Unorthadox query about tube amp circuits

[ali3nat0r]

Hi, this is an unusual query which I haven't found the answer to anywhere online, so here goes; I'd like to know what kind of voltage goes across potentiometers in a valve guitar amp. Mechanical pots can handle hundreds of volts, but I'm looking to build a valve amp with digital potentiometers in, and these can usually only handle 5-10v without burning out; I did find a "high voltage" digipot that can take 36v, but considering valves run at 400v sometimes, that's still not enough. So... is the voltage over pots in the preamp, tonestack etc at the high voltage tube level or the more managable line level?

Thanks,

-A

[sluckey]

A typical volume or tone control only sees a small AC signal voltage, usually only a couple volts. But this is no guarantee. Each circuit is different and some circuits may see larger voltages. Some circuits even allow high voltage dc on the pots.

I'd like to see a schematic or even just a block diagram of your idea. I don't see any application for digipots in a tube amp.

[92Volts]

Some datasheets specify "clean" signal levels in various circuits, see "Eo" on the second page: http://www.mif.pg.gda.pl/homepages/frank/sheets/093/1/12AX7.pdf

If 40v RMS (56v peak) can be produced at 5% distortion, it's plausible an overdriven guitar circuit could hit the next stage with more than 36v. Earlier stages will of course have smaller signals since the input hasn't been amplified as much.

If you use it in a location where the signal "shouldn't" be too big, you could protect it using Zener diodes or something to prevent issues during power-on, larger signal than expected from active pickups or faulty tubes, etc.

I'm assuming you're doing this so you can "remember" or remotely adjust settings? Certain functions can be accomplished differently, like increasing cathode resistance to decrease gain. You can also handle the signal with solid state components which can stay within the limits of the digital pot, which can later be boosted and passed through tubes to produce the type of distortion you want (for example).

[ali3nat0r]

I'd like to see a schematic or even just a block diagram of your idea.

It will basically be a Marshall 1987 clone with a few mods such as master volume, spring reverb, fx loop etc.

Some datasheets specify "clean" signal levels in various circuits, see "Eo" on the second page: http://www.mif.pg.gda.pl/homepages/frank/sheets/093/1/12AX7.pdf

If 40v RMS (56v peak) can be produced at 5% distortion, it's plausible an overdriven guitar circuit could hit the next stage with more than 36v. Earlier stages will of course have smaller signals since the input hasn't been amplified as much.

If you use it in a location where the signal "shouldn't" be too big, you could protect it using Zener diodes or something to prevent issues during power-on, larger signal than expected from active pickups or faulty tubes, etc.

Ah cool, it also states lower Eo with lower plate voltages... so could maybe run the preamp at a reduced voltage and the poweramp at full...

I'm assuming you're doing this so you can "remember" or remotely adjust settings?

That's correct, like those Line 6 modelling amps where you can store a "patch" and quickly switch between them with a button/footswitch, except using an actual valve circuit so it sounds better. I've already done a proof of concept by modding a solid state Fender Frontman, and the digital circuitry is completely transparant - it sounds exactly like it did before modding, with the exception of a slight pop during patch change (which can be easily fixed as well). Voltages inside the Fender's preamp are probably around 1-2v though, so well within the range of digipots.

[VMS]

Hughes and kettner uses something called smart rotary control:


http://blog.hughes-and-kettner.com/smart-rotary-controls-giving-128-real-tube-channels-one-amp/