Scratchy trem intensity pot: How best to proceed?

[Mango Moon]

Hi Folks!

I'm a repeat (amateur) newbee here with a brand new combo amp that works and sounds beautifully in virtually every respect, save one. Its got a particularly scratchy 'Tremolo Intensity' pot.

I've tried cleaning and lubricating the pot itself with Deoxit MCL followed by Deoxit Gold without success, and I hate like hell to return the entire amp for this seemingly small defect, but I don't know what I'd be up against in fixing it? Can anyone help advise me on this? Is it likely to be fixed by replacing the pot itself? Is it possible that the lone Orange Dot capacitor (soldered to the pot) is bad? Or, is something more complex likely to be involved? I have no clue!

From researching the matter on-line, it would appear that scratchy trem pots are rather common, and I'm perfectly capable in terms of good soldering technique, but I'd be reluctant to proceed on it without knowing what's likely to be wrong with it. Anybody?

Thank you.

[Enzo]

What brand is the amp? If it is a brand like Fender or Peavey with tons of service centers around the country, ANY service center can handle the warranty repair even if the purchase was made online. SO in those cases you wouldn;t have to return the amp somewhere across the world. On the other hand if it is a Musicians Friend Rogue or something, you'd have to send it packing. A defect is a defect, that is what warranties are for.


As to the problem itself, check for any DC voltage across the pot, or across the wiper and pot. DC voltage makes pots act scratchy. If DC is there - and it doesn;t take much - cleaning and replacing won;t help. If it is not there, and cleaning was no help, it might just be a bad pot - rough spots in the resist element maybe.

One remote thought, you cleaned the wiper part, but dab some Deoxit around the shaft to make better contact between shaft and bushing. That and a good connection between pot body and chassis.

Some place to start anyway.

[Mango Moon]

One remote thought, you cleaned the wiper part, but dab some Deoxit around the shaft to make better contact between shaft and bushing. That and a good connection between pot body and chassis.

Some place to start anyway.

Thanks Enzo. Nope, no warranty, no service centers, but I've got a decent multimeter here and I can certainly check for DC "across" the pot (as you've said). Problem is, I'm not quite sure how to do that (i.e., multiple wires and terminals vs. ground, etc.)? In other words, how do I determine which two points of contact to check?

And I hadn't thought much about the shaft/bushing area, so I'll give that a try too. Might as well be certain before I pull the trigger.

Thanks again.

[Enzo]

In this case it doesn't matter, there are three legs to the pot, and there ought not be any DC between any of them.

But in my parlance, across means end to end - the two end lugs. Like crossing a bridge over a river. And "across the wiper and pot" is a clumsy way of saying to check that the wiper has no DC with respect to either end lug, wiper being the center pin.

And if it is one of those pc mount square plastic pots with 4 or 5 legs instead of 3, then we have to look at a data sheet for the control, I sure don't ever recall which leg is which on those myself.

Would you identify the amp? It is possible there is some relatively unusual circuit that might suggest some other approach.

[Mango Moon]

. . . there are three legs to the pot, and there ought not be any DC between any of them.

Would you identify the amp? It is possible there is some relatively unusual circuit that might suggest some other approach.

Whoops! I apologize . . .

The amp in question is a 40w 3x10 Gretsch G6164 Variety, made for Fender/Gretsch by Mark Baier and crew at Victoria. Hand-wired, no printed circuit boards, all-in-all a pretty decent amp. Quite desirable actually. But also quite spendy, so I need to be confident in it. NOTE: I can provide a digital photo of the pot/cap assembly, and I also have (and can attach) a schematic if necessary.

Unfortunately, I won't have time now to mess with it until tomorrow, but if I've read you right, there should be no measurable DC between the two outer-most of the three total terminals, and there are indeed THREE, with a single (large) orange capacitor spanning those two outer-most terminals.

Problem is, if testing for DC across those two outer-most terminals as you appear to be suggesting, and the capacitor is also soldered to those same two terminals (with the left-hand terminal soldered to ground), how will such testing distinguish between a bad cap and a bad pot? Its testing both simultaneously, isn't it?

Many thanx.

[Steve Conner]

http://www.gretschguitars.com/amps/p...ic_Variety.pdf

Looks like a very nice amp. Interesting tremolo circuit.

One thing to ponder is:
DC always makes pots scratch.
The trem oscillator is a very low frequency.
Maybe it's low enough that it counts as DC from a pot-scratching perspective.

If this was built by Victoria, maybe you could ask them if the pot normally scratches.

If the answer is yes, maybe you could modify the circuit so that the .05 cap goes from the grounded lug of the intensity pot to the wiper, rather than going between the two outer lugs.

