Amp pots and master volume

[basschops1528]

Hey Guys,

Still gathering ideas and info for my Hoffman 100W Plexi Clone build.

When I started the amp I used whatever pots were lying around my HS electronics shop. Thinking about it now, I'm curious what power rating should be used.

Below is the project itself with all of their values but there is no mention if they need to have a minimum wattage.

I'm also thinking of putting a master volume in post-phase inverter as someone mentioned in another post. In the same circuit, any idea where that could go?

I plan to use a switch pot--either pull or twist on--that essentially removes itself from the circuit when not needed. Any idea for ohms and wattage on that?

By the way, for bonus points, the pink wire from the 8 ohm tap on the OT going to the 100k resistor, is that for feedback?

Thanks guys!

[hesamadman]

The only time you will have to worry about a "power" rating on a pot is if you're using a switched pot to turn the amp on, in which case typically higher rating than 120 will suffice. Your tone stack doesn't see any B+ and if it does, you won't like the results. An error in a schematic I drew once put a tone pot before a coupling cap rather than after and it made some serious unwanted noise when adjusted.

The presence acts as a negative feedback attenuator so to speak so yes the purple line on the CT of the OT is part of NFB

[Willabe]

the purple line on the CT of the OT is part of NFB

It's an 8 ohm tap, not a CT.

And yes the 100K is the NFB R.


             Brad   

[sluckey]

And it's pink, not purple. 

[basschops1528]

What would happen if you raised or lowered that value resistor, or even made it a 80k pot+ 20k resistor, or even a 70k pot+ 30k resistor? See where I'm going with it? 

[sluckey]

What would happen if you raised or lowered that value resistor, or even made it a 80k pot+ 20k resistor, or even a 70k pot+ 30k resistor? See where I'm going with it? 

The amount of NFB would change. If you use a pot you'll have one more knob to twist.

[basschops1528]

Would it be a cool experiment as a gain adjustment? Or probably a nightmare in oscillation land?

[sluckey]

Would it be a cool experiment as a gain adjustment? Or probably a nightmare in oscillation land?

It would probably be somewhere between "cool" and "not worth it". I would not expect any oscillation problems. Some people like to put a switch on the NFB line to totally disable it for a more raw and open sound.