

Thanks again.

From how it looks to be wired the tubes are the only things capable of blowing this resistor?

[Helmholtz]

From how it looks to be wired the tubes are the only things capable of blowing this resistor?

I think so.

[g1]

I think if power tubes lost their grid reference to ground, they could run away and burn that resistor. Or a bad coupling cap from the prior stage could do it.
But agree a bad power tube is most likely.

[DrTurbo]

Thanks, I'll continue checking things out in the meantime, but I believe the smoke was pretty instant after power on..

[Jon Snell]

I would replace the 150 Ohm 3W MO resistor and the output valve that uses it.
Common cause is worn/over driven/old valve flashes over internally but only intermitantly.
Check the screen grid resistor!

[DrTurbo]

I think if power tubes lost their grid reference to ground, they could run away and burn that resistor. Or a bad coupling cap from the prior stage could do it.
But agree a bad power tube is most likely.

I guess it would indeed cause a fair current to flow in the cathode(s) if the ground reference disappeared. I'll check out the HT fuse and make sure it hasnt been replaced with something silly!

[DrTurbo]

I would replace the 150 Ohm 3W MO resistor and the output valve that uses it.
Common cause is worn/over driven/old valve flashes over internally but only intermitantly.
Check the screen grid resistor!

It looks to be a shared cathode arrangement, I'll do some more checking and come back to you all..

[Helmholtz]

I would replace the 150 Ohm 3W MO resistor and the output valve that uses it.
Common cause is worn/over driven/old valve flashes over internally but only intermitantly.
Check the screen grid resistor!

It's a shared cathode resistor, so both tubes are suspect.
An open or burnt screen resistor might be the consequence (not the reason) of a shorting tube.

[DrTurbo]

Just to close this off guys, I never did get the correct schematic for the amp, even Koch sent the one with seperate cathode resistors..

Checked out pretty much everything else and couldn't find anything wrong, so replaced the common cathode resistor, the output tubes and alls well.

But a very nicely built and good sounding amp!!

Thanks for all of the input though, its appreciated.