Vox ac15tb > ok with a 16ohm spkr?

[Chuck H]

Ok... On your amp there is a voltage divider between the tone stack and the input to the phase inverter. Your amp and the TBX have reverb though. And the circuit looks about like the Fender type. Which means it adds a small amount of gain to the dry signal through the recovery and mix circuits for the reverb. But that voltage divider behind the phase inverter is cutting signal about 30%. Its R38 series (220k) and R41 shunt (470k). Several AC15 models as well as some boutique-y AC30 type amps do have the 220k series resistor. So let's leave that there. All you have to do is lift one end or just remove R41 (470k) and the overall gain of your amp will be the same as other AC15 models past and present. It looks like you'll have to lift the board to do this though as the solder joints are on the under side. The only other option would be to just snip one end of that resistor from the top. The down side of this is now you can't put it back if you don't like the change. However, if you have a 470k resistor on hand then I say just snip from the top. Then if you want to change the amp back you can put in the new resistor and you'll only have to lift the board once for the experiment.

Hope this helps.

P.S. There is no guarantee that an increase in gain on your amp won't cause instability at the higher gain levels. So this should be done at your own risk.

[Sea Chief]

Ok... Got it. What you're experiencing is that driving the power tubes harder by turning up the master volume is only adding a small amount of clipping.

I know this will be brought up by someone, so let's get it out of the way... Other possibilities for why you're not getting the clipping levels you know the amp is capable of are:

1) Your guitar has weaker pickups than the one in the video you saw.

2) Your guitar circuit is faulty and putting out a weak signal.

3) The amp is faulty and losing gain somewhere between the preamp and power amp.

4) The guy in the video either didn't mention or lied about a dirt box being in use.

And that's about it.

I do see in the schematic for your amp that the signal is padded down to the phase inverter (the tube that feeds the power tubes). I'll look into the wiring for other AC15 models and see if this is a circuit exclusive to the TB and TBX models. If it is then there may be a simple modification to bump the gain up to known AC15 levels.

Be back in a sec.

Hi Chuck. Thanks for ready my big post there. I understand all of your 4 possibles here. But I can discount them Im sure of it. In reply to both posts:

My two guitars are a strat, a tele, standard set ups, and a similarity in p/ u output. So no screaming humbucker types, no, but, putting out enough numnums. I had a diddy Laney 15 w with very decent preamp distortion, and both guitars sounded nicely saturated distortion just as youd expect, with the preamp vol maxed, MV low, from a single coil gtr. Stupid of me to sell the Laney now I think. Discounted.

Not faulty, no not both guitars simultaneously, i can definitely discount this too.

The amp is in great condition now. It was sold to me with no issues bar a crackly input (Just sorted), Ive used only a few times since, & nothing at all suggests it is faulty in any way now the wee reverb spring has been reseated. Its just been serviced as it were, clean bill of health. Discounted.

The chap in the clip was not using a pedal, this would be cheating & giving false info for no rational reason: he was an experienced player simply & honestly appraising a 90's rarely found, decent vox amp, not a private seller trying to sell one. The knobs were shown in their positions too iirc, plus, I can hear if its real distortion too even via my laptop and a youtube clip, I wouldn't be fooled by a pedal. Discounted.

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I will check what guitar he had, post the clip so you can agree no pedal, but even if it was an SG, the ammout of distortion he achieves ( which is perfectly expected, perfectly normal for a 12 to 15w tube amp, 5e3 to PR to ac15)... is sooooooo far in excess of what i find, even with my single coils plus the attenuator used with the amps volume and MV turned to max...... that something simply does not compute. Its not even close in terms of distortion levels. Its a 1.5 compared to an 8.5.

--

Your kind idea of snipping a resistor, might, I would surmise, get me from my level 1.5 distortion perhaps to level 2.5 as it were.... but the disparity, would still be soooo massively significant that even afterwards something still does not compute. Something isn't right. The mystery still remains.

The only way I can determine if this mystery lies with the amp, or, the attenuator, is to turn the amp up full leaving out the attenuator. Does the amp distort similarly to the clip eg? (absolutely no reason to think it wouldnt, nothing) if so then I have definitively concluded the attenuator is not doing what it is meant to do. Then it is massively likely my workmanship than any design aspect, of course. But i will get police visits if i do this. So i cant turn it up. Hence the attenuator idea made ideal sense.

afaict the attenuator is only correctly cutting the volume fir some unknown reason, but preventing (almost, apart from a smidge) all of the amps clipped tone from being heard. Its effectively only making the amp audibly ever pretty much a clean amp. Its really only the preamp distortion that can be added onto this clean amp sound. Which is exactly the same, for all intents & purposes, when you have no attenuator.

