Volume pot value for CBS Fender 135 watt twin

[Chuck H]

the Normal channel volume was replaced with an Alpha C50K. That seems like an odd choice, and a huge leap from original 1 Meg.

Not to mention that C is a reverse audio. It must be a terrible pot for that circuit. And a 50k parallel load with the plate load! Not particularly dangerous, but certainly not what Fender intended for performance. Sorry to say, but in good conscience it should also be replaced as a matter of pride in a professionals station.

[Leo_Gnardo]

I wonder what's up with these CTS pots?

Also, I should mention the Normal channel volume was replaced with an Alpha C50K. That seems like an odd choice, and a huge leap from original 1 Meg.

CTS - aren't shipping quality pots any more, not for over a decade. Meanwhile, Alpha's quality has improved.

50K reverse as a volume control? Previous tech must have his head up his bum. Put in an Alpha 1 Meg A taper & smile.

[Randall]

I don't have another Alpha 1M on hand, the C50K works surprisingly well, at least down on the lower usable end, there is no complaint about it, and I am not wrestling this chassis out and back in the box YET again if I don't have to. I already have way too much non-billable time into this beast for other reasons I have not mentioned.

[Enzo]

That the C50k works in place of a 1 meg or whatever is further testament to the fact that guitar amps just don't care that much about values.

I could be wrong, but I always thought a C taper was the one that tapered either way from center, like you'd use on a graphic EQ.

[Helmholtz]

A 50k pot in this place will significantly change the TMB response and lower available preamp gain by at least 6dB.
I'm sure I would notice the difference.

[pdf64]

I hate 'zombie amps' those where you think they're all done, get them all put back together, and there's a problem with the final test. I recall a Marshall built, Korg era Vox AC15, that amongst other issues had many failing solder joints. Having got it all buttoned up for the 2nd time, that crackly, fade out signal loss again became apparent on final test. It wasn't until I'd taken the pcb out and traced out a perfect signal that I realised the issue was the pickup selector on the guitar I'd been using to test it

This all seems like so much ado about nothing. Customer wants the pot replaced because it has a bad spot at around 1 or 2, where he plays it because it is so ungodly loud. So, taper seems like a moot point to me if it never gets to midpoint in the first place. The Normal channel pot is already replaced, I have not measured it yet, but it is an Alpha with no numbers visible, so we know that is not the special taper. And further, is this special taper pot even available from Fender now? Not that I can find.

Also, the master volume has been removed on this amp previously.

With the benefit of hindsight, if the above info had been available in the OP, then the 'much ado about nothing' would have been avoided; but that's hindsight for you

[Leo_Gnardo]

That the C50k works in place of a 1 meg or whatever is further testament to the fact that guitar amps just don't care that much about values.

I could be wrong, but I always thought a C taper was the one that tapered either way from center, like you'd use on a graphic EQ.

Ya learn something new every day - same here. Heck even a 5k pot would "work", sort of, as Helmholtz said at the expense of gain & EQ range. Randall, if you don't want to take the time to make it right, I can understand. But personally, I couldn't leave it that way although it involves unassing that PITA of a chassis, and maybe replacing one boogered-up bolt/nut as well. Do as you see fit, and in the immortal words of Forrest Gump:

[Randall]

Eh, who uses the Normal channel anyway? A little less gain on this mega-beast I don't see as a real problem. He says he plays the amp on about 1, and I hear no difference between channels up to about 5, past that I go deaf.

[Enzo]

Well the thing is, if you put the two side by side, of course you could measure and even hear a difference. But the bottom line is whether or not your amp works, plays well, and sounds OK to you. And at that level, they are SOOO flexible. This customer for example wanted a new 999k pot, but said nothing about the channel not being right where the "wrong" pot was installed. I lose some gain? OK, I normally run on 3, so I now have to turn it to 4? I can handle that. I never EQ by the numbers, I turn the knobs by ear, so again if my settings are a couple numbers different from the next guy, so what?

Am I advocating "wrong" parts? No, of course not. I am advocating for using reasonable judgement. Should that amp sit unused until the exact pot can be found? If we needed a 500k pot and all we had was 250k, should we cancel the gig until we can source a part, or should we stick in the "wrong" pot and play on.?

[Helmholtz]

I don't think the problem is availability of a suitable pot here. An audio taper pot will give better control at low setiings. Real audio taper means around 10% at mid position.

[Enzo]

Speaking in the general case, defending the poor guy who stuffed a 50k in there because it was all he had, and it worked for him. Yes, we are aware the 1 meg pots are available.

[Helmholtz]

Well, if the normal channel never gets used, it doesn't matter what pot is in there.

[Chuck H]

I don't think the problem is availability of a suitable pot here. An audio taper pot will give better control at low setiings. Real audio taper means around 10% at mid position.

Agree. I usually use the regular Alpha pots and I find their A15 (standard) taper fully acceptable for Fender circuits. Though I agree that 10% would be even better. IIRC the original CTS pots were A20 and on a few later models they used their "J taper" which is an A30.

But it's all subjective isn't it. Some guys want the original spec because they think there's "mojo" involved. I just want these amps to have a reasonably adjustable range before clipping

[The Dude]

Well, if the normal channel never gets used, it doesn't matter what pot is in there.

Yep. If that's the case, it doesn't even need the associated tube in there, and Twin PT's run extremely hot. (That is a hint. )