Vox Top Boost Circuitry...

[roger bayliss]

I would say they use the new Top Boost circuit design in most of their newer AC amps Erik if the truths known.

I believe the circuit will be the same for all AC4C1 designs as well. The blurry circuit would appear not to be correct either !

So we are back to re jigging the bass pot onto the earth somehow if we want the original behaviour.

[Twang46]

This one's not "blurry"



Is this helpful ?

Dick.

[roger bayliss]

It is nice and clear Dick, but the circuit according to what Vox told me will not be an AC4C1 or variant as they stated the Bass control is not connected to earth. The schematic does not state what particular amplifier it refers too either.

What we could really do with is a service manual with schematic for the AC4C1.

[Twang46]

Hi Roger

The cct is from a US Telecaster forum which is stated to be a professional drawing of a forum members AC4C1 amp

"Here is the schematic. The great guys over at Sewatt.com wrote this up for me off of my amp."

I found it on Google images while looking for a cct for an AC4TV one.

Dick.

[roger bayliss]

If that is the case Dick... but an email reply from Vox states that they can confirm the bass control is not earthed in the normal manner (maybe the casing to earth ?) but not one of the lugs as in the old Top Boost.

[ErikMAMS]

New information!
From a reliable source (to the best of my knowledge) I’ve been told that the AC4C1 has adopted a deviation from the original TB circuit that is also part of the AC30 Custom Classic amps. In the AC30CC the circuit deviation is a switchable feature: EQ-STD/CUSTOM (those interested can look up the CC-diagram). However the AC4C1 has the "non-original" circuit wiring only.

The changes from the original circuit are twofold:
- the bottom lug of the bass potentiometer is not connect to GND (as we thought - and verified by Vox)
- the bottom lug is wired back to the wiper of the pot (kindly not mentioned by Vox)


As the center/wiper lug and the end lug of the bass pot are connected on the PCB, changing it back to "classic" top boost circuit is a bit complicated. One way is to cut off the bass pot lug (to isolate it from the PCB/wiper) and then add a jumper wire from the cut off lug to the "ground" lug of the gain pot (on the small upper terminals).

Of course a more elegant solution is possible if one wants to unsolder the bass pot and mod the PCB traces.

Erik

[David Martin]

Thanks everyone for your contributions... Now back from holiday. Will need to disassemble amp further to move on...

[roger bayliss]

New information!
From a reliable source (to the best of my knowledge) I’ve been told that the AC4C1 has adopted a deviation from the original TB circuit that is also part of the AC30 Custom Classic amps. In the AC30CC the circuit deviation is a switchable feature: EQ-STD/CUSTOM (those interested can look up the CC-diagram). However the AC4C1 has the "non-original" circuit wiring only.

The changes from the original circuit are twofold:
- the bottom lug of the bass potentiometer is not connect to GND (as we thought - and verified by Vox)
- the bottom lug is wired back to the wiper of the pot (kindly not mentioned by Vox)


As the center/wiper lug and the end lug of the bass pot are connected on the PCB, changing it back to "classic" top boost circuit is a bit complicated. One way is to cut off the bass pot lug (to isolate it from the PCB/wiper) and then add a jumper wire from the cut off lug to the "ground" lug of the gain pot (on the small upper terminals).

Of course a more elegant solution is possible if one wants to unsolder the bass pot and mod the PCB traces.

Erik

Thanks Erik good research ! I think that the mod is simple enough to do and the likely answer to this.

[David Martin]

And now... The result!

Instead of cutting the leg on the pot, I removed the track joining the leg back to the wiper, and added the wire as shown. And... The bass control, far from exhibiting the classic top boost feature, didn't work at all... It did nothing.

So I reversed the change and paid more attention to the existing top boost eq.

To be honest, because I have more than one amp to use, I hadn't really experimented much with the AC4C1. The eq section seems to work with no effect in the centre position, but with boost or cut from that, depending on left or right rotation, whereas the "classic" behaviours as exhibited by the AC15HTV Heritage is no effect on minimum, but increasing boost with clockwise rotation.

And, strangely, on the AC4, the "sucked out middle" phenomenon now appears on the treble control approaching maximum boost - and that's where the Burns era sounds lurk...

[David Martin]

And now... The result!

Instead of cutting the leg on the pot, I removed the track joining the leg back to the wiper, and added the wire as shown. And... The bass control, far from exhibiting the classic top boost feature, didn't work at all... It did nothing.

So I reversed the change and paid more attention to the existing top boost eq.

To be honest, because I have more than one amp to use, I hadn't really experimented much with the AC4C1. The eq section seems to work with no effect in the centre position, but with boost or cut from that, depending on left or right rotation, whereas the "classic" behaviours as exhibited by the AC15HTV Heritage is no effect on minimum, but increasing boost with clockwise rotation.

And, strangely, on the AC4, the "sucked out middle" phenomenon now appears on the treble control approaching maximum boost - and that's where the Burns era sounds lurk... With Strat or Marvin.

Show it. Les Paul, however, and some gain,,then turn the guitar down to 3 and clean rhythms are there. Turn in up and it ROARS!

Comments?