Thanks, SC

[Chuck H]

Well, be sure that there's nothing about the attenuator circuit or it's function that can clean up clipping from the amp. It might mitigate speaker distortions only because of reduced volume. But speaker distortion isn't really a significant thing in any reasonably designed amp. So, as to gain with humbuckers vs single coils, here's a video clip Johan Segeborn playing a couple of old AC15's cranked up. He's a very good player with an affinity for cranked up, vintage amps and he has A LOT of videos doing this with every amp you can imagine. Very honest reporting and well respected.

Your amp isn't very different in gain structure from other AC15 circuits. Like I mentioned before, the reverb circuit likely adds a tad of gain. But then that gain is reduced again by the circuit feeding the phase inverter. So, notice in the video reporting that these amps are cranked to full volume. And notice that some crunch with humbuckers is pretty easily achieved. There is a lot less distortion when Johan switches to the strat. The typical AC15 will only crunch up a little with single coils anyway. Anyway, watch the video. Like I said, positively honest reporting and a definite testament to the tonal possibilities of the AC15 circuit.

EDIT: I should note that your amp has the top boost circuit and Johan doesn't mention this. So it's probable that the amps he's using are using the single EF86 "normal" channel circuit. That said, even using a top boost circuit a player like Brian May with Queen uses series wiring for his pickups and a boost pedal upfront to get his distortion tones. AND... I have a wiring diagram (somewhere) for series pickup strat wiring. There are others around on line too. If you're feeling ambitious.

[Chuck H]

Just thinking out loud...

Taken to it's logical conclusion, if you were to pull R41 and run your master at full and did a series wire for the single coils on your strat and use a boost pedal when you wanted even more gain, do you really think you wouldn't get enough overdrive? Sometimes it's the culmination of little things that makes a difference. I know you're looking for "the thing" that is missing. But perhaps there's not as much missing as you think. Most players have a couple of things they have/do that give their tone the oomph they want. Some have several. And there's nothing to lose if you don't have what you want now.

[g1]

Sea Chief have you watched the video in post #125 above yet? Full cranked strat distortion can be heard at 4minute, 5min, & 8minute marks.
Also please post the video you were referring to.

[Sea Chief]

Chuck H appreciate the clip, thoughts. But I'm just not on the same page. I really don't know how this amp, dimed, can be of relevance. I'm just not understanding why it's chosen to show me.

If it's to demonstrate the difference between a humbucker output vs a strat.. I understand this, the difference is as I expect, I've known this/ played sg's, Lp's, strats & teles for 35 years. On day 1 I could tell the humbucker's higher signal produces more distortion.

So I knew that my single coils wouldn't achieve as much distortion as a humbucker gtr with my ac15. Its simply a case of forgetting the humbucker eg.. because.. I do not have one.

The one thing in the clip that is of use, is showing what a fully cranked amp's tonal characteristics are: rich, full bodied, touch sensitive. Which mine (with the attenuator on) is the absolute antithesis of. Complete polar opposite. It is quite simply, poor quality 'ratty' thin preamp distortion. It is like playing a solid state peavy 10w, it's really not much more fun. And I have proof the amp can sound 'in the same approximate decent-tone ballpark' as your clip there. I am not getting anything through of the amp's clipped power tube tone. Nothing (bar the additional 3% distortion I mentioned, like a token gesture to laugh at me).

The question, the only only only question... is why I am not getting clipped power tube tone through. I mean I could say "how do you know jery ear goggles here isnt using a rat distortion pedal..? you can't tell" reversing your question to me, to you (but I wouldn't).

Everything points to my workmanship, if you can use this attenuator successfully to achieve a 15w amps cranked tone/ to hear a 15w amps cranked tone, or let's say "the majority/ the lions share of it" using this little attenuator, hooked up to such an amp.. with both its volumes (if it has two) or its single MV volume.. dimed.. & get some form of rich, full bodied, distortion tone rehgardless of a strat or an sg: then I should be able to too. There is no reason on god's earth why i should be prevented from it (but it just seems- that I am). I wouldn't suggest there is an iota wrong with the design of it either: If you say its proven to work.. then thast's more than good enough for muggins here. It works. But with me.. somehow.. it does not work.

I'm resorting to such head-scratching scenarios, as this: do you think it could be that I'm wrongly (?) using a typical 4m guitar lead, to connect between the 'amp' and the attenuator socket-?? its my last last gasp before I conclude.. I will never, ever understand how other people can use this attenuator, but with me unfathomably, it just does not work.