However try Enzo's suggestion, check for DC on the pot. The two ungrounded lugs should not show any DC with respect to the grounded one. But if you test with the tremolo on, your meter will show a fluctuating DC reading.

So there's another thought: Does the pot still scratch when the tremolo is disabled by the footswitch?

[Enzo]

Good point, does it still scratch with trem off, or does speeding up the trem diminish the scratch?

If my schematic is like yours, there is a cap across the pot and a 2.2Meg resistor in series with it from the trem oscilator.


Testing for DC voltage is done to determine if there is any DC voltage present. If there is, that explains the noisy operation, and directs us to eliminate the stray voltage. Such voltage would be coming from elsewhere in the amp. But such voltage reading does not identify the source of said voltage. A cap in parallel with the pot will not create or remove DC voltage. If there is voltage across the pot, it will be across the cap as well. SO if the cap leads are more accessible than the pot, measure for DC on the cap.

[Mango Moon]

Good morning guys and thanks for the added feedback here.

Enzo: It sounds like I've interpreted you correctly regarding the way in which to conduct the pot/cap testing, and yes, the stems from the cap itself (which spans the two outer-most terminals) can be easily checked for DC. I'll proceed with that testing and follow-up with the results here shortly, and before doing so, I'll also try to determine an answer to your other question re: does the pot act scratchy even when the tremolo is turned off. I've never tried the pot with the tremolo 'off', so I'm not quite sure on that one.

Steve: Yes, it is indeed a very nice amp, at least in terms of sound quality. I'm not ultra impressed with its overall build quality (seems rather run-of-the-mill in that regard), and frankly its a bit of a one trick pony in terms of sound sculpting, but what it does, it does very well. Nice cleans with loads of headroom and a very open, almost dimensional ambience - not unlike my vintage Magnatones (280/480), but "larger" sounding. Basically, I just "love" the amp, and to be honest, I rarely modify the trem intensity anyway, but given the high cost of these things, its still a bit troubling to me. Also, per your other question, I've inquired (via e-mail) of Victoria amps regarding the pot's scratchiness and what they might propose (if anything) in terms of a solution, but it was late on Friday, and I haven't heard back yet.

Anyway, I should have some answers to these questions here in the next 60-90 minutes, so please watch for a follow-up.

Thanks for your assistance guys - you've both been great!

[Mango Moon]

O.K. guys, I've gone ahead and conducted the above referenced testing here and I've also heard back from Mark Baier at Victoria, just moments ago, so here are the results of that bench testing effort and the response from Mark:

TEST RESULTS - USING MULTIMETER & TREMOLO/REVERB FOOTSWITCH:

1) The scratchiness of the Tremolo Intensity pot is consistent in both its occurrence and magnitude throughout the total travel of the Intensity pot, whenever the Tremolo is activated ('On'), but it is only audible when the Intensity pot is actually being moved (i.e., there is no scratchiness at any volume with the tremolo activated, when the knob/pot is stationary). NOTE: See response on this specific topic from Mark Baier at Victoria posted below;

2) There is no scratchiness of any kind from the Tremolo Intensity knob/pot when the Tremolo is deactivated ('Off');

3) There is a loud and distinct 'popping' sound (which hiccups or echos back through the reverb) whenever the tremolo is activated via footswitch, but no such 'pop' when being deactivated, and this is particularly troubling;

4) The cyclic tremolo oscillations (when trem is activated) are considerably more audible or noticeable (noisy) at idle when compared to other (similarly designed) vibrato/tremolo amps that I own. However, those other amps are also less powerful (i.e., <40w and EL84, 6V6 or 6973-based rather than 6L6-based like this one).

5) With this amp turned 'On' and tremolo activated, the multimeter readings (although unstable due to trem oscillations) appear to show DC leakage (??) across the two outer-most terminals (i.e., across the two capacitor stems, each soldered to the two outside terminals on the pot) of 3.5-5.0 VDC. NOTE: I took several readings at the same multimeter setting (or scale) and I took several more at different multimeter settings (different scales), with the same result;

And finally, here is the most recent communication I've received from Mark Baier at Victoria amps regarding this 'perceived' (pot scratchiness) problem:

QUESTION THAT WAS ADDRESSED TO VICTORIA AMPS:

"While I've got you, despite being reportedly "brand new", this particular Variety (amp) has an ultra scratchy 'trem intensity' pot, and Deoxit MCL/Gold (cleaner/lube) had no effect on it whatsoever. Any suggestions? I love this amp, but is it conceivable that its already got a bad pot or related component? And if so, what's likely to be involved?

I'd appreciate your feedback."

Thanx

REPLY FROM VICTORIA AMPS:

"The scratchyness is endemic to the harmonic filter vibrato circuit..Part of the "beast"..Taming the scratchy also lessens the trem effect..it should only be present when rotating the pot tho..no scrazthy (sp.) should ever be present on the signal path when in use...."

