Trimmer in place of plate resistors

[Structo]

I've been reading John Chambers amp repair site in Nottingham. He really has some great repair stories there if you ever want to waste a few hours.

Anyway, he is talking about this Deluxe Reverb amp he had on his bench.
He says that he likes to install a 200K trimmer pot in place of the plate resistors on the phase inverter.

He states:
Dave indulged in letting me do a little mod that I always recommend on any amp. This is to fit a 200k pre-set pot in place of the two plate resistors of the phase splitter, usually an 82k & 100k. This small but effective addition balances not only the splitter tube & associated components, but the output tubes (no matter how many) and any balance deficiencies within the output transformer too. It works wonders!

[img:750:563]http://www.chambonino.com/work/fender/fend2k.jpg[/img]

I know we like to include a trimmer on our PI plates in the Dumble amps.
But I hadn't ever heard of doing away with the actual plate resistor entirely and replacing them with a trimmer.
Anybody ever done this and what are your thoughts on this?

Website:
http://www.chambonino.com/work/fender/fend2.html

[ChrisM]

Same as what HAD is doing but he gets more range out of his trim pot. Is more range good? Probably not Id say.

Are you really going to use the whole range, say have one resistor 50K and the other 150K?
Also much better chance of burning out that little trim pot. I would not feel safe replacing plate resistors with a little (0.25W/0.5W) trim pot.

I like what HAD. Has a set resistor value in mind for best tone/performance. Then he adds a small trim pot to fine tweak the PI for best performance according to what tube your using.

[Bob-I]

We know that the composition of the plate load resistor makes a big difference in tone, so personally I wouldn't use a trimmer to replace the entire resistor.

[Structo]

Well that is what I am wondering, how much current is flowing there and is that a safe practice?

Say if you had the input side set to 105K @ 280v.
Isn't that .74 watts?

[Bob-I]

Well that is what I am wondering, how much current is flowing there and is that a safe practice?

Say if you had the input side set to 105K @ 280v.
Isn't that .74 watts?

You're only accounting for the DC, the AC swing at this location is significant as well.

[Structo]

True that.
It was just the way this guy said it was like his secret blackface weapon or something.
I had never heard of anybody eliminating the plate resistors all together and just using a trimmer.
I can see maybe using a 2 watt potentiometer there if that was your thing but a little tiny 1/4 watt trimmer just doesn't seem smart.
I mean the whole amp signal is going through that!

[David Root]

I agree, I wouldn't do it either, especially with a 12AT7, ~2x more current than a 12AX7 PI.

It does seem to be a pretty big trimpot, compare it with the blu molded caps and the sandbox resistor. Even so, I think it's risky. HAD's way is better, no risk.

[scotto]

I used appropriately powered pots in place of resistors to dial in my dropping string, but then I put resistors in once I found what worked best for my amp/tube combination. Never thought about doing it for the PI. I'd still go back to resistors with small trimmer once dialed in.