Why such high voltage?

[RyanVee]

This UltraLinear '79 or so Pro Reverb I've been working on - I changed it to a "standard" bias system today. While I was in there I decided to see what kind of high voltage was REALLY coming from the power supply. 517 volts at the plate!!!

The spec on the schematic shows voltage here should be 470 volts, and 6L6GC's MAX is 500 (as per RCA book...)

So what's the happs here? There's no way these tubes are going to last with that kind of plate voltage...

[rdjones]

This UltraLinear '79 or so Pro Reverb I've been working on - I changed it to a "standard" bias system today. While I was in there I decided to see what kind of high voltage was REALLY coming from the power supply. 517 volts at the plate!!!

The spec on the schematic shows voltage here should be 470 volts, and 6L6GC's MAX is 500 (as per RCA book...)

So what's the happs here? There's no way these tubes are going to last with that kind of plate voltage...

It's not just the voltage that will hurt the tubes, because MusicMan amps have 585-720 Volts on the plates.
It's more about total dissipation.
Those MM screens are at half the plate voltage and they are biased very cold.
They never have a chance to 'run away'.

UL circuits are hard on tubes because they stress the screens, not because the they run the plates high.

What is your line voltage ?

rd

[RyanVee]

In the house? If that is what you're asking it's 123 vac (or is that acv)

[ToneMerc]

The Pro Reverb UL schematic i've seen shows 520VDC B+. IIRC, that funky PT is around 373@120VAC, which would make it around 381VAC at 123VAC. This would equate to about 542VDC with no load, under load I would expect to see around 515VDC on the plates......sounds normal to me.

TM

[RyanVee]

Alright, I've got the schematic. I think what i was looking at is a resistor value over on what looks like pin 4. the "other" side of the STBY switch states 520v, so I guess that's correct.