Treble pot acting like a volume/high pass

[Gainzilla]

Hi all,

I wanted to add a "character" switch (3000v ceramic disc/silver mica tone caps on a DPDT switch. 240k pot with DPDT switch to be precise.) to my tone-stack while also changing out the tone caps for polyester film ODs. Putting the tone stack caps on the switch gave me a smooth/edge tonal option which was kind of nice.

The problem is this: The treble pot sounds great at about 12:00 on the dial. When I turn it down, it gets middier and the overall volume increases. When I turn it up from 12:00, it gets really thin... Like, more than just turning up a treble. It's almost like variably shelving everything below a certain point. I don't see anything obvious and even went so far as to reverse the "mod" and am still getting the same results (I had originally installed a 250k audio taper pot while waiting for the linear taper pot to show up. I chalked up the weirdness to the audio taper pot in that position). I pulled the board last night and checked all the solder joints. They all look good.

Any ideas what might be going on here or where I should begin troubleshooting? Amp is Jet City JCA20HV.

[Structo]

If you think about it, all of the signal is passed through the treble pot on a lot of amps.

Defective pots can cause all sorts of havoc.

Did this amp work properly before this?

[Smokebreak]

Do you have a schematic of your switching mod?

[Gainzilla]

240k pot with DPDT switch to be precise.

Lol. And by precise, I meant 250k, not 240k.

Structo: thanks for the help. I suppose that's true isn't it. I've actually tried 3 pots there (all new, 2 alpha/1 bourns, 2 with switches, and finally reverted to a standard 250k pot (completely reversed the mod). Still does it. I've freshened up all the solder joints on the board, but paying attention to cathode follower through the master vol. scratching my head over this one.

Smokebreak: I reversed the mod for the time being, but here is a quick drawing. The silver-mica looks weird in the drawing. LMK if you're not sure what it's doing.

Chees!

[Gainzilla]

Ok. A couple ideas I've read in other forums.

• - Check that all in the tone stack section are solidly connected to ground
  - Check the value of the volume pot. I've read that with a small value volume pot it can make the other controls function a full boost or extreme high pass filter

I know the vol is supposed to be 1M, but I've never actually verified this. Seems unlikely though, as it worked as expected before.

One thing that occurs to me is that there is a solid state effects loop in between the treble pot and volume pot. It's all on a separate PCB. In order to pull the main board I need to pull that smaller board as well. I wonder if a loose ground or failing connection could cause the tone stack to act weird. I'll check these when I get home tonight, but any thoughts on this or the above points?

FWIW, I had tried to build a solid state loop in another amp and something similar happened. Converted the loop to a tube buffered loop and the problem went away. Coincidence?

Cheers!

[Smokebreak]

Ok i think I see your intention, as far as the switch goes. I'll assume one side of the old treble cap goes to slope/CF, and the other side goes to top of volume, correct?
So you're switching in a ceramic on one function, and a mica on the other, in parallel with the original ceramic treble cap, to hear a difference between a small mica or ceramic boost? There's definitely a very subtle difference, but I would think that you're not getting the full effect, if the "old" ceramic is always "in circuit".
Also, if you've kept the 470p as "old treble cap", then you've also lost any "stock" treble cap setting, (unless you've managed to find an on-0ff-on pow/switch ). Not sure if that is important to you or not. If you're looking for an honest to goodness mid boost , try a 1n series 47K from cathode to wiper of T pot, on a switch. Now there's a nice bump !

[Gainzilla]

Hi Smokebreak. Not exactly (although switching between two values would be a more traditional mod). In my example, the switch literally just switches between two composition types of the same value cap. I had already pulled the 470p treble cap in favor of a 220p in trying to tame the highs of this amp a bit (cranky el84s). In the drawing the "old cap" has been removed and is only shown for context of where in the circuit the switch is connecting (you nailed it).

I initially just wanted to audition the differences between ceramic and silver mica but liked the option and found it changed the "character" significantly. In one position it's much smoother, in the other it's much more aggressive. I tended to like the aggressive character of the ceramic for rhythm but liked the smoothness of the silver mica for lead.

BTW, I found this thread (https://tubeamparchive.com/viewtopic.php?t=26075) where someone else had the same issue. In his case it was caused by the volume pot, but in my amp there is also the effects loop to think about. I feel like the answer to the riddle lies somewhere between the treble pot and the PI. Stay tuned...

[Smokebreak]

Ah I see, the old cap is a ghost! Thus the dotted lines. So basically, all you've done in the big picture(besides the ceramic vs mica discussion) is reduced the treble cap from 470p to 220p, correct? So technically, if everything is wired up correctly, and you put 470ps on the switch, the amp should sound the way it used to, before any mods, correct? (again, forget the ceramic vs. mica for a second). Or even if you just switched out the 470p for 220p on the board, before the switch mods, technically the amp should sound exactly the same with the 220s on the switch, and nothing on the board.(again, at least in the position with the same composition cap, tolerances and brand aside).

If it were me, I would listen to the amp with no switching mods. Either 220p or 470p. Then do the mod that you drew, with the same cap in one of the positions. If it sounds different, then something is wired wrong. If it doesn't, I would take a closer look at what reduction to a 220p cap actually does when you crank the treble. You end up losing a lot of mid/highs, compared to a 470p, which(depending on how other things are setup), can make the amp sound thin, as there's no upper mids to balance out the sound. Maybe you just don't like the 220p with the treble cranked?

[Gainzilla]

Thanks for the guidance, I'll definitely look at whether or not I really like the 220p. So, I reversed the switching mod at the first sign of weirdness. I'd swapped the 470p for the 220p months ago and I'm pretty sure I'd have noticed this. But who knows, maybe not!

looking at these values in TSC but changing the vol to 25k (as per Larry's advice in the thread I linked to) I feel like the treble behaves exactly like what I'm hearing. That's why I wondered about a loose connection or cold solder joint in the fx loop board. Thoughts?

I'll have a look at your suggestions tonight when I can poke around in there.

Thanks again!

[Smokebreak]

Right on. Lots of variables here. I'm sure you'll find it, but you don't have a 25K gain pot there, so yea, take some chopsticks to it and jab around. Hopefully that can narrow down a bad connection, if that's the case.