Hum pot implementation on 2a3

[andyjevans]

Hi guys,

I want to add a humpot to my 2a3 outputs with AC heating. My question is concerning the wattage of the potentiometer. I've seen mention of 2W and 5W potentiometers at 50 or 100 ohm. What I have in stock is 100R, 1W parts as here, datasheet says 100mA for the wiper:

https://uk.rs-online.com/web/p/potentiometers/5028564

Is this OK, padded with a 12 or 15 ohm resistor each side to the wiper?

 

[audiowize]

The wiper current will be whatever the plate current is for your amp, which is most likely 50-60mA. The heat dissipated in the pot by the 2.5V AC being across it will be 1/16 of a watt. The flanking resistors will reduce the overall resistance of the hum pot element and reduce the degenerative feedback from that unbypassed resistance as well as giving less twitchy feel to the hum pot (till you get toward the end of the rotation on either end at least). Be sure to use at least 2W rated resistors for the flanking ones if you decide to use them.

 

[andyjevans]

Thanks for the reply. So a 1 watt pot is OK. The current would be 60mA, not more. Yes - I can use 2W resistors, no problem.

So looks like I can go ahead with the parts I have.

 

[Bigun]

My way. The bootstrap not only adjusts the hum but also reduces the cathode feedback .

I have not seen this approach published or used before.

I'd be interested in other opinions on it ?