Carvin Stereo Tube 100 (TS100)

[Guitarist]

ISSUE: Hum on both channels

I made a test lead from a guitar chord. One end plugs into a mixer, the other has a 600 V cap/probe and a ground clip. The question is that as I put this onto a preamp filter cap I get some - rather a lot of noise - on a this amp. Shouldn't the B+ be nice and quiet? Maybe I should test the main rectifying diodes?
I don't have a scope nor know how to use one. It is not the tubes.

I've added the schematic.

[oc disorder]

Well a bit of long distance deduction.
If its common to both one can assume its in something that is common to both.
Ht supply
Bias supply ( C45 ? )
Filament supply
DC filament supply
Does the volume control affect the hum in any way ?
If it does then the HT filtering (C12,C49,C50 and C52)capacitors are suspect.
Also the filtering on the DC filament (heater) could be suspect C13 and C 14.
If it doesn't removing V1 and or V3 should make no difference to the hum.
If the hum is still there with only the output tubes and the phase inverter
in circuit HT filtering is still suspect as well as R78 and R79 in the ac heater supply and possibly C45 although you should check if there is a healthy bias supply after that point with a multimeter as this is least likely with both sides having their own bias filter cap.

Your probe is good to trace a signal path but not so hot on checking HT supplies !

Any internal connectors seem burnt ?
Possibility of a ground wire not making good connection.
Hope thats some help .

[Enzo]

Your audio probe is what we call a signal tracer. It is very useful for tracing SIGNAL through the amp.


COnsider that 1/2 volt of ripple on a 400v B+ is pretty darn clean. But as a signal, it is more than a guitar puts out. SO "listening" to B+ will be a louder signal than a guitar would be going into your probe amp.

Like oc says:
if both channels hum the same, then look for something common like power supply. Set your meter to AC volts and measure the B+. On AC volts it will ignore the DC part and ONLY read the ripple or hum. If it is a volt or less, I;d say you were in good shape. Don;t forget the bias supply is also common to both channels.

There are two sets of heater "virtual center tap" resistors. One on the main power supply and another on the preamp board. In each case a pair of 100 ohm resistors. A shorted power tube sometimes will burn one or more of those resistors open. Check for it. R78,79 and R22, 25.

[Guitarist]

Thanks a ton, I'm working on it now.
After the B+ diodes and at the first filter cap: 3.7 V AC
A filter cap or two after that is drops to .1 V AC ~that seems too clean?

At the phase inverter plates, pins 1 and 6, there is 2.8 volts of AC.
When I touch the controls or the case, doing that added a higher frequency buzz until I scraped and sanded paint off the painted case insides and then reconnected the beautiful metal front.
Reconnecting the preamp board power connector causes the hum to increase.
Those heater resisitors all measure 50Ohms in circuit.

I'll keep at it...

[Guitarist]

When turning off the amp the hum lingers then fades. I've heard this suggests that B+ is not th problem.? I disconnected the fan cap C22 to rules out that as I think I was able to hear the fan prior.
DC cap C14,15 are hard to access. filiment R's R22,23 test OK.

hmmmm..