new PT, rational voltage for the plates?

[Doug H]

Was looking at a transformer site, I forget which place, but they have a bunch of plexi PT s there, some with fairly high voltage.

What is rational to consider putting on the plates of TS el34Bs or some of the 6ca7s out there? Would 475-500 be out of the question?

And what's the general formula for PT output versus plate voltage once everything's hooked up. I think my PT puts out about 490, but there's only 45- or so on the plates once all is said and done. Maybe I read something wrong?

[Riscchip]

How many volts actually end up on your plates with a given transformer will vary based on the design of the amp, the rectifier, how many tubes there are, etc. If you measure the B+ with tubes installed it's going to be higher than it would be if you pulled out all the tubes. Also, adjusting the bias directly impacts the B+. Adjusting the bias hotter lowers the plate voltage (some) by stealing volts at the bias wind. I was just re-tubing a Fender Bassman that had 490 volts on the plates with no tubes installed and 450 after I put the new tubes in and re-biased.

500 volts is a lot of plate voltage in an amp that also uses a large screen voltage. Old Marshalls tend to have a lot of plate voltage and a lot of screen voltage because tubes could deal with it. Early 100 watts had even more. These amps will give currently produced tubes of any kind a run for their money, though I find SEDs survive just fine in my 1969 Superlead with 500 volts on the plates and a little less than that on the screens.

On the other hand, if you hold the screens a lot lower, you can get away with some higher voltages on the plates. For example a Sovtek Mig 60 I retubed a recently had 630 volts on the plates but only 330 on the screens. The average quality Russian tubes that were in there still worked, and had worked for years at that high plate voltage, only because the screens were lower. We just replaced them because they were old and sounded like crap.

Hope some of my babble was useful.

[Doug H]

Mine has about 460 on the plates, it's probably a 490 volt PT I would guess then.

trying to work out if I want to go higher, lower or stay the same. I have to replace it.

Thanks

[joey]

I wouldn't push it, especially with cp valves, which tend to less robust in the screen dept. than the old ones were. I would opt for lower if you can 440V-430V under load, Or else limit the screen voltage more if you opt higher, this will have the implication though that you will have more compression at higher excursion levels because of the increasing screen current dropping more voltage across the larger screen resistance, and thus squashing the grid curves further down, and closer together.

You might actually like it, or you might not.