is there a "best B+" for a high gain preamp?

[tedmich]

I have been discussing some high gain amp designs/mods with people at sloclone and a few maintain that Peavey amps often sound better running a higher B+ to the earlier preamp stages, with one person stating that the "standard" ULTRA/XXX PS resistors after the screen tap (4k-22k-22k) sounded better at (4k-15k-10k) or even (4k-4.7k-4.7k) and one fellow (Joey FYI) stating the old Rockmaster Preamp benefited from going from (10k-22k) -> (4.7k-4.7k) with the original voltage B+ as low as 260v and that the higher voltages giving 320-350v to the preamp tubes sounding MUCH better.

I run old NOS GE 12ax7a's and play with max gain, are there any issues giving the preamp tubes 320-350v, or is there a better high number for the tubes lifespan? Is higher voltages on the pre tubes a useful mod IYO?

[big_teee]

I have been discussing some high gain amp designs/mods with people at sloclone and a few maintain that Peavey amps often sound better running a higher B+ to the earlier preamp stages, with one person stating that the "standard" ULTRA/XXX PS resistors after the screen tap (4k-22k-22k) sounded better at (4k-15k-10k) or even (4k-4.7k-4.7k) and one fellow (Joey FYI) stating the old Rockmaster Preamp benefited from going from (10k-22k) -> (4.7k-4.7k) with the original voltage B+ as low as 260v and that the higher voltages giving 320-350v to the preamp tubes sounding MUCH better.

I run old NOS GE 12ax7a's and play with max gain, are there any issues giving the preamp tubes 320-350v, or is there a better high number for the tubes lifespan? Is higher voltages on the pre tubes a useful mod IYO?

Do you want cleaner or more gain?
I found that lower Preamp voltage gave more gain, but the higher voltage was cleaner.
I try to keep the 12AX7s 300V or less.
T

[tedmich]

Hmm gotta do the experiment, Mason's Ubergain at Ax84 runs at 360v and their Lead pre runs at 340v and I guess I'll read this thread
http://music-electronics-forum.com/t5020/

1st stage preamp B+
260V Fender 65 Deluxe Reverb Reissue
300V Marshall JCM-800
357V Mesa Boogie MK II
359V SLO
402V Mesa Dual Rect

(RMM has noted their "actual" plate/cathode voltage is around 200 V)

Daz says 108v on his Marshall's 1st preamp tube was magical...

[exclamationmark]

Personally I don't really hear a difference when I change the actual B+ hitting the plate resistor (up to a certain degree of course). Anything from 250-350 pretty much sounds the same to me. There is a noticeable decrease in something (clarity? can't really describe it) under about 200 volts, and I have a feeling this may be due to the bias point sitting pretty much on top of the -1 volt line. Grid current starts to flow at something like -1 volt (perhaps even less), and tends to mush up the signal somewhat. As you increase the voltage the transfer curves also get much less linear, which may contribute to what people hear as well.

Also, without adjusting the bias of the stages the dropping resistor feeds after this mod, it may just be the hotter/colder bias of the stages giving a more favourable tone. There's no real downside to running tubes at high voltages, just that you can't really set the bias point above 300v (well you can, but arcing can sometimes occur beyond this). So cold clippers or cold biased stages will need to have larger dropping resistors.

And yet another point to make is the contribution of tone from the dropping resistor itself. The load line of each stage the dropping resistor feeds will dynamically change depending on how much current is being drawn by each stage it's feeding (circular much?). Some think this may cause some compression or sag, although I'm fairly sure proper decoupling will somewhat mitigate this effect.

[diagrammatiks]

really depends on how the circuit is set up.

if you raise and lower the preamp voltages without checking to see where your bias is and the rail to rail characteristics at the input and output you might as well just be poking around in there with a chopstick.

[Enzo]

It is all about the circuit, not the individual parameters.

[imaradiostar]

I agree- it's all relative.

It would be really easy to set up a few small "switch boxes" with various dropping resistors to try in an given circuit as needed. One could even use a few small mosfets and 1 meg pots and vary the voltage infinitely to try different things.

I tend to favor smallish dropping resistors (1k to 5k) and varying each triode/pentode's operating point via plate and cathode resistor selection rather that playing games with voltage. Tubes seem to react most predictably with good filtering and steady supply voltage. I don't buy the whole "5uf sounds better than 50uf" as it applies to preamp tubes- you shouldn't see major changes in preamp supply voltage unless it's some esoteric vintage design that relies on the whole amp sagging for its tone. Anything master volume marshall or later (ie: preamp distortion) should be well filtered.

/rant.

jamie