Vox AC15 (TBX) general Q's

[Dave H]

Ok roughly on board with 1st bit: but so I read & the manual says.. it -is- a 'top boost' type vox ac amp (but you say its got "one more" gain stage than this.. which would suggest to me, that I should definitely have some OD available using the MV knob).

What exactly top boost is, even from reading some xyz about it is still totally lost on me.

It has an extra gain stage but it only has a gain of about 10 owing to its large unbypassed cathode resistor (R35) then most of that gain is dumped by the 3M3, 470k divider before the following stage. You could try bypassing R35 with a 22u cap.

Historically the AC30 was a double AC15 with an EF86 front end. The EF86 was found to be too microphonic and was replaced by an ECC83 which unfortunately reduce gain. To combat this Dick Denney came up with the “Top Boost” circuit which was an extra ECC83 to boost gain and drive a Treb, Bass tone stack to boost treble. Initially the “Top Boost” circuit was an add on bolted to the back panel but later on when it became popular it was incorporated into the amplifier.

[pdf64]

The queries raised in post 1 might well indicate malfunction, hence my response. Unfortunately, after a couple of decades use, I find that some pcbs need reflowing in order for the integrity of their connections to be reliable, especially for lower spec, non through hole plated types.

The master vol is after the phase splitter, so with it set low and the preamp controls set high, I would expect a reasonable degree of crunchy overdrive, even with lowish output pickups.

Please clarify what is ineffective about the master volume control. eg with the preamp volume and tone controls set high, the master should be able to adjust the sound level from zero to deafening, does it do that?

Please clarify which preamp socket the 12AT7 is in, eg V1, V5?

I don’t think that the Vox Blue characteristics / rating have been published, but I’d expect them to be the same as a Celestion Blue of the same age, wherever it was made.

[Sea Chief]

With respect pdf64, having just bought it on ebay & given vg feedback etc.. its working fine/ no indicators as to otherwise: its just the query of how relatively little OD this amp/ this circuits preamp just gives out, is just a bit strange. Similarly the reverb is very lacking too though, but this is a known-fact on these amps; the small preamp OD ammount is not written about anywhere hence my thread.

But it still sounds vg. Its just the 'vol' knob is useless bar set at 10 & then only delivers a minimal OD ammount. I find this rather strange (& after my 30 yr attempt at amp OD.. pretty "I just cannot believe this!" infuriating).

Most are probably lucky enough to dig into the real power-tube OD getting amp loud & treating the preamp 'volume' just as a gimmick they don't use.. but not me: its permenantly on 10 (& I just need it to go to 15 as it were).

[pdf64]

If you're happy it's all working fine and there are no actual faults that require resolving, then that would seem to be that, and it's time to get on with enjoying your new amp

[Sea Chief]

If you're happy it's all working fine and there are no actual faults that require resolving, then that would seem to be that, and it's time to get on with enjoying your new amp

Yes fair point. And I do enjoy it, the OD I get is great you pick light/ its clean, you dig in/ it gets -a little- dirty. Its the 1st amp Ive ever had like so (we all know is the essence of a good player's amp). Its so frustrating is so little tho. And I wonder how from a design pov this might have been deemed "finished/ we're all happy/ all done" when it seems weirdly 'uncomplete'.

IO guess the further Q is whether increacing the range of this volume knob by a mod, might be possible (but if so why wouldn't it have been done prior to 'signing it off' in the design dept-?)

[pdf64]

I've tried to be clear in stating my view that your amp may not be working as intended, my apologies if you haven't understood that; as such, it would be bad practice to consider modification to it.
Whatever, as Dave alluded to earlier, gain may be increased somewhat by bypassing R35.

[Sea Chief]

I've tried to be clear in stating my view that your amp may not be working as intended, my apologies if you haven't understood that; as such, it would be bad practice to consider modification to it.
Whatever, as Dave alluded to earlier, gain may be increased somewhat by bypassing R35.

I have understood it pdf64, but unless you say why you think this I can only say that its not the case as there's nothing to suggest anything wrong with it. I may not have found the ideal amp in 30 yrs of trying, but I know & I buy an amp that is working as it should.. not agree to leave vg feedback having spent £500 if I have a smidgen of doubt its not. Its just obviously a curious design with a severe limitation deemed by the designers "fine- let's put it out". I just wanted to find out if the circuit/ schematic tallies with what you might expect (levels of OD on the preamp vol knob @ max)or if not.. then it must be just an odd amp design.

[pdf64]

I have understood it pdf64, but unless you say why you think this I can only say that its not the case as there's nothing to suggest anything wrong with it...

Please consider that all I can know about your particular amp is what you have written, eg -

...the 'vol' knob is useless bar set at 10 & then only delivers a minimal OD ammount. I find this rather strange (& after my 30 yr attempt at amp OD.. pretty "I just cannot believe this!" infuriating)...

... if anyone could second this/ know this amp at all/ or could decipher from the schematic if the small ammount of OD is to be expected it would be helpful: that's mostly what the thread is asking...

...I just wanted to find out if the circuit/ schematic tallies with what you might expect (levels of OD on the preamp vol knob @ max)or if not.. then it must be just an odd amp design.

One last time, no, it is not what I would expect. The low preamp gain you describe suggests that the circuit may not working as intended (ie by its designer).
Please don't carry on like this, every one of your threads always seems to follow this same pattern, rephrasing the same query over and over, and ignoring the response

[Sea Chief]

Ok pdf64 point taken there might be a prob with it although I think its working ok. How would I establish if there's a problem tho without comparing to another amp I wonder.

If anyone has this amp or knows it & can say if the small OD ammount with 'volume' maxed is normal on theirs, or not, Id be grateful.

Its not the cc1 chinese one. That one's rubbish compared to this, I had one once.

[eschertron]

Have we ruled out lifeless tubes?

[Sea Chief]

Have we ruled out lifeless tubes?

If replacing v5 with a fairly new jj ecc83.. yes/ same. And swapping v4 for another.. yes/ same. Similarly v1.

Not changed out v3, v2 but these are reverb & tremolo.. unlikely to add any chutzpa, I think.

[pdf64]

What tools do you have available, eg multimeter, sig gen, scope?

As there was a 12AT7 in there, it may be that other changes were made to further reduce gain.

[g1]

I think with master volume set low and which ever channel volume full up, should give a reasonable amount of crunch. Tone settings will also affect this, so they should at least be up half way.

I'm still not convinced we are all on the same page as far as physical tube layout (which is V1, etc.). A drawing of the layout would be quite helpful.
I take it this is the C1 version of AC15.

[Sea Chief]

What tools do you have available, eg multimeter, sig gen, scope?

As there was a 12AT7 in there, it may be that other changes were made to further reduce gain.

That's a good point pdf.. I have multimeter (rebuilt my twin rev '78 up from scratch) but not the other scope things.

All looks pristine/ as stock on the pcb. Im pretty sure the AT was just put in to reduce gain yes.

[Sea Chief]

I think with master volume set low and which ever channel volume full up, should give a reasonable amount of crunch. Tone settings will also affect this, so they should at least be up half way.

I'm still not convinced we are all on the same page as far as physical tube layout (which is V1, etc.). A drawing of the layout would be quite helpful.
I take it this is the C1 version of AC15.

Hi g1 no this is not the C1.. its the "TB" basically you know those decent korg ac30 "TB" reissues circa mid 90's? this is the ac15 version. Made in UK (in the marshall factory in fact.. even more reason to think the preamp vol maxed should kick out some dirt, in my book!).