Vox Top Boost Circuitry...

[ErikMAMS]

Although Vox claims that the latest generation of Vox amps has the "classic" top boost circuitry, it certainly doesn't. '60s TBs have a real quirk in that the last eighth of a turn on the bass control sucks the middle out of the sound. Add a dose of treble, and there lies the classic Burns sound - even on a Strat.

So what caused that effect, and, can I modify my two tone AC4 bass control to do the same?

Hi David
I'm a bit confused - maybe I'm missing something? The quirk you are describing is due to the treble/bass tonestack which is part of the orig TB circuit.
But AFAICS none of the recent AC4's has bass/treble controls - just a TONE knob. Hence you won't get the treble/bass interaction with the middle suck as in the original. But still - and this it what confuses me - you are refering to the BASS control on your AC4???

I recall seeing a AC4 diagram - can't remember which model except it was one of the RI's - and I'm pretty sure the TONE control is a simple treble bleed (in essense somewhat similar to a CUT control. Anyhow, if Vox claims it has the TB circuit it's just a marketing feature.
The one TONE knob on the RI's are in line with the original 1958 AC4 circuit in their different versions (AC2-AC4-AC6 which was pretty much idential).

Unless you have a AC4 model I'm not aware of I don't think it would be possible to mod the AC4 to feature the classic treble/bass tone stack - as it would require a triode set up as a cathode follower to get the right impedance to the tone stack, and the tonestack itself would introduce a heavy signal loss which would have to be made up for somehow. Well - everything is possible I guess if you really won't to do it

Erik

[RogerCook]

I think David's amp does indeed have the traditional top boost circuit though this diagram shows the bass control connects to ground and it looks like David's amp is not quite like this.

http://www.tdpri.com/forum/amp-tech-cen ... ideas.html

[ErikMAMS]

Up until now I've totally missed the existence of this particular and now discontinued AC4 RI version with treble+bass controls. My bad, sorry.
From a quick look at the blurry schem it does indeed look like it has the orig TB circuit with the tonestack. Will have to have a closer look at the component values.

Erik

[roger bayliss]

From the blurry schematic that is available I have checked it against a Top Boost circuit from the 60s and the component values seem to be the same on the AC4 as far as the bass side goes. The bass pot would appear to connect to ground the same way as far as the schematic shows.

One thing that may be worth considering is the tone pot ... is it the full 1meg ? sometimes they come of production with lower values.

Also David Martin said he had changed his valves a while back, so maybe it is worth trying the original valves to see if that makes a difference especially the one in the tone circuit.

[peter roest]

Yes, the TONECONTROLsection components in the AC4C1 are the same as the old and new AC15 and AC30 amps. In the AC15/30 there is as I told lowcut before the tonecontrolsection and highcut in the poweramp. Only Vox do this. What happens is that in the AC4 the "equal" signal is feeded to the tonecontrolsection and in the AC15/30 a low cut signal is presented to the tonecontrolsection. The tonecontrol is interactive. That's the reason you can't get wat you wish. An equalizer in the amp is the solution.

Peter

[ErikMAMS]

......In the AC15/30 there is as I told lowcut before the tonecontrolsection and highcut in the poweramp. Only Vox do this. What happens is that in the AC4 the "equal" signal is feeded to the tonecontrolsection and in the AC15/30 a low cut signal is presented to the tonecontrolsection. The tonecontrol is interactive. That's the reason you can't get wat you wish.......
Peter

With all due respect I have to disagree on that!
The blurry schem (in combination with the debate in the forum thread from which the schem originates) seems to indicate that the AC41C has the low cut before the tone stack alright (470pF/500k).

IMO the first thing to try is to get the bass potentiometer grounded as per the schem (which I understand it is not). Looking down on the circuit board and into the back of the potentiometer casings it's the rightmost lug of VR2 - the lug closets to VR3/treble pot (according to a pic of the PCB I've found).

Erik

[Iain Purdon]

Apologies to all the electronics experts here. I'm not one. This may be a stupid question but I'll ask it anyway. If Vox wants to market its old top boost should it not reproduce it exactly, component for component, connection for connection?

[rogera]

The problem here Iain lies in the fact that the circuit diagram of the Vox AC4 that David has seems to show (it's rather blurred) that the circuit is as per the old original circuit.

David knows how the old top-boost circuit behaves and in his current amp he does not hear the same reaction when the bass control is moved through the last quarter of a turn.
After talking to him and suggesting some checks it appears as though the circuit is not the same due to the fact that one side of the bass potentiometer is not connected to earth/ground.

[ErikMAMS]

Roger - watch your email

Erik

[Iain Purdon]

The problem here Iain lies in the fact that the circuit diagram of the Vox AC4 that David has seems to show (it's rather blurred) that the circuit is as per the old original circuit.

David knows how the old top-boost circuit behaves and in his current amp he does not hear the same reaction when the bass control is moved through the last quarter of a turn.
After talking to him and suggesting some checks it appears as though the circuit is not the same due to the fact that one side of the bass potentiometer is not connected to earth/ground.

That's what I meant by "connection for connection". So, essentially, it's an error?