In closing, I'm most grateful to Mark Baier for responding as he did over the weekend, and his statements regarding the expected performance would appear to be exactly what is occurring here (i.e., the scratchiness only occurs when the pot is actually being turned). However, not being an experienced amp tech myself, I'm not quite sure how to interpret the apparent 3-5 VDC leakage across the pot/cap, nor am I particularly comfortable with the loud 'pop' that occurs when the tremolo is being activated.

Any additional thoughts?

Thanks All!

[Enzo]

OK, so it sounds more like Steve's scenario than the possible leakage I was looking for.

With the oscillation present, you will be unable to measure DC offset, your meter will average out the moving waveform voltage.

DC on a pot makes it noisy only when turned, not continuously noisy. As Steve surmised, I'd have to say then that the very low frequency of the trem oscillator is close enough to DC as far as the pot is concerned. Turning off the trem ceases the noise, and looking at how the turn-off is done, any of the possible leakage I was thinking aqbout would have remained, trem on or off. SO my theory is no good.

Once in a while you will find an amp with a perfectly quiet volume control, but if you turn the control while playing loud, there will be a certain amount of noise from it, for similar reasons.

[Mango Moon]

Thanks Enzo! I greatly appreciate your continued insights and opinion on this, and I'll certainly look forward to anything further that Steve Conner may care to add (assuming he visits the thread again), but it sounds like the consensus is . . . this Gretsch amp is (by design) perfectly healthy (i.e., that the scratchiness is both normal and explainable, given the amp's somewhat unique design)? Is that a correct interpretation?

I've gone ahead and contacted Mark Baier again to inquire about the 'popping' sound (at volume) when the tremolo is activated, because this particular quirk is indeed troubling from a purely functional or practical (live performance) standpoint, and we'll see what he has to say. Perhaps I just have a bad footswitch?

Anyway, thanks again for making the journey with me on this. I'm certainly thankful for having ready access to experienced folks like you, Steve and Mark. I wouldn't be able to solve it any other way.

Cheers.

[captntasty]

Perhaps I just have a bad footswitch

it's possible... in stompbox building a common practice on the input/footswitch is to use a pulldown resistor. When using a mechanical switch, switching on allows a sudden voltage spike... pop! The pulldown resistor prevents the sudden voltage from discharging all at once. The pulldown resistor is just a large value 1M, 2.2M, etc. installed from the the input jack(hot lug) to ground. Maybe ask Mark Baier if this is something that could be implemented into the footswitch jack...

[Mango Moon]

Thanks Cap'n!

I think its important to reiterate that I don't have this problem (popping sound) with several other tremolo/vibrato amps that I own including several Fenders, Magnatones and a Goodsell Super-17, and significantly, they each use different types (brands) of foot switches.

However, your explanation makes perfect sense. So I'll make a point of asking Mark when I can, and meanwhile, I'm also going to try a couple of other foot switches or actuator switches that I've got laying around here including a brand new one. I'll try to post the results when I can.

Many thanx.

[Steve Conner]

Hi all

So, it sounds like the scratchiness is just a feature of Mark's circuit and there's nothing wrong with it.

I can see how the pop might also be a "feature", since the footswitch node is referenced to the cathode rather than ground. The footswitch won't just ground out the oscillations, it'll also ground out that DC offset, causing a transient.

Any DC measurement you made across the pot with the trem oscillator running is meaningless, for reasons I explained. If you want to make a leakage measurement, stop the oscillator with the footswitch.

[Mango Moon]

Hi all

So, it sounds like the scratchiness is just a feature of Mark's circuit and there's nothing wrong with it. Yes, it appears to be so.

I can see how the pop might also be a "feature", since the footswitch node is referenced to the cathode rather than ground. The footswitch won't just ground out the oscillations, it'll also ground out that DC offset, causing a transient. I'll be trying-out some other switches here soon including a brand new one, just to make certain that its not the switch, but Mark Baier has surmised elsewhere (via direct e-mail communications) that this 'feature' and others may have been . . . "why Leo Fender abandoned this topology in '63."

Any DC measurement you made across the pot with the trem oscillator running is meaningless, for reasons I explained. If you want to make a leakage measurement, stop the oscillator with the footswitch.I did not know this, and I'll certainly repeat the exercise with oscillator 'Off' to insure a good reading, but I would imagine the reading(s), though more stable, will still be in the range of 3-5 VDC, yes? And this raises yet another question in my mind: I've read elsewhere that typically speaking, "leakage" of that magnitude in any other (normal) tremolo circuit would be considered rather substantial. Am I wrong on that score?

Many thanks for your continued contributions.