Vox ac15tb > ok with a 16ohm spkr?

[Chuck H]

I don't or will never be in a position to use a closed back cab you see.. it makes no sense for me, even if I might 'slightly prefer one to the other' I cant help but think if one's more bassy (I assume this is the main difference- I don't know) then if you have a tone knob on the amp (on the guitar too).. then I can't really understand why closed back cabs exist.

So to just to boil this back down. Will using a 300uf cap instead of a 400uf.. make it emulate more a closed back cabinet then-?

So.. that means -less- bass. Is that all there is to it, or am I missing something?

thanks, SC

Actually the bass response of a closed back cabinet is more solid, but with a slightly higher and more focused resonance. Not less bass per se. The 300uf alone will be fine. It will raise the resonant peak from 70 something Hz to 80 something Hz. And since this is just to represent a dynamic load to the amp and what you'll actually be hearing is from the speaker I doubt you'd hear a difference using 300uf vs 400uf in the attenuator.

[Sea Chief]

Actually the bass response of a closed back cabinet is more solid, but with a slightly higher and more focused resonance. Not less bass per se. The 300uf alone will be fine. It will raise the resonant peak from 70 something Hz to 80 something Hz. And since this is just to represent a dynamic load to the amp and what you'll actually be hearing is from the speaker I doubt you'd hear a difference using 300uf vs 400uf in the attenuator.

Ok good so Ill go for a 300 uf. Im having as much trouble with the resistors, and inductors too, not being able to find identical values. Will fairly close do?

Here's a photo of my attenuator, maybe Chuck could recognise it as his other design-? Worth me checking the wirings ok 1st, before I pull it apart. It seems only partially useable, or rather partially effective, for some reason.. surely likely my build of it.

thanks SC

[Chuck H]

Ok good so Ill go for a 300 uf. Im having as much trouble with the resistors, and inductors too, not being able to find identical values. Will fairly close do?

Here's a photo of my attenuator, maybe Chuck could recognise it as his other design-? Worth me checking the wirings ok 1st, before I pull it apart. It seems only partially useable, or rather partially effective, for some reason.. surely likely my build of it.

thanks SC

Well... I'm going to guess that you were making my attenuator because all the parts are there. But it looks to be wired entirely wrong. Interestingly it will still "attenuate" as wired. But it's doing it by gradually grounding the speaker output. You could actually damage an amp using it at low volume settings.

[Chuck H]

Ok... Relative to YOUR layout (at some expense of my own time) I drew up a diagram. Wire it like this and THEN maybe you can form an opinion...

[Sea Chief]

Chuck H

Ok thanks for that- it looks like I added a bypass (red wire) on the resistor: I must've been prompted to, & can't have misjudged the wiring so simple this is).

But apart from that isn't it the same though?

The only difference is the wiring to the lower pair of switch terminals I have the wrong way round (but as its a switch terminal it'll be the same either way.. isn't it?)

Many thanks SC

[Chuck H]

Haha. It looks like I am the one that got confused on this. I just re evaluated it and it seems fine. With the switch terminals and the load resistor mount being done on "opposite day" I must have got lost the first time around. So... Which jack are you plugging the amp into and which jack are you plugging the speaker into? (relative to the picture you posted, top/bottom)

[Sea Chief]

Ok... Relative to YOUR layout (at some expense of my own time) I drew up a diagram. Wire it like this and THEN maybe you can form an opinion...

I have managed to switch the two wires round (on btm pair of switch terminals), & snipped the bypass wire. What I wasn't expecting.. is that it sounds no different this way?!

If it's now wired correctly - I must now have surely- then I'm just not obtaining anything much 'extra' in terms of amp distortion, from the small ammount of preamp distortion I get (on this amp- it's known for annoyingly minimal ammount, with the 'vol' knob maxed) without having the attenuator in place.

What I was expecting, knowing this is a 'top boost' circuit (it says on it & is marketed so, & shares -some- true top boost features, so I'd expect it to at the least distort pretty well if dimed, if not identically to an ac30 top boost amp).. was when I introduced the attenuator, then dimed both the amp vol & master knobs.. was, well some pretty serious distortion. But I do not find this. Instead, I find only a 'useable OD' "just-into-distortion" tone. With everything maxed. Eh? I don't understand. And, furthermore which is near identical to how it sounds without the attenuator on (& the master low). So if I'm straining the amp using the attenuator with it's vols dimed, & achieving only a fractional difference/ addition of distortion.. it doesn't seem very sensible to use it.