I know this amp --can-- achieve a very decent clipped power tube tone. How? because of 1 clip. This is the only clip of relevance (surely). It shows not A) how close this amp is to a 1963 ac15 (it is completely different), not B) how fabulous an amp it is (for £750.. it isn't gonna be: its decent though, but it aint a 1960's plexi). The one thing it shows/ the only thing of relevance it shows.. is that when turned UP it produces a decent, fairly well-saturated distorted tone with all the usual attributes with it we all love, touch sensitivity, sustain etc etc. Actually in my book (which does not have the expectations of your book) it's actually a really pretty darn good tone for me, perfectly if not more than acceptably-so.

The only thing of relevance.. is the saturation of the tone, with the amp dimed. On the clip here. Can this amp do this turn the vol up distortion trick? yes it can. Nothing else is of relevance to simpleton muggins, just for now, just to get me to base1. From this.. when I have the same amp.. & I put an attenuator with it... I EXPECT some (if not a fair-share-degree) of similarity of tone. This is not over-expectant. This is not me 'aiming too high in my expectations'. I just -should- get there or thereabouts with the attenuator & a similar type of guitar. Some parity of tone. Something, anything comparable is EUREKA! for me (bc my expectations are so damn low, having spent 30 years on this distortion quest). But I get nothing remotely, not even ballpark close to it. A completely differene tone. No distortion really at all bar just the feeble preamp distortion, as you do without the attenuator. Fun? about as fun as a £15 charity shop fender 15r. 3 chords.. turn it off. Makes you sooo annoyed.

There has to be a reason for this disparity.

The only clip I think that is of use is here: (just skip to 3.15: amp in full monty mode, with tele). https://www.youtube.com/watch?v=4hBoeY6hjIQ

[Sea Chief]

Sea Chief have you watched the video in post #125 above yet? Full cranked strat distortion can be heard at 4minute, 5min, & 8minute marks.
Also please post the video you were referring to.

Hi g1. Thanks again for enduring/ reading my posts.. really appreciated. I have seen the clip, but don't understand why it was posted. A vintage ac15, without a top boost (or with mine's silly reverb), has no correlation with my amp. Its a very different animal. There's only a vague connection, which, if Im trying desperately to pin down specifics, I can't quite see as being much help (with due respect to Chuck fir kindly putting it up).

I've posted the clip of my amp. One before this post, at the very btm.

Actually to me this amp sounds as good, if not better than the vintage one with the builder in his ear muffs in his lunch break. So it is an excellent clip, excellent reference point of what my amp is capable of with single coils.. & the only clip of relevance unless Im not seeing the wood for the trees (always entirely possible). The only difference is the country the blue spkr was made in.. negligible for all intents & purposes for this thread.

Thx SC

[Chuck H]

[IThe only clip I think that is of use is here: (just skip to 3.15: amp in full monty mode, with tele).

Very good clip and I did see that one in this process. It could be noted that the actual saturation level isn't that deep and that VOLUME is at least partly responsible for the dynamics. He's a hard attack player too. That's significant. So...

Remove or clip R41!!! You can always turn down the gain and volume knobs. Nothing to lose. If you're not hitting the guitar as hard as this guy but you want the same saturation level then you'll need to increase gain a little. That's absolutely NOT a high degree of clipping going on in the video. He's just playing with authority. And also playing to the amps strengths in that area. Not a saturated tone at all. I know. I use saturated tones as my own MO. And also more dynamic tones for emulating players that use gear differently than I do. As in cranked tones.

You mention that your Tele has "Texas Specials" Are these Texas Special tele pickups? Or are they just single coils strat pickups/ There is absolutely a difference. The standard Tele pickup has A LOT more output than strat pickups.

[Sea Chief]

Very good clip and I did see that one in this process. It could be noted that the actual saturation level isn't that deep and that VOLUME is at least partly responsible for the dynamics. He's a hard attack player too. That's significant. So...

Remove or clip R41!!! You can always turn down the gain and volume knobs. Nothing to lose. If you're not hitting the guitar as hard as this guy but you want the same saturation level then you'll need to increase gain a little. That's absolutely NOT a high degree of clipping going on in the video. He's just playing with authority. And also playing to the amps strengths in that area. Not a saturated tone at all. I know. I use saturated tones as my own MO. And also more dynamic tones for emulating players that use gear differently than I do. As in cranked tones.

You mention that your Tele has "Texas Specials" Are these Texas Special tele pickups? Or are they just single coils strat pickups/ There is absolutely a difference. The standard Tele pickup has A LOT more output than strat pickups.

