Strange symptom, noise, master volume/preamp volume pots interacting?

[dwp]

I have a house build, Fender blackface, Twin Reverb preamp, stage one and two. (is the Dumble 1997 revision) The Master Volume comes off the second stage of V-1. When the master volume pot is 100% up/on, (must be dimed, 100%) and the preamp volume is turned down 100%, at the moment the pot bottoms out/off, I get loud rattle/hum/buzz. crack the pot just a hair up, the noise stops. It doesn't matter which one you do first to get the same noise. If the master is backed off from full on slightly, with the preamp volume 100% off, I get the same noise as soon as the master hits 100%. either pot, backed off ever so slightly, no noise. The instant both pots are (MV)100% on/(PV) 100% off, I get the noise. only takes a slight bump of either pot to make the noise stop. Amp sounds great in normal play, have been putting it through the paces, no issues, no background noise, except when/as described above.
     I figured I had a bad pot, I replaced the preamp pot, (both are 1MA's) last night, this build is fresh BTW, problem solved, I check it many times. Now today after a couple of hours of playing with settings, it started again. Not as extreme of a noise, but basically the same rumble, (very loud last night) as soon as I turned the replaced preamp volume pot all the way down with MV all the way up.
Before I just change the master pot out now, am I missing something here? Anyone else have similar issue? Can't see how the circuit could be causing this. And it did stop for a while with the new pot.
Thanks in advance, dwp

please forgive the longhandle .sch, only review V-1 and pots. there are shortcomings on the drawing that havwe been fixed.

[Geezer]

this build is fresh BTW

Just to eliminate a common problem as the cause, disconnect the NFB......any better? If so, swap the OT primary leads.

G

[bobmegantz]

Could be DC on the pots - check the coupling caps for leakage.

[dwp]

NFB? c'mon.....
Actually I think I have defective pots, mojo in-house brand, but I'm going to go in and poke around, will check for DC, etc. Would the wiper coming off the winding at the end, inside the pot, and somehow touching ground, or just going open, would this cause the noise? Will post my findings, hoping to generate alittle more interest in this post.
Thanks again, DWP
Except geezer, NFB? c'mon.......(lol)

[jeff]

I got a suggestion, but after the way you treated geezer, whose only trying to help, I don't think I'll share it with you.

I mean c'mon.....

[sluckey]

Can you post a Biggg pic of that scan? I can't read it.

[dwp]

hey lighten up, I did not mean to offend the geezer, did I offend you geezer? I apologize if I did. I was trying to make a funny.
 But NFB? C'mon.......(lol..lol..lol)
I'm a rookie, connect the dots guy, plagerizing fool, but I never get a slight intermittent noise with OT leads reversed, I get full blown growl that has to be corrected instantly, NFB builds of course.
Steve, I will learn the .sch soon, I promise. I hate posting my drawings for this reason, the non-professional in me is apparent. I converted the 1997 revision of the Dumble posted on Dougs page, but then went back to Twin Reverb values with V-1, some switching changes, etc. the drawing posted is not complete or accurate
I DISCOVERED THE CULPRIT. I cannot explain it though.
A PREAMP TUBE.....HOW CAN THIS BE?
I liked my first build attempt of this so much, I built a second. This one being the second, the one with the pot issue, I decided to plug #001 in, to see if I had a similar symptom I had overlooked. I did not. I then swapped the preamp tubes exactly, the noise issue moved from #002 to #001. V-1. Is a new production TAD 7025, sounds good in both builds, but for some reason, master 100%, (pin 7) and preamp volume off 100%, (pin2 grounded with pot closed, yes) I get a nasty rumble. Tube issue, sound impossible? input appreciated as always.

[Geezer]

Glad you got it figured out......

geezer, NFB? c'mon.......(lol)

Well, I've seen NFB (or PFB) do some weird stuff, & since you said it's a new build, it needs to be eliminated as a potential cause.

Most folks think PFB will cause the amp to squeel loudly, & if that didn't happen, then all must be OK. That's not always the case. If you haven't tested it, how can you be SURE you have NFB, & not PFB?

The easiest way to test if you have the OT primaries correct is:
*Disconnect the NFB resistor & connect a clip-lead to the loose end.
*Turn the amp "on" & turn it up to the point that you can hear a good "hiss" (or if your amp is really quiet, plug a cord into the input jack to create some noise).
*now re-connect the NFB resistor (via the clip-lead)....does the amp get quieter, or louder?

If quieter, then you have it correct....NFB is present.
However, if the amp actually gets louder, then the OT leads are reversed & you have PFB.

It's a 50/50 shot, and and I'm convinced that alot of folks don't get it right & that causes the amp to never quite sound right & they chase their tale trying to find it......it's a simple thing to test.

G

[dwp]

thanks geezer, I didn't know all that. I did get the loud roar instantly on this build, and did reverse the OT leads to make it stop. I will check this out as well. any thoughts on how a preamp tube can do the thing it did? I moved that same tube to V-2, which has preset on one signal lead, no issues, yet.
Thanks again, dwp

[Geezer]

I will check this out as well.

No need to, as you already confirmed it's correct (by the loud squeal)

any thoughts on how a preamp tube can do the thing it did? I moved that same tube to V-2, which has preset on one signal lead, no issues, yet.

I guess that tube just has "issues" that preclude it from working well in that position. Simply use it where it DOES work & keep it out of the spot that disagrees with it.

[jeff]

What I was going to suggest, but didn't want the "c'mon..... that's just silly", was to check how the volume and master are grounded. I can't tell from the schematic(vs layout) but are the grounds daisy chained and grounded from one spot(ground of the volume goes to the ground of the master then to ground) or does each go to ground? Try grounding the master and volume to different spots, or a wire from each to the same spot. This way your MV and vol aren't sharing a path to ground. I had the same problem and this did help.

P.S. Good lookin' amp.

[dwp]

again, I apologize for the drawing. I draw it for my benefit to get the components right and/or to make sure I leave the correct amount of slots on the rails. It does not reflect how I finish wiring the thing. The pots are all grounded hardwire down the backs, connected at far left end of chassis, along with preamp tube cathodes, input jacks.
Never experinced this with a preamp tube before, was considering wiring issues, but it's the same scheme I always use, with this method I have amp noise at nill, am attributing it to the tube.
Thanks again, dwp