Budda superdrive V40

[Wakculloch]

Anyone on here have any experience of these? I'm at a loss. The output valves are red plating, tried new set, same problem. Checked HT voltage, 470V. By my calculation each valve is dissipating 15.4W at idle in a 6v6 that is rated at 14W max. Have i missed something or has someone designed a valve destruction machine?

[g1]

Is a schematic available? Is it fixed bias, cathode bias? For fixed bias check the bias supply voltage at pin5 of power tubes, for cathode bias check for shorted or leaky cathode capacitor at power tube cathodes.

[Jon Snell]

From memory, these amplifiers use quite an odd phase invertor coupling arrangement.
If the phase splitter fails or has a faulty valve the grid voltages can go to an unstable level and cause this issue.
I would remove the phase splitter valve and check again.

[Wakculloch]

Unable to locate a schematic. Cathode bias with a bypassed 250R resistor shared by all four 6v6's. 35 volts across the resistor giving 35mA per valve. This seems way high. Is it supposed to have special or selected 6v6's?

[Wakculloch]

The phase splitter seems to be just a voltage divider followed by an inverting stage. Checked the grids on the outputs and they are at zero to 3mV. Gonna fit a set of those Russian 6n3c's that used to be sold as 6L6's for testing purposes so i can leave it on without fear of a meltdown.

[g1]

Cathode bias with a bypassed 250R resistor

Disconnect the bypass cap and see if it makes any difference.

[Helmholtz]

The phase splitter seems to be just a voltage divider followed by an inverting stage.

Please post a drawing.

A plate-to-cathode voltage of 435V with 6V6s certainly is excessive..
Redplating at idle indicates abuse and short tube life.
You might try the JJ 6V6 S with a larger (e.g.330R) cathode resistor.

[The Dude]

Yes, I agree with raising the cathode resistor value. But, first check the cathode bypass cap as g1 suggested. Raising the value of the resistor won't help if the parallel cap is shorted or leaky.

[Helmholtz]

Lift one end of the cap and see if the cathode voltage increases.
If it does, the cap is leaky and increases cathode current.
Normal plate dissipation with cathode bias is 100%, i.e.14W with 6V6GTs.

[Wakculloch]

Lifted the cathode cap and voltage is constant. Checked the phase splitter resistor values and it is the same as the Superdrive 80 model; the schematic is on this site.
Drove it to clipping level and cathode voltage increases to 52V, so it may be cathode biased but it's not class A.
I'm reluctant to change the cathode resistor. Amps shouldn't need modding to fit the valves they were designed for, but if that's the only way to get it up and running it may be the only option.

[Helmholtz]

It is not unusual to see cathode biased guitar amps running at 110% PD.
But what would make me nervous is redplating at idle.

And yes, the amp is running in class AB like most cathode biased PP amps.
Full class A would mean much lower B+ and output.

[Wakculloch]

Ahh; i get what you mean now. Sorry if i appeared combative. Still not sure what to do as this is a sale item and don't want it coming back to bite me.
I was playing through it this afternoon and it sounds good. Marshallesque.

[Mick Bailey]

The amp would have worked fine when it left the factory, so I'd be reluctant to make any component changes. One thing to look for is to scope for ultrasonic oscillation, which can cause excessive current draw where there is no other sign of component failure. Even poor lead dress can cause this.

What's the B+ with the tubes removed, and also the heater voltage? If the current draw is reduced, you may see an increase from 470v. Checking the heater voltage gives a good indication whether the PT is correctly configured, or whether it's a 230v amp running in a region with high mains (ours is usually 248v or more, which raises the B+ considerably).

[The Dude]

It is not unusual to see cathode biased guitar amps running at 110% PD.
But what would make me nervous is redplating at idle......

Agree. It seems to be a "thing" these days. There have been several amps come into my shop that were designed this way. AND, just because some designer decided to build an amp that kills tubes doesn't make it right. I've often raised cathode resistor values in new amps. I don't see it as any different than rotating a bias pot in a grid biased amp.

Edit: Also remember to subtract cathode voltage from plate voltage when looking at tube plate voltage limits.