Vox AC15 (TBX) general Q's

[Sea Chief]

Hi chaps-

got myself a lovely AC15 TB (C'96) with a blue 'vox' speaker: I assume this was an addition as amp says only TB on, & therefore can now be called a TBX, & is this the usual monkey celestion G12 alnico just rebadged?

Maxing the 'volume' (the preamp.. not overall 'master') I hear only very little OD.. weird I thought. Looking inside I find a jan philips 12AT7 in V1. So knowing all meant to be 12AX7, I swapped it for a JJ 12AX7.. expecting a bit more OD. Not so. Actually its a little -less- OD than with the 12AT7.

I'm a bit confused.. not only the master doesn't seem to do much at all (& the reverb severely lacking too), but this tube swap not as expected. Its a fine amp, very well made, & looks gorgeous but I'm scratching my head a bit.

Any thought? cheers, SC

[Dave H]

Maxing the 'volume' (the preamp.. not overall 'master') I hear only very little OD..

I don't think it has a lot of gain in the pre-amp before the master so there won't be much OD. (Assuming I'm looking at the right schematic). It's like a Top Boost circuit but with lower gain. Cathode resistor R35 is a high value and not bypassed and the 3M3 reverb mixing resistor is also going to reduce gain.





Edit: Ignore what I said above. I've just noticed the input tube triodes are cascaded

[pdf64]

V1 in the preamp is the phase splitter.

My experience with these amps is that they’re plagued with dry joints; it’s best to reflow the pcbs.

[Sea Chief]

Ah.. not sure what is meant by your edited line then DaveH. Thanks for schematic. I'm so used to fender ab763 etc types. pdf64 saying v1 is the phase splitter is totally weird.

So what are the 'chain' of tubes from two inputs.. to power tubes then? is v5 the 1st preamp tube then? isa it maybe 'backwards' to fender v5,4,3,2,1 > power tubes?

[Dave H]

Ah.. not sure what is meant by your edited line then DaveH. Thanks for schematic. I'm so used to fender ab763 etc types. pdf64 saying v1 is the phase splitter is totally weird.

So what are the 'chain' of tubes from two inputs.. to power tubes then? is v5 the 1st preamp tube then? isa it maybe 'backwards' to fender v5,4,3,2,1 > power tubes?

I meant that it's not as low gain as I first thought because the two sections of the input trube (V5) are in series i.e. It has one more gain stage than the Top Boost pre-amp.

I think the chain is V5 section A, V5 section B, V4A, V4B (cathode follower), V1A, V1B (phase splitter)
V3 reverb driver, V2A reverb recovery, V2B trem oscillator.

[Sea Chief]

V1 in the preamp is the phase splitter.

My experience with these amps is that they’re plagued with dry joints; it’s best to reflow the pcbs.

But unless I hear anything suggestive of a dry joint, there's not much point doing so surely:

or,

are you suggesting the things I find/ hear -are- suggestive of dry joints?

(Its a bit of an ambiguous post/ sentence).

[Sea Chief]

I meant that it's not as low gain as I first thought because the two sections of the first triode (V5) are in series i.e. It has one more gain stage than the Top Boost pre-amp.

I think the chain is V5 section A, V5 section B, V4A, V4B (cathode follower), V1A, V1B (phase splitter)
V3 reverb driver, V2A reverb recovery, V2B trem oscillator.

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. How on earth any' normal gtr player' (IE without having tinkered with the guts of a tube amp as I have) would have any idea I just do not understand. I mean its got a treb & a bass. Most gtr amps have a treb & a bass. If I want any gtr amp I almost expect a treb & a bass. pg1. What 'top boost' means.. god knows, but its something to do with the treb, & the bass is all I know.

The chain I can get more sense from/ can see what you mean, looking at the schematic- thanks, I guess its just labelled 'opposite' to fender to be deliberately different.

[Chuck H]

But unless I hear anything suggestive of a dry joint, there's not much point doing so surely:

or,

are you suggesting the things I find/ hear -are- suggestive of dry joints?

(Its a bit of an ambiguous post/ sentence).

And what do bad solder joints sound like? The answer is that it depends. It depends on the circuit in question and the nature of the joint failure (open, intermittent, low continuity).

I don't think pdf64 would have mentioned it unless he thought it was worth doing. Are you looking for a reason to disagree? He's had more than a little experience at this.

[Helmholtz]

Consider using freeze spray to identify cracked/bad solder joints.

[Chuck H]

What exactly top boost is, even from reading some xyz about it is still totally lost on me. How on earth any' normal gtr player' (IE without having tinkered with the guts of a tube amp as I have) would have any idea I just do not understand. I mean its got a treb & a bass. Most gtr amps have a treb & a bass. If I want any gtr amp I almost expect a treb & a bass. pg1. What 'top boost' means.. god knows, but its something to do with the treb, & the bass is all I know.

It's simpler than you're making it. On the early Vox amps the "top boost" channel had more HF gain than the "normal" channel***. Later incarnations that imitate the top boost channel were naturally called "top boost" as a name for the design. That's all there is to it.

*** Actually what the top boost design does (among other things) is reduce LF through the circuit and boost overall channel gain. But that's beyond the subject of it's name.

[pdf64]

But unless I hear anything suggestive of a dry joint, there's not much point doing so surely:

or,

are you suggesting the things I find/ hear -are- suggestive of dry joints?

