Can a dirty pot effect tone of a tube amp?

[212Mavguy]

One of the fun parts of having a modest amp stable is the chance to get under the hood for maintenance and adjustment. I had an amp that had languished in the basement unused for the better part of 18 months. It is a Siegmund Midnight Blues Breaker head glassed with Brit Kt66's a la the "Beano" EC/Mayall rig. Old stock in the pre's and rectifier, this JTM45 has a tube fx buffer/loop, bypassable master volume single channel amp with the Radiospares oversized OT. Baaaaaaaad amp. www.siegmundguitars.com I had been running it with some 40's RT 6F6G's, (coke bottle 6v6 type) but after my ears took lessons in bias adjustment from a trio of my Dumble clones I wanted to put the Emitron black glass brown base '66's back in for the thang that the amp was designed to do. The last time I thought the sound was a bit too dark and middy, and I wanted to see if a cooler bias setting would help open up the upper mids and top end to balance out with the HUGE bottom and lower mids. Another consideration was operation at high volumes without too much nasty distortion happening in the tones. The original amp was operated at very high, to dimed volume levels to get that recorded live tone. Was looking for a 1-2 MV sweet spot in bias and bias balance, this amp has pots for both.

 

Fired it up and man, a horrible noise started coming out of this thing, sounded exactly like a noisy tube, that splattery sci fi movie space station radio static. Not musical at any volumes. I knew the power tubes were fine, and I rolled out every preamp tube for subs and STILL the "bad tube" sound continued. I kept playing through it anyway, because I could dial out the fx loop where the trouble was. The design always allowed a bit of noise when turning one of the loop's pots, but this was horrendous. I had owned this amp for a few years, it was not new anymore. Then the "aha" moment...hearing a fluctuation in signal at certain settings on that pot, and quietest operation at maxed, so I deplugged, decabled, decabbed the chassis and flipped it over and gave it a pot job...lubeing the pots and cleaning them with a product called Jiffy Bath I got at the local old school electronic store in Salt Lake. The worst pot got turned back and forth 50 times and got shot three times with the stuff while doing it. All of the knobs got twisted back and forth a good 30 times and shot twice. Used a paper towel stuffed in the catch the stuff, wiped everything down getting the residue off the chassis, dusted the top and stuck it back in. Stuff evaporates quickly and you don't want to huff it.

 

Fired it up and WOWEEE!!! Everything peeeeeeeeeeerfect, and I'm thinking that not only was the noise gone, but the amp was a bit louder and definitely more open and "Airy" in it's already articulate transients and tonal response, dirty pots over time sneakily sucked tone out of this magnificent blues amp. one that sings and screams and sustains like a Dumble but has its own formidable voice. At small house to outside stage LOUD volumes (great master volume circuit) going from crystalline glass edged fat cleans to a full on throaty roaring howl that the Brit KT equipped Bluesbreaker amps have. This amp would pair well with a Dumble in a stereo rig, as well as a Harry Joyce Hiwatt clone would now.

[H]

Nice bit of detective work

[bolero]

ha, I always enjoy reading your experiments etc.. that's pretty interesting I wonder how many other old amps could be improved with a pot cleaning

 

never heard of those amps before, followed the link...this is pretty wild....300B output tubes?

 

http://www.siegmundguitars.com/soundking.html

 

 

and the blinking eye intensity thing, funky! must be similar to the old marantz tuner eye?

 

[smurph1]

Didn't understand much of that..(I'm from West Virginia..) but COOL!!!

[212Mavguy]

Chris builds nice stuff. The Midnight blues won Amp of the year for a major guitar rag a while back, can easily hear why.

 

300b is normally a hifi weenie tube, but occasionally sees duty in a guitar amp, Chris builds some tube amps for acoustic and jazzbox guitars, as well as this early Marshall 1961-ish circuit build. The magic in this amp is in the tube effects loop/buffer controls, with or without use of effects in the loop. It allows some amazing voices in guitar/cable/amp playing as well as with stomp boxes galore. One thing I've learned how to do is to balance the amount of drive between the preamp and the effects loop, with or without using effects. Some great tones, and this amp can do cranked amp tones at small house volumes as well as roar loud and proud. Sustain is like an opera singer.

[rockabilly69]

The clip on the Siegmund page of the ES335 really float my boat...

 

http://www.siegmundguitars.com/music/SK/SKclip9.mp3

[212Mavguy]

That was a nice clip!