JMI Era AC50: Tone Controls Work Backwards - Normal?

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I've only come across 3 JMI era AC50s, 2 MkIII and a MkII just now, and they all seemed to have quirky operation of their tone controls. On this MkII they both definitely work backwards, ie full CCW = max treble and bass.
Usually with Vox top boost circuits, the tone controls are 1M and tapers, where noted, are log (ie 10% audio), eg https://el34world.com/charts/Schematics ... _tbmod.pdf https://el34world.com/charts/Schematics ... _ac100.pdf
But if wired backwards, log would be completely wonky.
This MkII has linear tone pot tapers, I didn't check that of the others unfortunately.
More strange still, the treble and bass pots have different part # labels, no nominal values marked but treble tracks measure about 650k, bass about 800k. So the treble may have been 500k and drifted up, bass 1M and drifted down?
This MkII Joe Piazza drawn schematic shows both pots as log https://el34world.com/charts/Schematics ... x_ac50.pdf
Part of the official JMI MkII schematic, shows log tone controls https://www.voxac50.org.uk/images/schem ... as_053.jpg
The official JMI MkIII schematic shows treble as lin, bass as log https://el34world.com/charts/Schematics ... ac50_1.pdf

Edit. Just found the official JMI MkIV schematic, shows treble and bass controls as lin https://www.voxac50.org.uk/images/schem ... ematic.jpg

[Alnico]

I have a MkI. Both bass and treble are 1M lin and yes, wired backwards.

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I have a MkI. Both bass and treble are 1M lin and yes, wired backwards.

Many thanks for confirming that
Have you ever measured the value of the pots in your amp?

[Alnico]

No,I haven't measured them. I'll have a look in the morning.

[pdf64]

That’s amazingly helpful, thanks!
Would you mind noting any (readily apparent) labels/markings on them also? Apparently the codes on this one can be cross referenced https://ibb.co/s9XfMCg
Does yours have vertical can caps on the power amp chassis like this https://www.voxac50.org.uk/images/AC50s ... /sc_05.jpg
Or horizontally mounted axials https://www.voxac50.org.uk/images/AC50s ... _10_06.jpg

[Alnico]

That’s amazingly helpful, thanks!
Would you mind noting any (readily apparent) labels/markings on them also? Apparently the codes on this one can be cross referenced https://ibb.co/s9XfMCg
Does yours have vertical can caps on the power amp chassis like this https://www.voxac50.org.uk/images/AC50s ... /sc_05.jpg

That's my amp.

IIRC, the date codes are BL - Feb '64, but the markings are definitely 1 MEG LIN.

[Alnico]

Bass and treble pots both measure about 710k. The 500k volume pot is 397k. Interesting... I'm used to seeing old pots drifting higher.

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Yes, value drifting higher with age seems the norm, for CC fixed resistors at least. And at face value, whatever the material, wear on a pot’s track would seem likely to cause its resistance to increase.
I guess they could have been specified with a loose tolerance, perhaps to cut cost. I seem to remember that some Omeg 1M pots that I ordered back in the 80s had a 40% tolerance as standard; even at that, I still was able to reject a couple that came in at about 1.5M!

[Roe]

for the bass pot a low taper and a low reading pot is desirable imho, at least for guitar purposes

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The owner uses it for bass - AC50s do make amazing bass amps, they seem to have a 'larger than life' effect in the room, perhaps due to the open loop operation?
Testing it with guitar, I thought that the linear taper would awkward to use, but they're actually fine, a nice range of control as it's swept, no hair trigger / abrupt changes.

Apologies, I misread the (somewhat blurry, indistinct) printed 6 digit code labels on the tone pots; on close examination with strong light and visual magnification, they all had the same 66520 code. Apparently, from https://www.voxac100.org.uk/vox_ac100_p ... _codes.htm that indicates they're 1M lin.

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That's my amp. ...

How are those 4 jacks wired up on the diamond input models? It doesn’t seem to be documented anywhere

[Alnico]

That's my amp. ...

How are those 4 jacks wired up on the diamond input models? It doesn’t seem to be documented anywhere

Good question. IIRC, they're supposed to have high and low sensitivity inputs cf. the AC30 and Marshalls. Mine doesn't; all four have a 100k series resistor and they're connected in parallel. There are other anomalies on mine compared with the factory circuit diagram, such as a 10nF snubber from grid to earth on the first stage, a 500pF coupling cap off the first stage's anode, 500pF bright cap on the volume control and a 150pF treble cap in the tone stack.

[Roe]

vox changed the 50pf in the tonestack to 220pf in the 70s, at least on the 100w models.

I had two vintage ac50s on my bench from around 66-67. the only real difference between them was the shared screen resistor, which varied (I remember one value as 750R)

For guitar, I tend to prefer my ac30 and ac100

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Yes, this one has a 750 ohm shared screen grid resistor too.
At the owner’s request I replaced the ‘drifted down’ tone control pots, using Alpha USA 1M Lins, wired backwards.
I’ve documented its quirks and the few ‘reliability’ mods I made on a schematic
https://ibb.co/XJWKKLN