Vox Top Boost Circuitry...

[David Martin]

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?

[RogerCook]

http://www.voxshowroom.com/uk/amp/ac30_tb_hood.html

The mid scoop effect is probably due to the "mistake" on the bass control wiring. I don't know if this could be done with the AC4 circuit.

[rogera]

Hello David - I'm assuming that the two tone AC4 you have is the AC4C1 TV model and from what I can tell the top boost circuitry in that amp is identical to the original top-boost circuit.

There are a number of things that could be influencing your opinion that it is somehow different, but in my opinion it is unlikely that the amp could be modified to give similar results to those heard on an old AC30 top-boost.

The only circuit diagram of your amp that I could find on the internet is rather blurred, so if you have one, or as an owner if you could request one, then I could be more certain that the top-boost circuitry is the same.

[David Martin]

Thanks both... More anon...

[roger bayliss]

Does it have anything to do with the fact there is no Cut control on the AC4 series ? I have the same amp as David and am interested in this thread.

[David Martin]

Having fiddled some more, I am still of the opinion that the TopBoost circuit does not behave in the same way. Will a visual inspection of the wiring of the pot reveal whether it is grounded in the same way as the old ones, and what should I look for?

[rogera]

One side of the bass pot should be connected to earth/ground.

It may be difficult to check that visually with a printed circuit board so using a multimeter set to the lowest ohms range would be the best bet.

[David Martin]

Thanks Roger. I checked and only the pot body is connected to earth. With a little fiddle, I can get to the two outer pins of the pot. Which should I connect to earth?

(I think my phone number for you is out of date...)

[peter roest]

What is Top Boost? In the preamp (AC15 & AC30) between de first half ECC83 and the input of the second half ECC83 there is a capacitor with a value of 470 pF (or 500pF).
Normally this value is about 0,01-0,1 nF (much higher). This means the low tones in the AC 15/30 amps are cut off (low cut). The higher tones have more volume. This is called by Vox Top Boost. Well indeed "Top Boost" sounds commercial more attractive than "Low Cut" . Well what to do whith the overdosis higher tones? Well in the poweramp between the faseshifter ECC83 and the EL 84's, there is a filter tonecut ( in fact high cut) to eleminate the overdosis higer tones.
About the question of David. You have nothing to do with the tone control. The fact is that in the AC15/30 amps the same tonecontrol get another tonespectrum to handel than in other amps.
If you use the normal channel there is no top boost. You only have tonecut. I call it the dull channel

Peter

[David Martin]

Thanks Peter. I think you have misconstrued my posting.

I am referring to the Top Boost circuit which started life as a discrete add on for AC30 and located on the back panel initially, then later built into the grey control plate models. This is not related to the Cut control.

The latest generation of AC4s claim to have this Top Boost circuit built in, but I am finding that it does not behave I the same way. I now know that the bass pot is not grounded at all. So my next step is to experiment with grounding it to see if that causes the change in behaviour I am looking for.

Thanks for your interest.