Possibilities for switch or Pot adjustable preamp plate voltages

[pjd3]

Hello,

While reading some threads here on "Tone vs plate voltage" type topics, I noticed that a few folks had remarked on say, having a pot in series with the first power node resistor as well as a comment on power scaling.

Over the next month or so I will be experimenting with largely preamp plate voltages as I see there is notable range of voltages found in schematics of the old Fender amps.
This is for basically an AB763 circuit with a pair of 6L6WGB output tubes. I want to see (hear/feel) for myself what the differences is over a range of preamp plate voltages. I can't deny the possibility that there will some subtle or not so subtle and useful differences between the lower and higher range of safely applicable preamp plate voltages and should that be the case, I "know" I will be pondering a way to reasonably have both available.

Do you think that say, the use of a really high-power and robust potentiometer in parallel (or series I suppose) with the first power node resistor is something feasible? I haven't yet made it clear in my head if this would effect power tube bias settings but, I don't think so since the 1st B+ filter cap voltage should be left unchanged from what is hitting it from the GZ34.

If there was a way to do this with safety, it seems as though it be a very useful feature in an amp. There are for instance often times, when using a distortion pedal of some sort, that you'd like to be inputting into the cleanest amp possible but then on the next song, you would like to have a more sweeter and grainier clean sound than the punchy clean presentation of a more clean headroomy amp. Happens all the time.

But, back to the question. Are you aware of a safe way to achieve that? Having adjustable preamp plate voltage somewhat on the fly?

Thanks. I'm really looking forward to the next phase of experimenting a little with this. (just received my moderate selection of 3W power resistors from Digikey).
Thank you all!
Best,
Phil D.

[martin manning]

I would try a switched resistor in parallel with the dropping resistor feeding the preamp stages. An on-on-on switch would give you three settings.

[pjd3]

Thats sounds like a good idea martin.

I was just looking at Ohmite power rheostats and they are upwards of $90.00 for a 25 watt or 12.5 watt 1K rheostat. I'm sure they are good but, ha, it makes the switch solution sound darn good. Plus, when I think about it, 3 voltage options sounds more than enough.

thank you for the suggestion.

Best,
Phil D

[TUBEDUDE]

Or you could use a little MOSFET regulator.

[maxkracht]

https://theelectronicgoldmine.com/produ ... 7b58&_ss=r

These 10k 2w pots go on sale for $1. each just about every month. I got a handful for similar experiments but haven't gotten around to using any yet.

[pjd3]

Thanks for stopping by.

Now that I think if it, I may have been playing it "way too safe" looking at those super power Ohmite pots. It occurred to me that the resistors in the power nodes are often are sufficed by 3 Watt resistors.

I was just finishing off the Bias control - with bias balance that I made and realized that there isn't alot of room on the Princeton reverb chassis that I am building this is, so, should I insist upon having something like a switchable plate voltage, I may want to investigate using Mosfets to control something like this. I really just started getting versed on the possibilities' of Mosfets in these tube amps - just finished a B+ Mosfet reducer (Mosfet follies inspired) and have been studying their uses in driving power tubes and EQ's. That way I might be able to keep high voltage stuff a little further away from small signal panel items.

Thanks again, good feedback. Whew weee !!!!! Its Bruins time !!!

Best,

Phil D