Strange Artifacts....

[zozoe]

G'evenin' all~ What you see here is my new old 1973 Davis Model 106 PA amp, that under the hood sits a pair of EL84's, a 6EU7 & a 6CA4 recto,,, and this amp is no firecracker, but more like an friggin' M-80!!,,, 1 Mic. Volume, that middle 'Music' Volume for the audio RCA jacks in the back, & a Tone control, that could use a value change of sorts to let the amp breath a bit better, & brighten things up a tad... BUT when I had the chance to take it for a full out test-ride, besides it's awesome o/d, what at 1st sounded to be just a dirty, noisy volume pot on the guitar, proved to be happening on all 4 guitars I tried (all tone controls were unaffected), and was clearly evident along the almost full sweep of the pot,,, Regardless of the amp's volume, from 0 to 1, and from 9 to 10 on all the guitars, were dead silent,, but along the whole remaining sweep, from 1 to 9, it was real bad,,, again, only on every guitar's Vol pots...
Lastly, when I'm plugged into the main MIC input, that middle AUX vol. control seems to add a bit of (pleasing) hair when I turn it up from 8 to 10,,, Only it's own, that AUX volume does nothing... I LOVE this amp, but it's got some strange stuff going only... Thanks in advance for your attention & expertise...
kenny ॐ

[Ewizard]

You have DC on the input grid of the tubes ( at least V1 anyway ). This is what causes the scratchy pots on your guitar. The DC riding on the input grid of the tube causes it to shift bias, causing several operational and audible problems. DC on the grids of tubes is caused by leaky capacitors, or turret/eyelet boards becoming conductive. Given the age of the amp, it is most likely leaky capacitors. I would bet it is point to point too, so that would weed out conductive boards. Sometimes, but not always and not very often, the tube itself causes DC to get on the grid. This is rather rare though. You can try another tube in the V1 position and see if the scratchy guitar volume issue goes away, but I doubt it will.

Unfortunately, in order to get this thing back to 100% operational order, you will need to do at the very least a cap job. While you're at it, you may as well check every resistor for its value against spec and replace any that are not in spec. You may as well buy new tubes and bias the power tubes as needed. It will sound much better when you do all that.

[zozoe]

Thnx, Ewiz!! Fantastic sounding beast of an amp, btw.

[Ewizard]

I have no doubt about that! Now imagine what it would be like if it were operating at 100% tip-top shape! It is always hard to let go of an amp to allow repairs that better its operation. A good tech can make it sound the same as it does now, while still achieving safe and prudent operation. It may sound great for now, but without proper service, it will eventually **** the bed; the damages done later when it does, are what will make you wish you just serviced it earlier.

[zozoe]

Thanks again,, Ewizard!! And just in today & off to my tech with the Model 106,,, the Model 100,, 6EU7,
Single Ended 7591, 6CA4,,, Controls are MIC, MUSIC, & TONE.... Identical look, but no top vent, & nice original chicken head knob!!!! This amp I will NOT be turning on until it's checked out!! Be well~