So, I can only conclude that, if you happily add these into amps (therfore they must be effective/ not innefective like mine seems to be) then, I must still have it wired incorrectly. Or something is still amiss.

As a result I'm not really understanding what to expect of this little attenuator, with the mystery of it not seemingly doing much tonewise (apart from it effectively cuts the volume very well- it does this fine). Very confusing.

[Chuck H]

Well now, having switched only the bottom two terminals, it actually IS wired wrong. Switch them back. Which jacks are you using for amp and speaker respective to the photo?

It seems we were posting at the same time.

[Sea Chief]

Well now, having switched only the bottom two terminals, it actually IS wired wrong. Switch them back. Which jacks are you using for amp and speaker respective to the photo?

I'm baffled Chuck. You said it was wired "entirely wrong" a few posts ago. The only two things different to your diagram, are:

1) this pair of wires to the lowest two switch terminals: on mine they are opposite to your diagram.

2) I have a bypass wire on a resistor.

I can't equate these two minor differences as it being wired "entirely wrong" (so I do not therefore know what you were referring to). And now I've rectified these differences, you are now saying it was correct before/ switch the wires back. So Ive rarely been so confused!!

[both switchcraft 11A types/ mono 2 terminal sockets, not switcheable ones with a 3rd terminal].

thanks. SC

[Chuck H]

See post #111
It was my error.

Now what I am asking is which jack you are plugging your amp into and which jack are you plugging the speaker into relative to your photo (top/bottom)?

[Sea Chief]

See post #111
It was my error.

Now what I am asking is which jack you are plugging your amp into and which jack are you plugging the speaker into relative to your photo (top/bottom)?

Omg.. the time & effort to switch those wires over!! took me hours (seriously- I'm not kidding).

Ok. I can't rewire it back it's exhausting/ plus I snipped the brown short to clean up the end, so now I need to extend it to reach back where it was, which is not possible.

--

The upper socket is 'speaker'..the lower one 'amp' Chuck. Thanks.

[Sea Chief]

Well I've snipped the leads/ swapped & crimped them onto the correct switch terminals.. so reverted back. Connection is not soldered, but solid, for this test.

It is identical sounding. It attenuates the volume fine.. but I get very little distortion: well, I hear only the preamp distortion (which is pretty lame really on this amp) plus I'd say an added tenth of "other/ extra" distortion, but really such a minimal addition like I said it makes no sense to strain the amp for such an additional scrap. So whether or not my amp does not distort at full volume, I do not know, will never know. Incredibly frustrating.

Amp distortion is a quest I have spent I'd say over 2 decades on, I find it unfathomable & utterly impossible to achieve.

So to sell this amp, buy the only one I know, from experience having played one, has decent-enough-useably-so preamp distortion. If I can find the certain one again (ss preamp/ valve power). It had amp OD that was toneful, full/ rich enough, generally a touchy feely playing amp, & a very toneful fendery clean sound: since then for 30 yrs Ive tried all manner of amps/ ideas, builds, small watt amps, big watt amps with all sorts of od pedals.. all the distortions were unuseable. Now I've tried an attenuator & for whatever reason does not seem to work- so I'm in a final corner & I'm spent.

Getting distortion seems like riding a bike, once found you're off/ away & everyone finds perfectly easy.. but for some bizarre reason, I alone, just cannot do it (coincidentally & adding mystery- I'm in the same boat with soldering unless I can find a solution): hugely demoralising. One last try with one more gtr amp for me.

Much appreciate your help Chuck.

[Chuck H]

Well your amp does have a master volume. To get more distortion from the amp than just the preamp distortion the master volume has to be turned up so that the power tubes can be driven into clipping. When using the attenuator you should have the master volume turned up or even full and then use the attenuator to reduce volume.

