Tweedle Dee 10k phase inverter pot safety question

[oldmica]

I'm in the late stages of working up a 5E3-like circuit for my brother (2x5E3 with some tweaks). I'm thinking of putting Dumble's 10k balance trimmer in the phase inverter.

I do not want to add any risk of electrocuting him into the plan. ATM anyways.

Would it be safe enough if I used a good quality 10k pot with a nylon shaft?

There is already a B+ fuse in there, which should blow if the wiper shorted to the case. I cannot think of anything else that could happen.

I know it isn't good design to bring anything HV close to beer potential.

[pompeiisneaks]

In most cases the PI balancing only needs to be done rarely. Normally this is why it's a small pot on the board, and should be adjusted with some measurement device connected to the outputs of the PI. Would the intention be to just let him tweak it by hearing instead? Also, the tweedle dee PI balance is in the cathode section, so it's not running at a very high voltage, in the 10 volt range maybe? (you should measure this to be sure/safe, i don't recall taking one on mine, so maybe I'm wrong and it's higher.)

~Phil

[sluckey]

You should measure the cathode voltage on a cathodyne PI such as your TWD. You may be surprised at what you discover.

[pompeiisneaks]

Oh guess I should I'm guessing that means it's much higher. I went off of the more typical stage which I've seen 1-10VDC. Maybe I'll do that hehe.

the other point, though, is still pertinent. You normally don't change the PI tube often, and it needs to be done with something to measure the 'balance' or intentional imbalance as some do.

~Phil

[oldmica]

^^ thank you both.

Yes, the concept is to be able to tweak it from the front panel. I've read that the best clean PI tone setting is different than for the best overdrive tone. If it can be done safely, of course. I was reading the schem as one side of the pot being a little less than 1/2 B+3.

I would like for him (and me) to not have to crack it open unless it requires repair, such as eg replacing a burnt screen resistor. I'm planning out the front panel and parts, so I can ensure I have room. If I go with smaller pots and knobs, that will solve any room problems, of course.

Great point on measuring the im/balance. Will put in any parts and/or test points for metering that, if it is easier than doing it by ear.

A better question would be: in general, if someone was considering putting high voltage dc on an external pot, would nylon be good enough to do it safely? Another example could be a pot-variable conjunctive filter (B+1 through the primary windings).

I can't think of any failure scenario where that could cause lots of volts to the front panel/chassis. A relay powered by the filament supply, a front panel switch, and two internal pots or rear panel external pots would work too.

The other consideration would be tube rolling in the PI position.