Voltage on Preamp Tubes?

[booj]

Originally Posted by booj
Also, why do these amps almost always seem to ratchet up the voltage for the later pre-amp tubes? How does it affect the qualities of the overdrive?

No, this was posted by the chipster; the originator of this thread. It was inside a pasted copy.

Cool info about the V1 stage of yer Marshall. 105 volts duly recorded.

[Robert M. Martinelli]

Hi All,
I'm continuing to think about building a stand-alone pre-amp for exploring high-gain ideas. In my last post we discussed power supplies. One important design choice is how high in voltage I want to go. Looking at examples out on the web, there is a huge range of voltages applied to the pre-amp tubes. Here are some example plate voltages used on 12AX7 pre-amp tubes:

260V Fender 65 Deluxe Reverb Reissue (my current amp) schematic

300V First stages on Marshall JCM-800. pre-amp, power-supply

357V First stages on Mesa Boogie MK II. schemtic

359V First stages of Soldano SLO-100 schematic

402V First stages of Mesa Dual Rectifier schematic

So, my question is...what is the effect of the voltage supplied to the pre-amp tubes? How does it affect the sound? Also, why do these amps almost always seem to ratchet up the voltage for the later pre-amp tubes? How does it affect the qualities of the overdrive?

Thanks,

Chip

Hi Chip,
AFAIK the maximum applicable operating plate voltage for 12AX7s is 300 V, and some of the voltages you referred to are not the actual plate voltages, because they're measured before the plate resistor - as to the others, some of them are indeed very high ( referred to GND ) but you can see the cathodes are lifted from GND by resistors with pretty high values ( on the dual rect. one of the cathodes is at 216 V, which BTW is way over another limiting value, the cathode to heater voltage ) and, being the cathode the reference point, the "actual" plate/cathode voltage is around 200 V.

IMHO higher DC voltages on the plate are an attempt to increase a stage's gain trying to keep the stages number as low as possible....

FWIW, my opinion is that tinkering with tubes is funny, but there are limits I would never trespass, If in need of more gain there' s always the chance to add another stage, I' d always prefer that rather than having to be soooo rough on the tubes and make reliability worse. ( but maybe I' m just getting old and losing the courage to "dare". I am sure many tubes have been "cooked" by Jim Marshall or Dick Denney or by other designers by trial and error before getting "that" sound ).

[booj]

108... sorry.

[Enzo]

SOmetimes, just for science, get out your sine wave generator, and plug it into your amp. Leave the volume down so you don't have to listen to this. Now cranks a signal into the input jack and watch the plate of the first stage with your scope. See the amazing amount of signal you can have coming out of that tube before it runs into anything. Now compare the input signal level you just used to the level a guitar could possibly deliver. I don't think you are going to hurt the 12AX7 much extracting gain from it.