The amp settings you've been using with the master in use don't necessarily apply to what you're after. Turn some knobs up! The attenuator does not ADD distortion. It allows you to overdrive the entire amplifier, power tubes included, and then still be able to turn down the volume with the attenuator. Without the attenuator your only option is fully cranked up amp volume.

[Sea Chief]

Well your amp does have a master volume. To get more distortion from the amp than just the preamp distortion the master volume has to be turned up so that the power tubes can be driven into clipping. When using the attenuator you should have the master volume turned up or even full and then use the attenuator to reduce volume.

The amp settings you've been using with the master in use don't necessarily apply to what you're after. Turn some knobs up! The attenuator does not ADD distortion. It allows you to overdrive the entire amplifier, power tubes included, and then still be able to turn down the volume with the attenuator. Without the attenuator your only option is fully cranked up amp volume.

Hi there Chuck. Indeed the amp has a MV, & I know that this volume controls the overall 'power tubes' volume, or let's call it the real McCoy volume/ the daddy volume (& the other one, the 'volume'.. the kiddy one, I know is the preamp volume).

I understand, from using the amp without an attenuator, that turning the 'volume' to max (& the MV low) produces a "so-so" quality distortion. In this amp, for some bizarre reason, the saturation of it @ max is fairly feeble (equates let's say to a distortion pedal at setting '2' out of 10.. not bad, useable? I don't really think so: if it was more saturated.. perhaps so.). Ok so the amp is calling out "I only really work with the MV up dummy, the preamp vol is just a gimmick really!". Understood.

So this amp, with its preamp vol so fairly useless, & at 15w.. an attenuator seems to make real sense. Almost a hand-in-glove example, I would surmise.

So, the amp is 15w. It has a tube rectifier, 2x el84's, a rack of 12ax7's (including an added preamp tube(in some form of 'top boost' configuration/ addition/ it is marketed as such too). So the potential afaict is all there in front of me, for a cracking rollickingly good sound, if dimed; & with a vox blue speaker too.. I have some pretty decent ingredients in my book. Not cordon-blue vintage marshall ingredients perhaps, but damn fine on my plate. All there in front of me.

Now, I see a very good clip of some lucky chap in a huge room demonstrating the amp with the MV up (quite clearly from simply the richness/ complexity of tone.. & knobs are seen too). Like my recipe book, it fulfils my expectations. I would expect it too, even via a Youtube clip. Fine. So, I know, this amp is -capable- of producing a good well-saturated distorted tone, with the MV up. It's a concrete fact.

So I introduce the attenuator, & would expect -some- degree of similarity of -saturated distortion- (not full complexity of tone, because the broken-up speaker is redundant chiefly) between having the amp dimed... a similarity in the amp in this settings state/ exactly the same knob positions, with the attenuator ON > and with the attenuator OFF.

My expectations are: that there will be 1) a huge drop in volume, 2) a decreace in the complexity/ overtones/ quality of the distortion, but, 3) a similarity in saturation (if not, furthermore, some additional similarity in sustain/ touch sensitivity, & some degree of quality too).

But I do not find this. Furthermore, & uncomprehendingly, I do not find anything remotely close.

What I find set to it's lowest three notches on the dial [more & its very loud- defeating the primary objective to the tamp the amp's volume] is: no saturation, no similarity to the youtube chap's dimed amp clip, very very little additional distortion added in fact to the preamp distortion/ the equivalent to having the preamp volume maxed, & the MV on say 2, with the --slightest-- almost imperceptible addition of extra "real" distortion.

I just can't understand why the attenuator, which seems to attenuate the volume spot-on (so it's primary function, is functioning), is only letting through say just 3% of the amp's dimed tone. Now if I'm only adding 3% "rich complex true amp's dimed distortion tone" onto my 'so-so' preamp volume @ max (IE the fairly rubbish distortion).. the end result is pretty much the same. For alot of effort. The amp sounds, well average preamp distortion only. Nothing more. And clips galore of others' attenuators bleating out their 15w dimed amps lovely saturated, sustainy, touchy-feely distortion tones to rub my nose in it too!

I am completely defeated. But I still strive strive strive to understand how distortion can ever possibly be achieved, for me. 30 years spent on this & not a step closer it seems! thx SC

[Chuck H]

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.