Fender Blues Deville Treble control problem

[sonicmojo]

I've got an older Blues Deville on my bench that has a few problems. I fixed the main power tube problem and the amp sounds pretty good, but when I turn down the treble control, the amp gets a lot louder. When I turn it up, more treble but way thinner and much less volume. I really hate pulling out the boards on these to change out parts but what do you think would cause that kind of operation? I am thinking maybe the pot itself is okay and it is something else? If I can try to swap a couple of things quickly, it might be worth the effort. Other tone controls seems to work okay. What do you think I should target? I can always try the pot itself but thought I'd ask around a bit first.

blues_deville.gif

[xtian]

It looks like a pain but really only takes five minutes to get the board out and flipped. Look for cold solder joints on the pots, and resolder the input jacks too.

[sonicmojo]

It looks like a pain but really only takes five minutes to get the board out and flipped. Look for cold solder joints on the pots, and resolder the input jacks too.

I hear you. I hate doing in though! Not very possible to test with it pulled out and this one has had the original jacks replaced with jumpered switchcrafts so they will be just flopping around like fish and probably break.....

I will probably...

[sonicmojo]

The treble pot seems to measure okay and I replaced the 250pf treble cap but no help there. Hit all the pot solder joints for good measure. Calling it a night. Will try again with the bass and mid caps in the morning and check those pot measurements also.

[Stevem]

Well are you seing any DC voltage on the groundward leg of the Treble pot as if c6 or c7 was leaky?
If not then a circuit trace on one side of the pot may be bad.

[sonicmojo]

Well are you seing any DC voltage on the groundward leg of the Treble pot as if c6 or c7 was leaky?
If not then a circuit trace on one side of the pot may be bad.

No DC voltage that I can measure on C6 but I can't measure C7 as it is pinned tight on the board. I'm not turning this thing on when it is hanging out.

I don't see any problem traces and I metered from the treble pot lugs to other places. Will keep looking. Maybe I should just replace the pot before I guess too much here.

[sonicmojo]

I pulled the treble pot and other caps. Everything checks out. I think there may be an excess of bottom end coming in to the tone stack and is giving a perceived loss of volume. This is the first time I've played around with these older Devilles. Or maybe the filter caps are failing?

I'm going to check the bypass and coupling caps at V1 just in case something is off there....

[Stevem]

Try it thru different speakers if you can.

[sonicmojo]

Try it thru different speakers if you can.

I just tried. No change. I've called it done on this amp as the customer does not want to replace all the electrolytics which would be my next step to see if that changes anything. Also, the power amp in jack does not work and the customer does not care about that either but I kind of wonder if something is happening downstream.....

It's not like the amp is broken and it can be dialed in to some really nice sounds. The only weirdness is the treble pot gives a pronounced bass response when turned down and a bass cut when turned up. If you keep the treble between about 3 and 8, everything is fine so that was my suggestion. I hate cutting things short when I feel it is not perfect but I'm trying to balance perfection with time spent. I spent way more time than I probably should have on this one but I fixed the main issue which was the amp was only running on one tube due to that screen grid resistor. Next! Thanks guys, I always like to bounce things here and any feedback is immensely appreciated. If it were my amp, I'd strip it and put in XTIAN's Dumble board!

[Stevem]

Power supply filters would have no issue on that tone stack problem, but it would interesting to model that stack and see what it takes to get a treble pot to respond like that!

[martin manning]

...it would interesting to model that stack and see what it takes to get a treble pot to respond like that!

It's a straight-up 5F6-A tone stack, so I wouldn't expect anything odd to happen... unless the pot is failing.

[Stevem]

Yup!
The more I think about it , the more it just plain sounds like a bad pot!