Re: Turning MV up - nothing...nothing...Loud!

On Jan 21, 7:54 am, Charmed Snark <sn...@xxxxxxxxx> wrote:
Xtrchessreal expounded innews:12f33fc6-2622-4d26-849c-4a21b848cdf9@xxxxxxxxxxxxxxxxxxxxxxxxxxx:

On Jan 20, 2:17 pm, Charmed Snark <sn...@xxxxxxxxx> wrote:
JP expounded innews:hqkvm4tnd59ekf5kbp74fq3eu759os7ups@xxxxxxx:

On Thu, 15 Jan 2009 18:02:49 -0500, Peter Alerich
<scoo...@xxxxxxxxxxxxx> wrote:

On Thu, 15 Jan 2009 13:27:23 -0600, JP
<J...@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

Try a linear pot instead of an audio...

I think it's the other way around, chief.

I think he said it already has an audio pot in it..

I think he is either wrong about that or
has it wired wrong. Each end of the pot has
a different response. Try switching the ends
(the non-wiper connections) and see if that helps.

I would also be inclined at this point to measure
the pot with a meter and check if it is really
an audio (log) taper pot. If the ends (non-wiper
connections) are not reversed, then my bet is
that this is not an audio taper pot.



I wonder if this is my issue.  The third replacement pot is doing this
too.  I thought about changing the position of the terminal points but
it is exactly how the schematic shows it right now.  And I figured
that all it would do is change the pot to opposite rotational

While I was trying to get to sleep last night, I realized
that I might be wrong about the pot ends reversal. After all,
if you did have those connections reversed, the "no volume"
setting would now be at the opposite end of the knob rotation
(full clockwise instead).

As long as full volume and no volume is in the same place,
then provided you have the wiper wired correctly, then it
_is_ correctly wired. The same principle applies to your tone

BUT, if you find that any "action" is reversed, then where
log (tapered) pots are concerned, that would explain it.

Linear pots don't care which of the non-wiper ends are
which, because the resistance material is, the same
throughout. But a tapered pot will act differently at
each end, which is just nature of the beast.

If you bought parts off of ebay or some other less than
known source, it might even be possible that the pots
are reverse tapered. Who knows where stuff comes from?
I can see reversed taper pots being cheap because few
people have a use for those. I've never seen one, but
I wouldn't doubt that someone somewhere made some for
some special purpose (military equipment?).

But if it were me, I'd yank the one giving you trouble
out and confirm it, once and for all, with an ohm meter.

1) Is it really tapered?
2) Which end is beginning of the log scale?

To test it, rotate the pot to normal volume zero,
and then go clockwise 25%. Take a reading between
the pot and the counter-clockwise connection. (to
double check this, a reading between the opposite
and and the wiper should be very high).

Now turn the pot fully clockwise (full volume), and
then back it off 25% (counter clockwise). Take a
reading between wiper and the clockwise end. THIS
reading should be much higher than the earlier
reading if it is a audio taper pot. If they are
nearly the same, then you have a linear one.

So if they differ enough, then it is a tapered pot.
But if the 2nd reading is much lower than the
first, then you might have a reverse tapered pot
(which I've never seen, but may exist somewhere
in this universe). Normally the first reading
should be much less than the last reading.

IOW, what you should find is that the pot varies
by a small amount of resistance near volume zero.
It should vary by large amounts of resistance
the near full volume end.


Alright,...I know nothing compared to the most awesome Lord!

So, swapping the terminals from abc to cba merely reverses the
rotation - some logic I had huh? Yeah I am thankful that it played
out so much in my favor.

I spoke to LV today. It turns out that pots work very differently
than I had previously believed. The truth about that is I have never
looked at any electronics books that explain a pot. I never really
thought of them as directional until now.

Well I wish I knew more. But, I am learning. During our conversation
it came up that I should open my Marshall 2204 and compare and verify
the wiring with "MY" 2204

When I put my amp together I used a layout from MetroAmp for
reference. There were a couple of wires missing in that layout and I
guessed where they went by using the schematic. It turns out one of
those guesses was wrong. Not wrong enough to blow up the amp but the
tone stack was seriously affected. In turn I believe the tone stack
wiring as I had it was the bigger issue that was affecting the Master
Volume. The schematic shows the terminals from Treble pot and the
Bass pot sharing a connection from the Bass wiper and a .22uF cap,
four wires in all. Well, I figured the missing wire in the layout
from MetroAmp was connected from the cap on the Bass pot terminal.
Then I ran a jumper from there to the Bass wiper terminal, and a wire
from the Bass wiper to the Treble terminal. Pots having three
terminals and three wires it all looked symetric and proper so I never
gave it another thought. Initially turning the amp on I was more
interested that it worked at all. Later I realized the amp was really
over bright. I had a really high oscillation tone and some AM radio
interference. I came here and asked for help. It was suggested to
place a small cap on the input which fixed that. Two years later
now. I start looking to fix hum issues. I fixed those and found some
wrong wiring during that. Then I am tweaking the amp for less hum,
found high ripplle on the B+ and the bias. I move the bias from the
high voltage to the bias supply tap and rebuild the bias filter. I
rebuild the input filter on the B+ as well, find a bad MV pot in the
process, it popped and lost signal at times, I replace that. Then its
too sensitive, well it was sensitive before I replaced it but it was
faulty so who knows. I start this post. FF>> to "Hey why not open up
the Marshall and take a look at what they did?"

Wow theres a thought.

So Marshall has the wire from the cap going to the Treble terminal and
the Bass terminal is open, the Bass wiper runs to the Treble
terminal. But...wha...tha...That's not how the schematic had it!
Think about the way it was. The signal following the path of least
resistance, as always, means there was much more signal at the MV than
perhaps designed for. Think of it like this..if there was no tone
stack all the signal at the MV would be higher and the pot would allow
more signal per degree of rotation of the wiper.

Was it coincidence that the MV started acting up and giving me issues
after I found wiring issues on the input signal? Or was there
suddenly more signal showing up at the MV because of the other wiring
issues. Is it possible to run the pot too hot? Even burn it or damage
it from too much heat over time? In the past I had a higher floor,
hum, hiss, I even shared the signal with the chassis. Now I have
eliminated all of that so the signal at MV is much higher. And,
Finally, the tone stack is wired in properly - or at least the way it
is on my Marshall - so the tone is better. The MV was wired correctly
all the time until last night when I flipped the outside terminals and
re-wired the Bass and Treble pots to match Marshall. That's right I
flipped them anyway because I figured if the pot was bad too it would
reveal itself. So now I need to go and put that back the way I had

BTW a Log pot is a Log pot regardless of rotation because the log
curve is the same from infinity to 0 as it is from 0 to infinity. I
think but not sure if you put ground on the wiper you would have an
exponential e^(x) function pot or what may be called an antilog pot.
I am probably wrong but it makes sense this way to me: Log pot under
normal operation wiper functions above ground or zero in positive x
and y directions. With wiper on 0 or ground the x and y are forced to
work below or in the negative x and y direction which inverts the

Okay thats all the drama I have for now, tune in next time for the
next chapter of the X-files