Suddendly Scratchy Bass Pot

[rp]

My 5C8 clone developed a scratchy bass pot. BTW It has sat unused since summer, it's the cold and humid season now and I've been powering up all my amps up for a few hours to keeps the e-caps healthy and hopefully drive any moisture out.

I tried some tube pulling and rolling hoping for a dirty contact, it happens from V3 onward (6J5 on the schematic). Should be the coupler after the 6J5 plate but I find it really hard to believe an orange drop failed. Modern coupling caps just don't fail, especially these indestructible orange epoxy dipped things.

Could it be the mica cap? And if there's DC leaking after V3 why isn't the Treble pot affected? Could it be a carbon comp resistor? Guess first thing to do is re-flow the V3 stage solder joints but thought I'd ask before I start replacing parts.

FYI build in the pict differs from the schematic a bit, has an added filter stage, V2 is cathode biased, elevated heaters, and notably V3 has been rewired for a paralled 6CS7 rather than 6J5.

[martin manning]

I have had pots become scratchy with disuse. Usually rotating them up and down a number of times will clear it up.

[rp]

Hi Martin, I always spin the pots, along with sticking a plug in and out of all the jacks, clicking selectors and flicking switches a couple times on idled things. I'm such a dork I sit down and make/break and clean all my hifi connections once a year. (Try it sometimes!)

So I sat there and spun the pot a few dozen times more. It's a 2MA NOS, off ebay and prolly as old as me. BillyZ had a post here strongly advising against going dexoit crazy (and ruining one of my favorite pastimes) so I've sworn off, but I just hit the pot with Caig, plus the bushing carefully w/ WD40 for good measure.

It's a bit better especially the bushing, but still a little scratchy. Hard to tell with this hoary circuit and microphonic octals. Anything you touch comes through the speaker. Sounds like DC to me though - has a circuit caused rather than mechanical vibe, but it's possible the pot is crap inside.

Do I just go after caps or can it be a resister too?

This nonsense is the reason to not build PTP, be 10 minutes w/ a board.

[Malcolm Irving]

Did you try swapping the 6SC7 PI tube? If it has become 'gassy' it could put out some DC through its grid.

[Firestorm]

There's always a little DC on preamp grids. Tubes vary. If they sat unused, it might increase. Plug it in and let it sit for a few hours.

[Malcolm Irving]

.... Plug it in and let it sit for a few hours.

Yes, the 'getter' in a tube, which is there to soak up any gas molecules, works best when the tube is hot.

[Leo_Gnardo]

I find it really hard to believe an orange drop failed. Modern coupling caps just don't fail, especially these indestructible orange epoxy dipped things.

Except the ones that do fail. I've had the devil of a time with a 20 year old Mesa MkIV 0.1 uF 400V OD's, 2 failed & more on the suspect list. (xx)it happens, things break.

Sometimes it's the pot shaft, somehow makes a click noise, sounds like static. Mostly I see this in 70's Ampegs like V-4's, etc. A tiny dab of conductive goop - same graphite & Lubriplate mix I use in nut slots - pressed into the gap between shaft & bushing cures this. You can "help it in" with a single small drop of oil. Not too much now, you don't want it to find its way onto the pot track/wiper.

[Stevem]

Just give the control a good blast with the Euro equal to WD40!

[rp]

Did you try swapping the 6SC7 PI tube? If it has become 'gassy' it could put out some DC through its grid.

There's always a little DC on preamp grids. Tubes vary. If they sat unused, it might increase. Plug it in and let it sit for a few hours.

Good points, might be something here, w/ only the PI and output tubes I still got the scratchy pot. Next, I rolled the glass 6SC7s I had in it on Malcom’s advice and discovered that they all suck, they sound fine but impossibly microphonic, rolling the PI to V2 had the amp ever so slowly swelling into feedback with the volume way down and the guitar totally off. Tapping the tubes doesn't just ring way loud it actually causes the amp the start feeding back. Those glass tubes were econo-pulls off ebay so they might be suspect.

I subbed in some metal tubes I've had for years, real NOS, nice and crisp, and while still microphonic are much better. BTW I couldn’t stop playing the amp once they went in, they sound great, bright and full of vim. Don’t ignore the metal tubes!

The pot stayed scratchy with all rolling, I was also getting ever louder waterfall rush and crinkling newspaper sounds the more it stayed on (even with V1 & V2 pulled) so I replaced the plate resistor on V3, an old inelegant generic cc with a proud Allen Bradley 1W and cleared that up. I re-flowed the solder joints on V3 and the pot. The bass pot is no longer scratchy. Hard to tell what did it but good riddance.

While I had it open I swapped the dropping resistor on the extra filter cap I had added to 10K (from 4.7K) to better decouple the the last stage, something I been meaning to try for a while, and low and behold it nixed a fair bit of hum.

Good, fast, productive intervention. I've learned a lot reading TAG!

[rp]

Euro equal to WD40!

Hahaha I think that stuff is in every corner of the earth. Most bodegas in NYC even carried it, need WD40 at 3AM? It’s in every hardware store here. I bet the WD40 family are the secret rulers of the planet, they’re just smart enough to stay low key.