pots for amps

[peterg]

I realize this is not a stompbox question but I'm hoping someone can help. I'm building an AX84 amp and am wondering if I can use 16 mm pots.

As per the schematic (attached page 2) the voltages in the pot sections are 168 vac and 140 vac.

http://www.ax84.com/static/p1/AX84_P1_101004.pdf

Do I need to use the larger pots for these voltages?

[R.G.]

Pots have maximum voltage ratings. For voltages in the 100s and up, you need to consult the datasheet for the pot itself, not just worry if it's big enough, although bigger pots do tend to have higher voltage ratings. But there are exceptions to the general "bigger is higher voltage" notion.

[PRR]

> the voltages in the pot sections are 168 vac and 140 vac.

No.

The plate of the first tube sits at 168V _DC_. But C15 blocks this DC voltage. The DC on VR6 is (eventually) zero.

The _AC_ voltage on VR6 is not given on that plan. We must assume an input signal and also derive the gain of the first tube. A worst-case might be 1V in and gain of 50. Thus 50V AC/Audio signal on first plate, coupled through C15, is 50V on VR6. Another technique is to note that this stage's B+ is 238V, plate at 168V, 70V difference, so maximum 70V peak (probably less due to VR6 + R20 R21 loading), which is 50V RMS. Somewhere in that zone.

Similar study of second stage, tone network, and master volume pot suggests zero V DC, 100V peak AC.

> Do I need to use the larger pots

You need to read the datasheets, or else have snorted a lot of pot smoke.

From my pot-smoke days, I would not worry about 100V peaks on any mechanically GOOD pot in these resistances. However it is your life and your smoke-repair labor. So find some potentially suitable pots and read their datasheets.

That said... I don't see much reason NOT to use good old one-inch pots in a tube amp. Mini-pots won't make the build smaller. Knobs closer than one inch are hard to finger.

There are MANY types of pots. Even of "one size". I'm not even finding a correlation between size and voltage rating. One small pot is rated 20V DC, another a bit smaller (and MUCH more expensive!) is rated 2,500V.

[peterg]

RG and Paul. Thanks for the comments