Volume pot value for CBS Fender 135 watt twin

[Randall]

Among other things, customer wants the volume pot on the reverb channel replaced. He is convinced it a special value or taper, but he can'r really remember, and he wants to exact right one. I would just throw a CTS 1 Meg audio pot in there and call it good, but he sees on the schematic it is "999K-HD Fender part 014038". What can I tell him to convince him it is the same thing?

And why call it 999K? And what does HD stand for, heavy duty?

[Enzo]

I don't know, but it is entirely possible the drafting software couldn't support more than one MEG at the time, so he drew it as 999k. COnsider the difference between 999k and 1 meg is 1/10 of 1%.

There is nothing special about it.

I would suggest the customer come up with the part and you would install it.

[pdf64]

When I’ve checked those pots, they’re 1M linear.

[Randall]

They are 1 Meg Audio on all the other preceding Twins AFAIK, why would they be linear on this one?

[Enzo]

SO go with audio taper.

[pdf64]

They are 1 Meg Audio on all the other preceding Twins AFAIK, why would they be linear on this one?

Maybe because (boost mode aside) system gain wasn’t increased but an additional gain control, ie master vol, was introduced.
A lot of musicians don’t want to be fiddling with controls to get a loud balanced sound from the amp. Fender clearly put effort into designing things so that there was plenty of range of control but not too much, and setting all controls about midway produced a loud, neutral tone. The previous single gain control had a taper of about 30%, such that a midway setting resulted in a loud stage volume. Now that a second gain control was added, for a similar user interface, a steeper taper would be required. Hence both were 50%, such that a mid way setting of both provided a system gain of 25%, ie loud stage volume.
If both 30% taper gain controls had been used then midway settings would only have elicited a combined effective 15% gain, probably not as loud as might be expected.

[Chuck H]

Does the customer think the wrong pot is in there now? Or is it just acting funny so he wants a more functional one installed? If the correct pot is in there now you can just test it for taper and value, then replace it with the a new part of the same spec. Otherwise you could just use whatever he thinks is correct. If he knows what that is. And otherwise from that you could just use a "J taper" and tell him he got his special spec.

The 30% taper Pete is talking about is commonly called a "J taper" in Fender discussion forums. I can't say what Fender used in the 135 TR but some guys make a big deal out of this taper being the only "correct" one for specific applications. So this is probably what your crustomer is talking about. He could, of course, be wrong and just holding onto his little believies. I hardly see that it matters much beyond maybe the "bright cap" function at a given volume setting.

Ask him for references to his reasoning for that control being a special spec. Research it briefly from his angle and see if he's got cause for reasonable doubt.

[Chuck H]

Ok...

Looking at the schematic I see that the master pot is indicated as "1M JT" This is, as we know, a push/pull pot that also has a resistance tap for the bright cap. A very special pot/switch. I assume the "JT" stands for "J taper" and "tapped" So I have no idea why the 999k spec on the volume controls or what "HD" might mean. If either channel volume control is original (they are both the same on the schematic) you should just test them for value and taper.

[Chuck H]

Ok... Again...

With the exception of the ultra linear g2 connection and the specifics of the pull boost circuit the 100W Master volume TR and the 135W UL TR are the same in that they both use the standard Fender preamp AND a master volume. The 100W TR has the older style pot spec and is indicated as a J taper. So I'd bet the 014038/999k/HD pot is pretty damn close to the slightly earlier 1M J taper (30%) pots. That, or linear as Pete suggested. But that's all I was able to discover.

[pdf64]

Just rechecked a couple of the 014038 999HD pots from late 70s Fenders, that I happen to have kept in a 'good,scav'ed' parts box.
R1377922 track measures about 870k, midpoint about 440k.
R1377930 track measures about 1050k, midpoint again about 440k.

[Randall]

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.

[Chuck H]

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

In this light I would use a typical audio pot (Alpha standard is 15%). I might measure the taper on the normal channel and if it's an A15 I would duplicate that just for symmetry.

EDIT: Wanted to state clearly that the customer wanting the magic part spec for that amp design no longer applies because the design has been changed. It no longer has a master volume. IMHO an audio taper pot is better suited for that control than the original part spec now.

[Randall]

Well, as it turns out, I have a slightly conductive fiber board, not a bad pot. I was getting up to 70mV on the tone stack and volume pot. Moving the treble cap to the treble pot, and lifting the pot side of the bass cap off the board solved the scratchies. It may also have solved another complaint of, "at the same settings this Jetter pedal sounds great in the Normal channel, but lousy in the Reverb channel." They bboth sound the same to me now.

[Chuck H]

You're getting good at hunting down and fixing these obscure failures And...

I'd still like to see all the black paper boards at the bottom of the sea. Replaced, of course. Gotta keep the sweet old amps.

[Randall]

Not so fast! After buttoning up the amp and taking the final test drive, the pot failed again. I don't know why it worked on the bench, but not once it was back in the cab, but it's good to get dc off your pots. So I opened it up again and put in a CTS 1 Meg A taper. Closed it back up (one of those chassis nuts is a real PITA to get threaded), fired it up again, and guess what? Pot does nothing from 1 - 2, then jumps a lot to 3. This customer is not going to be happy with that. I checked two other new CTS pots from the same batch on a meter, and they all three did the same thing. And these are the nice ones. I tried an Alpha, and it was much more of a gradual increase. So out comes the chassis yet again to install the Alpha. It works a lot better, and I don't want to go near this beast again for some time.

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.