Blackface Champ Issue

[Jeffj]

I have a blackface champ that is having an issue. I noticed when I am playing it at say 4 on the volume, after awhile it suddenly gets ALOT louder....I never noticed it being overly quiet on 4 (then sound normal when the volume increase). So, I started messing with the volume pot (moving it back & forth, as in volume up & down) & noticed that sometimes with the volume control all the way down there is still volume coming through, sometimes pretty loud. I am leaning towards a bad volume pot. Does this sound like it could be the issue? The amp was completely serviced before I bought it, so I assume it is in good shape cap wise, etc. It doesn't seem weak or noisy at all.

[PRNDL]

It could be a bad preamp tube acting funny.

[Jeffj]

I'll give that a try. I just thought it was odd that when I would work the volume knob back & forth real fast, sometimes with the volume control all the way off I would still have sound. Is that not a sign of a bad pot? Does that happen when you have a tube acting up? thanks

[Jack DeVille]

i'd check/reflow the solder joints at and from the volume pot as well....

[Jeffj]

i'd check/reflow the solder joints at and from the volume pot as well....


Will do. I plan to pull the chassis tonight, so I will check the connections on the volume pot. If it needs to be replaced, what pot do I need?

[donnyjaguar]

If the volume pot gets its ground from the chassis, it could just be a loose control.

[phsyconoodler]

It could be just a dirty pot.Try some cleaner first.Then check the body of the pot to see if it's loose.Sometimes the crimps on the body come loose and they can be re-tightened.

[Jeffj]

well....I have tried swapping all tubes, tapped around inside the amp with a wooden stick & still cannot find anything. It seems as though the problem is getting worse. It seems that now it really doesn't matter where the volume pot is relative to amp volume. I can turn it on 10 & sometimes it is not as loud as when it is on 3. I can turn the volume to 0 & it sounds like it is on 7. It seems to start doing this after it has been powered on for a few minutes & is warmed up. I am thinking it must be the volume pot? Can someone tell me the pot that I need to purchase & where to get it? Thanks

[phsyconoodler]

It's a 1 meg audio taper pot.You can get them at many amp supply shops like Mojotone,Webervst,Tubes and More and a few others.

www.mojotone.com
www.tubesandmore.com
www.webervst.com

[WaltC]

it *might* be a leaky coupling cap as well.... careful inside that little sucker, small as it is it still has over 300DC volts on the filter cap can...

[Jeffj]

Is it different than a 1 meg audio taper pot for a guitar? Thanks


It's a 1 meg audio taper pot.You can get them at many amp supply shops like Mojotone,Webervst,Tubes and More and a few others.

www.mojotone.com (http://www.mojotone.com)
www.tubesandmore.com (http://www.tubesandmore.com)
www.webervst.com (http://www.webervst.com)

[phsyconoodler]

Yes,it's for amps.A guitar pot has a split shaft and uses push-on knobs.Yours has a solid shaft and the knob has a set screw.Usually guitar pots are 1/4 watt and amp pots are 1/2 watt.
In a pinch the guitar pot will work.

[Trout]

Got to thinking about this post,

Those Blackface amps have been a topic that seems to be appearing more and more as of late.
Many of the issues boilded down to the a drifted/old under rated cathode resistor.
Several also blew the bypass cap as well as the resistor. Sudden volume changes could occur when that resistor starts to overheat and run to far off spec. They only used a 1W 470 ohm resistor there and really with todays higher mains voltages it should actually be a bit higher
(560-680 ohm) and for safety/reliability a 5W ceramic is commonly used.

It is possible that a pot is bad, but my money is on the need for replacement of drifted parts.

[mark norwine]

consider this: the volume pot isn't even "in the circuit" if the ground [left tab] isn't sound.

Since the tab is usually just folded back & soldered to the pot body, your ground is only as good as the pot's mechanical connection to the chassis. Bad connection = bad ground = vol boost.

The problem could be a similar situation with the mid resistor on the bass pot.

Tighten the pots. Better still, remove the pots & clean the connections, then reinstall. Best of all, get rid of mechanical connections altogether & solder in good ground connections.

[ohmslaw]

It could be just a dirty pot.Try some cleaner first.Then check the body of the pot to see if it's loose.Sometimes the crimps on the body come loose and they can be re-tightened.

What he said. Before you start soldering and replacing parts try cleaning the thing.

[Jeffj]

Just wanted to add a another "symptom" that I noticed while playing it today. When I power the amp off, the sound gradually gets quieter & is very "fizzy" sounding. Obviously the sound will fade, but none of my other amps "fizz" during powering off. I will double check\clean all the connections this evening, just wanted to mention that in case it indicated anything else. Thanks for all the help so far.

[Jeffj]

ok...problem solved. Pulled the chassis again & re-soldered the leads on the volume pot. To the eye they looked fine & did not feel loose at all. I went ahead & took the wires off & re-soldered them anyway. I have had it running for a almost a couple hours now & all seems to be well. No more volume "bleed" & the amp sounds great. Thanks again for all the help.

[DH82C]

Good to hear.. Gerald Weber has a vid with a section on cleaning the brass ground bus.. something that should not be overlooked.