Who wants to play... Guess the amp mod?

Wizard of Ozz

Well-known member
Our subject in question: 1993 Mesa Triple Rectifier

... but what is this:

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Someone added a 15K ohm resistor off the last 30uf/500V preamp filter cap... then connected it to a small mini-pot... then to ground? Why? I've never seen something like this off a 30uf/500V large cap? Thoughts?

I think another resistor was replaced/added right next to the 2 orange ceramic disc caps too (right next to C52).
 
Someone doing the unthinkable...making a Rev F a Rev G!!!!! AAaargh!!!!!

J/K. I have no clue. How does it sound compared to stock? More or different gain structure?
 
Racerxrated":dur9rrkj said:
Someone doing the unthinkable...making a Rev F a Rev G!!!!! AAaargh!!!!!

J/K. I have no clue. How does it sound compared to stock? More or different gain structure?


Not sure... yet ;) TBD. :yes:

Here's a stock 1993 Rev F Dual for comparison:

YwzX6MD.jpg
 
Not a clue... but were they trying to drop the voltage to V1A? Just a guess, and not a very educated one without knowing more about the B+ supply.
 
Yeah, that makes no sense. The resistor and mini pot are connected to the ground side of the filter cap.
If the ground leg of the filter cap is still connected to ground, then the resistor and pot do nothing.
If the ground side of the cap has been lifted, then maybe it's a crude attempt to alter the filtering at that node :dunno:
Are you sure the other end of the resistor / mini pot is connected to ground and nothing else? Hard to tell from the pics.

As for the changed resistor @ C52: If you have high res shots of a stock amp, zoom in and see what the color code is on the stock resistor vs your amp. I can't tell from the pic you posted. Also, I don't see C52 on the scheme you posted. If we had a scheme showing C52 and what's in the vicinity, it would help.
 
fusedbrain":1svhv87m said:
Yeah, that makes no sense. The resistor and mini pot are connected to the ground side of the filter cap.
If the ground leg of the filter cap is still connected to ground, then the resistor and pot do nothing.
If the ground side of the cap has been lifted, then maybe it's a crude attempt to alter the filtering at that node :dunno:
Are you sure the other end of the resistor / mini pot is connected to ground and nothing else? Hard to tell from the pics.

As for the changed resistor @ C52: If you have high res shots of a stock amp, zoom in and see what the color code is on the stock resistor vs your amp. I can't tell from the pic you posted. Also, I don't see C52 on the scheme you posted. If we had a scheme showing C52 and what's in the vicinity, it would help.


I’m guessing that the mini pot is going to ground as it’s going to a trace that runs under the board... I would think it runs to a common ground somewhere. You can see the open joint where the mini pot terminates in the stock pic. It doesn’t look like anything is protruding up from the bottom side of the pcb.

After further marination on the topic... what about some sort of hum trimmer? Signal shaper? Since the filter caps in the preamp are filtering the signal... the mini pot would have to be increasing, decreasing or shaping the signal. Many of the older techs are used to looking at a Marshall Plexi or Fender Black/Silver face on a scope... maybe he didn’t like the shape of the wave on his scope and added the pot to shape/smooth it?

The pot is too small to be a bias pot... definitely in the wrong section... it is not connected to any preamp tubes, and that would be the easiest/quickest way to add gain by swapping or adding a cap to a preamp tube socket leg...hmmm

The other extra added resistor near C52 is not present in the schematic or in the stock unit either. I guessing they are somehow connected in providing addition filtering/shaping... but why?

The amp was looked at by a very experienced tech... and I’ve heard clips of it and it sounds great. But I’m picky... and spotted this in the pics of the board fast. Just covering all my bases before buying. Lol.
:yes:
 
Wizard of Ozz":205l55tv said:
I guessing they are somehow connected in providing addition filtering/shaping... but why?
Think you nailed it there... why? Filtering? Balancing? Shaping? Who knows... In my admittedly limited experience, I've never seen anything like it.

It does look like it'd be pretty easy to undo, though. Just curious... what did the tech you had say after looking at it?
 
ChurchHill":1sd8wc28 said:
Wizard of Ozz":1sd8wc28 said:
I guessing they are somehow connected in providing addition filtering/shaping... but why?
Think you nailed it there... why? Filtering? Balancing? Shaping? Who knows... In my admittedly limited experience, I've never seen anything like it.

It does look like it'd be pretty easy to undo, though. Just curious... what did the tech you had say after looking at it?

The seller said the his tech told him it was 100% stock... but I spotted it in the pics pretty quick. I’m detailed like that lol.
:yes: :LOL: :LOL:

I’m betting on some sort of hum trimmer. Not seen any sort of pot, unless added in any Mesa. It’s a clean job at least... should be easy to remove. I plan on replacing all the filter caps anyway... the 4 30uf/500V caps included. Just curious and was wondering if it was some sort of band-aid covering up a bigger issue.
 
Yeah, the only pots I've seen in Mesas were added as part of the adjustable bias mod, which this one is quite obviously not. I'm curious as to what they were trying to accomplish. I mean, there's just not that much in that part of the B+ supply, just the filter caps and dropping resistors.

You definitely have a point about not wanting it to be a band-aid fix. Maybe they were trying to reduce hum or just had a bad batch of tubes. Can't think of anything else they'd be trying to fix there.

Now that (presumably) replaced or modified (and not added) resistor next to the two ceramic caps... maybe they had a bad filter cap that blew and took out that resistor? Maybe they were trying to modify the voltage to that filter cap in an attempt to keep it from happening again? This doesn't make sense, either, as it looks to me like all of those caps are the same age. Maybe they replaced them all??

All of this is just wild speculation on my part. I'm really clueless. As you said, at least it's a clean job and should be easy to remove. If you do find out anything more about this, let us know, OK? Now I'm curious, too... :)
 
Perhaps some additional power supply hum filtering?

Typically in DC powered guitar fx pedals, one would place a 100 Ohm resistor in series with a 100uF electrolytic, to minimize ripple and noise coming from the power supply.
Not sure if it would apply here, but who knows...
 
This wouldn't be an attempt at changing the heaters to DC, would it? The pot could be a hum balance for the heaters... :dunno:
 
ChurchHill":1y23hz29 said:
This wouldn't be an attempt at changing the heaters to DC, would it? The pot could be a hum balance for the heaters... :dunno:
Yeah I was wondering about this too.
Elevated heaters??
But it's connected to the wrong side of the cap.
If you figure this out, be sure to let us know WTF is going on here. :confused:
 
Well... I heard back from someone very familiar with Mesas that it is indeed a bias pot. Never seen one placed here myself.

It is indeed sending the additional current to ground, lowering the current going to the power tubes. That’s why the pot is connected to a common ground. Most current production 6L6GC power tubes (and even many nos) run too hot (they draw too much current in the stock Mesa Recto circuit, which I’ve seen first hand myself). The pot must look a lot bigger in person, as it appears to be itty bitty in these pics.

If we were playing golf... Tim was closest to the pin. :yes: :thumbsup:
 
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