Vox Top Boost Circuitry...

[ErikMAMS]

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

It might not be that simple Ian. The AC4C1 is a smallish 4 watt practice type amp using a single ended power section (one EL84 valve). The AC30 TB has a totally different design with a phase splitter driving a push pull 4xEL84 power stage. It's like a eco city car versus a 6 cylinder Range Rover. Lifting the TB section from the workhorse AC30 to a small amp with a different topology might not work without a few adjustments.

We don't know if the AC4C1 diagram floating around is 100% correct - it's unofficial and unverified in obviously incomplete - and apperantly Vox doesn't bother issuing a factory schem or service manual. If the AC4C1 doesn't follow the orig. TB circuit "connection for connection" - which could seem to be the case with the missing bass control grounding reported in DM's amp - you could call it an "error". The way I see it it's more likely to be an adjustment found necessarry by the development team to make the AC4C1 design with it's limitations perform best possible (or as intended) with the TB circuit.

Obviously it also means that the infamous unverifed schems has flaws (I'd trust the actual amp circuit more than a schem with unverified origin).

Erik

[roger bayliss]

I have sent Vox a support ticket to see if they can comment and asked about a service manual. I live in hope !

Copy of the message sent.


Hi

Regarding the Top Boost tone stack in the AC4C1 TV you say it is based on the original AC30 TB circuit. I am on a forum and it has been noted that the bass control does not quite work the same way as the original especially with regard to the last 1/4 of turn. The original seems to pull the mids out and go a little more nasal. The AC4C1 does not appear to do this. We were wondering if the bass control is earthed on the one side as it should be ? Is there a service wiring diagram we could see ?

Thanks in anticipation.

[ErikMAMS]

Good move Roger - fingers crossed Vox can/will deliver.

I've managed to get in contact with the guy who had the infamous schem drawn up. He's been doing a lot of work and modifications on his AC4C1 trying to get the best possible sound out of it (to his likings). The guy confirms that the schem, although incomplete, is CORRECT in what it shows (as per his AC4C1)! I specifically asked about the bass pot grounding - and he said in his amp the bottom lug of the pot is grounded right where it's soldered to the PCB, absolutely no doubt about that.

The plot thickens.

Erik

[roger bayliss]

nice one Erik that seems to confirm it is the same circuit. I await contact from Vox and will let everyone know what they say.

[roger bayliss]

I do have a schematic drawn by J Bell on 11/12/1961 and it is the Top Boost Mod add on, but it has a note on the grounding connection...

it reads "Isolate this line and earth to tag at V.1. only." against the ground connections line.

Might be relevant ? just thought I would mention seeing that.

[rogera]

That note regarding where to connect the earth line of the top-boost unit is only to avoid multiple earth points which can lead to hum being introduced.

[ErikMAMS]

So, how will grounding of the bass pot vs no-grounding influence the way the tone controls work on the freq spectrum?

Duncan's Tonestack Calculator doens't give the option of disconnecting the bass pot from GND. However, setting the bass pot value as 10 megaohms, and only using the first 10% of the available pot rotations would approximate a 1M pot with a a resistance of 9M to ground. Still not a total disconnection of the ground, but close enought that we can get a useable comparison - or so I guess.

Below are the resultant 2 plots from Duncan's TSC. I've chosen 5 settings of the treble/bass pot - identical in both plots - giving 5 response curves in each plot. To the left for the standard/correct tonestack with the lower leg of the bass pot at GND, and to the right for the "disconnected ground" circuit version.

The 5 combinations of bass/treble settings are:
A: bass MAX - treble MAX
B: bass MIN - treble MIN
C: bass MAX - treble MIN
D: bass MIN - treble MAX
E: bass 50% - treble 50%


It's clear that the plots for the B+D+E settings seems pretty close in both circuit versions (bass pot grounded or not), but A+C shows substantial differences.
Most interesting point in this relation: the "mid sucking quirk" only happens to the full degree with the bass pot grounded.

Erik

[roger bayliss]

That note regarding where to connect the earth line of the top-boost unit is only to avoid multiple earth points which can lead to hum being introduced.

That is noted Roger. Thought I would mention it in case it had any bearing.


Erik

What you show does seem to confirm that the correct bass pot grounding is where the issue may be. It should be noted that the treble also has a big part in this as both interact.

Still no response from Vox on my query as yet.

[roger bayliss]

Here is the response from Vox...


Hi Roger,

Thank you for your enquiry.

Unfortunately we cannot provide you with a copy of the service documentation for the AC4C1 but we can confirm that the bass pot on the AC4C1 is not grounded.

Based on what you described below, it is very likely that your amplifier is working as it should. If you feel that is not the case, please contact Andertons directly and they will be able to arrange a return for it. We can get it check out and ensure that it is working as expected.

Best regards,
Saulo Valerio

[ErikMAMS]

Very interesting Roger.

The verification of the deviation from the original TB circuit explains DM's findings, and the "fix" should be easy - unless it introduces unwanted side effects in the AC4C1 package. Hence Vox's decision to depart from the original schem.

In the light of the reply from Vox the reports from the guy in US - who made the blury schem - is odd/confusing/doubtfull. I will present him the answer from Vox and await his comments. I would think that we can rule out the possibility of different versions of the AC4C1 - but then again...?

Erik