(Its a bit of an ambiguous post/ sentence).

Sorry, you mentioned a few issues (eg insufficient preamp gain for noticeable overdrive, master vol having little effect) that may indicate circuits not working as intended, which may well be due to poor connections; of course they may be due to something else, eg tired ecaps. A failing solder doesn't just cause crackles etc.
One approach is to track down each perceived issue.
Another is to blitz the boards and then reassess.

Regarding the Blue speaker, I seem to remember reading that more recent Vox Blues (than this range, later than 2002ish) were made by Celestion in their Chinese site, whereas regular Celestion badged Blues have always been made in the UK

Regarding the tube swap in V1, did you mean the tube noted as V1 on the schematic (ie phase splitter) or the tube that include the input stage (ie V5 of the schematic)?

[Sea Chief]

And what do bad solder joints sound like? The answer is that it depends. It depends on the circuit in question and the nature of the joint failure (open, intermittent, low continuity).

I don't think pdf64 would have mentioned it unless he thought it was worth doing. Are you looking for a reason to disagree? He's had more than a little experience at this.

I'm certainly not looking for a reason to disagree, I'm sure he has a wealth of experience & I didn't for one moment suggest otherwise; I'm just looking for a reason he mentioned it.

1) If it means I can dig into more OD.. great/ will do it.. but I don't think this is why it was mentioned.
2) If it means I can ID my speaker.. great.. but don't think the reason it was mentioned either.
3) Nor why I find an 12AT7 in V1.

These are the 3 things I mentioned in the OP. Arbitrary opinions on other aspects non-related.. even if you mioght think tyhe amp total cr*p, may well have worth, but they don't tie with simply what I'm asking in th OP, & as a consequence just confuse me.

[Chuck H]

1) If it means I can dig into more OD.. great/ will do it.. but I don't think this is why it was mentioned.
2) If it means I can ID my speaker.. great.. but don't think the reason it was mentioned either.
3) Nor why I find an 12AT7 in V1.

You mentioned that the master volume and reverb didn't seem to have much affect. Making the amp work properly should certainly allow you to dig into more OD. Don't be obtuse about things.

Perhaps what you meant was that the master volume doesn't seem to do anything to the tone of the amp? Volume not withstanding? Since the master volumes function is to set the master volume, when you say "it doesn't seem to do much at all" we can only interpret that to mean the control is not working.

[Sea Chief]

Sorry, you mentioned a few issues (eg insufficient preamp gain for noticeable overdrive, master vol having little effect) that may indicate circuits not working as intended, which may well be due to poor connections; of course they may be due to something else, eg tired ecaps. A failing solder doesn't just cause crackles etc.
One approach is to track down each perceived issue.
Another is to blitz the boards and then reassess.

Regarding the Blue speaker, I seem to remember reading that more recent Vox Blues (than this range, later than 2002ish) were made by Celestion in their Chinese site, whereas regular Celestion badged Blues have always been made in the UK

Regarding the tube swap in V1, did you mean the tube noted as V1 on the schematic (ie phase splitter) or the tube that include the input stage (ie V5 of the schematic)?

Hi pdf64,

I will consider this, but a bit drastic? I wasn't suggesting anything wrong with the amp. More I was puzzled by the design, which -afaict- & so far as I am using it correctly, & have the right tubes, seems to have such a small ammount of OD from a circuit I read alot about "top boost.. extra tubes in preamp.. more gain" & with an MV knob too.. that I'm left scratching my head. But every amp I have I find totally confusing, the OD infuriatingly impossible to achieve (without taking out half the village).. bar my small 10" Laney vc15, which is great but a bit like a toy version of this 90's UK made vox in terms of tone complexity.

[Sea Chief]

You mentioned that the master volume and reverb didn't seem to have much affect. Making the amp work properly should certainly allow you to dig into more OD. Don't be obtuse about things.

Perhaps what you meant was that the master volume doesn't seem to do anything to the tone of the amp? Volume not withstanding? Since the master volumes function is to set the master volume, when you say "it doesn't seem to do much at all" we can only interpret that to mean the control is not working.

I'm not being obtuse. I'm trying to be as clear as possible, sorry if this is not coming across.

The amp is working as it should, even with the small ammount of OD I achieve with volume maxed (the one near the gtr inputs). afaict there's nothing to suggest the amp is -not- working correctly.. but 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.

From MV amps I've had or played, this function/ this very knob's use, is to achieve OD without turning the other 'main' volume knob up to destroy half the village.. with the caveat of having a 'lesser' OD tone. I think, this is what it is for (but the manual makes no mention of its use, nor mentions what on earth 'top boost' means) this is the secondary Q of my thread. These other amps, including a Twin Reverb you turn this knob to max, have the 'main' volume low.. & there's a decent ammount of OD (the quality dependent on the amp: for some reason the TR hopeless, & a diddy Laney vc15 excellent in this dept).

If a very decent quality amp has this extra vol knob (one nr the gtr inputs) & its maxed, but only a very small ammount of OD achieved (& the amp is working as it should).. what is the design principle? why if the preamp circuit has "two extra tubes! top boost! extra gain!" do I seem to get the opposite of what I'd expect IE a very decent ammount of OD with this vol maxed-? this is the point of the thread.

The 3rd minor Q is the speaker: its blue & it has 'vox' on it.. but what is it? its wattage, sensitivity?

thanks, SC