Some considerations about the AC30 TB Treble & Bass controls...

[archaos]

As far as I'm now addicted to this forum, I wanted to talk about the different AC30 TB schematics available either from VOX, Hoffman or again S. Grosvenor whose I own the AC30 Service Engineer's Guide (which is very useful for a AC30 builder like me).
It's nowadays a well-known fact that D. Denney/J. Bell  had used the "Brilliance unit" of the Gibsons high gain channel GA-70/GA-77 from the 50's in the AC30/6 TB.
Indeed, the original Gibson diagrams for the GA-70 & 77 happen to have an error on the Bass pot, grounding one end that should be left floating. This error was carried across to the Vox unit. This fact, and the very unusual 50p high pass cap (along with every other component) show that this was certainly the circuit used for the JMI 'Brilliance' unit.

But here's the schematics attached below :

1) J. Bell's Brillance unit schematic
2) The Hoffman AC30 layout
3) The Hoffman AC30 modded layout
4) A pot diagram

If I'm not wrong there were 2 blunders on Bell's diagram :
  - the grounding error
  - the fact that the Treble pot schematic is wrong since its cold lug should be connected to both the Bass pot cold lug & one .022µF cap, instead of the 50pF cap. Actually the cold & hot lugs hadn't been rightly drawn, & were inverted with each other.

On the Hoffman layout, the Bass pot hot lug is connected to the Treble pot cold lug whereas the Bass pot cold lug should be connected to the Treble pot cold lug. Yet, true to Bell's schematic it has the grounding error too.

Therefore I have suggested a "corrected" Hoffman layout...

Finally, all those thoughts are based on a schematic representation of a pot : when both the cold & hot lugs formed a horizontal line with the wiper pointing from the bottom, the cold lug should be on the left & the hot lug on the right.

Any other thought ?  

[OldHouseScott]

The grounding "error" is part of what gives the Vox tonestack its quirky uniqueness. As you crank up the treble, the mids are scooped and the bass is reduced. I believe the recent CC1 and CC2 versions of the Vox AC30 had a switch to lift the ground off the bass pot for a traditional tonestack action, or to connect it for the classic Vox sound.

As for the pot tabs, usually they will either be numbered, or have an arrow showing CW orientation, but not always. Don't expect that they will always be drawn in a particular way. Doug's schematic layout is correct as originally shown.

[archaos]

I believe the recent CC1 and CC2 versions of the Vox AC30 had a switch to lift the ground off the bass pot for a traditional tonestack action, or to connect it for the classic Vox sound.

Yes CC = Custom/Classic : w or w/o the error thaks to the switch you have described.

Don't expect that they will always be drawn in a particular way.

Why not ?

Doug's schematic layout is correct as originally shown.

Actually both cold & hot lugs are inverted on Doug's schematic compared to the Bell schematic, but I assume it gives the same result.

[sluckey]

Actually both cold & hot lugs are inverted on Doug's schematic compared to the Bell schematic, but I assume it gives the same result.

Huh? I don't see anything on that Bell schematic to indicate which end of those pots is which.

[archaos]

Huh? I don't see anything on that Bell schematic to indicate which end of those pots is which.

My answer was based on the fact below :

Finally, all those thoughts are based on a schematic representation of a pot : when both the cold & hot lugs formed a horizontal line with the wiper pointing from the bottom, the cold lug should be on the left & the hot lug on the right.

[sluckey]

You quoted yourself? I've never seen that fact before. Where did it originate?

I'm not trying to be confrontational. I'm just wondering if this is a generally accepted industry standard that I've just never been aware of. It would sure help when figuring out Baxandall controls, etc. The only convention I'm aware of is using an arrow pointing to CW or CCW, or numbering terminals.

[archaos]

Outerspace perhaps ? 

[archaos]

I've tried for fun the Hoffman AC30 layout -see attached above- just to hear how it sounds when tweaking the Treble & Bass pots :

          - Treble fully CCW = full treble
          - Treble fully CW   = no treble at all
          - Bass fully CCW   = a round & bassy sound + some mids
          - Bass fully CW     = no bass

>>> The pots act in the contrary way as they should ; so be careful when you wire these pots...
       

[Tone Junkie]

Interesting discussion my friends i love learning something everyday you guys never fail me.
Thanks Bill                             

[markmalin]

I just wanted to add this link to help close the loop on this topic.  Interestingly enough, the grounded bass control was NOT an error copied from Gibson.  It was copied, but it was there on purpose.  Fascinating!

http://www.vintageamps.com/plexiboard/viewtopic.php?f=2&t=94991&start=0

[roseblood11]

I just wanted to add this link to help close the loop on this topic.  Interestingly enough, the grounded bass control was NOT an error copied from Gibson.  It was copied, but it was there on purpose.  Fascinating!

http://www.vintageamps.com/plexiboard/viewtopic.php?f=2&t=94991&start=0

7 years later.... The link is dead and wayback machine can`t help. What was "fascinating"?

[pdf64]

It may be that it provides a wider range of control, scoops the midd when bass is up full.