Hi Chuck.. ok good that post feels like progress, & that I'm not totally losing my mind here! (tho i am). No it's nothing like a fully saturated distorted tone: & I dont actually ever want this, the ammount in this clip is absolutely perfectly adequate for -all- my needs (I'm not a spotty herbert trying to emulate brian may with an 'affordable' vox ac top boost amp!!).

Just in terms of what little I have ever been able to achieve in distortion, in any amp I've owned, it is fairly saturated. It is just a benchmark. Its all I need.

I actually attack a guitar like JMascis. I abuse my acoustic/ heavy hitter.. & attack my tele similarly to the gurning chap in my clip. Grinning too.. you can tell how this -feels- to play, the sustain is there, the complexity, the overtones, & its my flippin amp! this is the frustration. And you can just tell by the sheer sound too there just aint no dirt pedal he's hiding away either producing it: I'm sure you could agree at the least with this.

Ok I could try your R41 idea.. but.. the question is being bypassed tho as to why I can't achieve even a fraction of gurning tele chap's tone, with the attenuator, if he (presumably its stock) hasn't done your R41 mod. But I need to wait on solder xyz bits to arrive, because I must do a tip top job/ pcb remove etc as I don't want to snip if I intend to sell (if my distortion quest now in its final guise returns no results: I am resigned [almost already] to seeking out the safest-bet-alternative instead; an amp with -decent/ useable- preamp distortion.. a hugely poor substitute, but a huge step up from my current dismal, untennable position). A week.

And my amp now is ticking a bit more prominently, & the reverb has stopped again! so some more work needed on this front.

The pickups given to me kindly by Rift Amps, who told me they were fender orig texas specials. Mex or US? i couldnt care tbh.. its only an £80 ebay tele. I had a 57 reissue (is it?) fender japan butterscotch tele's a few years ago (tex specials) which sounded the same to me. Only difference I know of, is that gtr's p/ups had wax cloth wires coming from them (black & white pair > to pots).. & my donated pair on this cheapo gtr, have thinner pvc coated wires > pots instead. Mex versions maybe?

Anyway, thanks for continued input- & glad my clip gives me hopefully a bit of kudos to my spiel.

[Sea Chief]

Attenuator/ amp update.

I did briefly turn amp UP, both knobs full (in a rain deluge/ hoping sound muffled it!).. & got a similar if even slightly more distortion to Gurning-Gary in my clip. I never want/ need any more distortion. I couldn't enjoy it due to volume & my nerves on edge, but nonetheless I ploughed into my strings for 15 seconds!

Ok so I know now 100%, that I should be able to add the attenuator, & get the majority/ if not the vast majority, of this sound.. at a much reduced volume: now I think that expectation isn't too high: it seems from general attenuator clips, that it is not.

Is it possible the rheostat is faulty (being the only moving part/ 'complex' component)? if it tamps the volume as expected, would this simply rule that idea out?

[Sea Chief]

With the new volume pot I noticed a bit of difference in tone, something felt it 'clicked' better too. No idea what. Exactly same wiring as the cheapo mini it replaced.

So I try the attenuator again, knowing now the amp -can- sound distorted at full pelt (as was always expected it to be able to).. & I get better results from the attenuator: a dose more distortion coming through. Very strange. It still doesn't replicate the amp at full pelt without it.. but it all sounds better.

I think I can only achieve this. I'll now rebuild it with capacitor & inductors, to make doubly sure of my wiring if nothing else I think. And to scratch the itch/ see if it is better this way. And if it lets through only a similar dose of distortion/ not I think all its meant to.. if it can be said to 'work' (whether or not its working as it should I cannot establish) then I could sell it. If Chuck would let me.

thanks SC

[Chuck H]

That's great progress. And I hope you got my PM regarding my earlier mistakes. I didn't edit the thread here to avoid being mistaken for any prideful reason. I just didn't want the bad advice followed by anyone reading the information in the thread.

So, good progress and yes, the reactive load version of the attenuator will sound better. I have both the resistive and reactive versions and I always choose the reactive load version for playing. Since this attenuator has the reactive load parallel with the purely resistive rheostat it is technically still largely a resistive attenuator. But the reactive load is still a great improvement.

Let's revisit the guitar volume pot on the dedicated thread.

[Sea Chief]

Chuck H thanks for PM, just seen it in fact.

I think it makes much sense me starting over, rebuilding the 'daddy Chuck' attenuator.

Just inductors to source (maybe someone can give Chuck a break/ I might need to ask the odd Q on these: not so easy to find here & various types I'm unfamiliar with).

Thanks- SC