Vox AC4C1 schematic and mod ideas

[PurpleStrat]

Here is the schematic. The great guys over at Sewatt.com wrote this up for me off of my amp.

A couple people have asked abou my post in another thread about removing the bright caps. They are C6 120pf ceramic and C20 100pf ceramic. Removing these will not only make your pedals sound better throgh the amp but also help some of the much needed bass come out to play. The small cab and speaker will still hold it back but you may find this gives all the bass you will need. You will lose some treble but the amp still has tons to spare. Be careful with the pcb as it is a bit more fragile than even a Valve Jr. pcb. I have lifted two traces including taking out the clip that goes to the led. Be very carefull pulling and replacing that clip! The clip is unsupported by anything but the solder holding the little metal pins.

 

[cakey]

Awesome! Do you happen to have a higher resolution? Its a bit blurry

 

[PurpleStrat]

Sorry thats all I have but if you can't read somethig just ask.

 

[Ike286]

I personally would remove the bright caps, and to tell you what to mod, you gotta tell us what you want, so have at it. If you want more bass, I'd say to replace the cathode cap on the EL84 (assuming that's what that is) with a 500uf instead of a 220uf. That shoudl fill it out a little bit. and then it might ("might") not sure with this amp, but it might get a little bit flubby, in which case I'd either replace the .1 in front of the master volume with a .047 (maybe even a .02) or just put it back, just try different stuff. It's a simple circuit, have at it. Also, you could raise the value of your master volume to a 500kA pot for a little more clarity, you probably wouldn't notice it. And if you want, you can replace the 5k6 grid stopper on the EL84 with a 1k5 for more brightness, or a 8k2 for a darker tone. And The first two components on the input jack. They look like a 22p and a 10M resistor, which feed into a 27k and a 1M impedance resistor to ground? Is that correct. The 22Pf and the 10M, I would remove the 22Pf and jumper the 10M. I can't tell for sure, but whatever that very first resistor is, I'd jumper it.

 

[PurpleStrat]

Sorry I have not added much more to this. I have done quite a few mods already.

Bright caps gone: Major improvement! Takes pedals well now and has more bass.

Changed the .022 tone stack caps to Mallroy: Did not hear any change but I had the caps so why not.

Changed the two 100K plate resistors to carbon comp: I plan to do the 220K but don't have one that will fit. Not sure how to explain it but it sounds better with them.

Changed the 470pf coupling cap to Mallroy .001: Lets more bass in and sounds better over all. Really changes the amp though.

The thing I am trying to figure out now is how to get rid of the static ghost note fizz that seems to be present in thia amp and The Vox Night Train. Not sure what causes it. It sounds like the clip in the first post of this thread http://music-electronics-forum.com/t27536/

 

[Ike286]

If you have funny fizz, IME, replace the 47PF on the tonestack with a new one. Usually it's a treble cap. I have had that several times. And again, I don't know exactly what those values are on the input jack, but that resistor that looks like a 10M in the picture, and the 22pf cap, I would say remove the cap, and jumper the resistor, but don't touch the 1M or the 27k. That would probably be quite beneficial to you.

 

[PurpleStrat]

If you have funny fizz, IME, replace the 47PF on the tonestack with a new one. Usually it's a treble cap. I have had that several times. And again, I don't know exactly what those values are on the input jack, but that resistor that looks like a 10M in the picture, and the 22pf cap, I would say remove the cap, and jumper the resistor, but don't touch the 1M or the 27k. That would probably be quite beneficial to you.

Thats a great idea and i'll try it tomorrow. The 22pf is over a 10k. This circuit has so much treble and gain that I haven't thought of messing with the input to much. I did change out the metal 10k and 27k with a 10k carbon comp and a 22k carbon film just becouse I read some where that they sound better there.

 

[Ike286]

On the input, the stock values of 1M (leave that one alone) and 27k will sound fine. I personally use a 33k, but to follow suit, you could use 68k. But the deal is that with the dual input amps, (fender amps etc.) when you are plugged into input one, you get about 34k, so the high input would equal 33k instead of 68k, besides I always used 68k and found that 33k had a little more clarity and sparkle. Anyway, just leave the 1M and the 27k alone. I personally would replace the 10k with a jumper wire, and get rid of the 22pf, but if that renders not preferable results, just put them back. Getting rid of those will make it more like the vintage voxes (regular design standards for any amp for that matter, my suggestions are based off of experience and precedent for 60 years on this part (not 60 years experience, just precedent there... LOL)

Anyway, this shouldn't be shrill or anything, maybe a tad bit bright, but Vox top boost amps have a lot of treble anyway, but they sound great. I have other suggestions in case that is too bright of course. And you could have a really cool amp. BTW, I would suggest putting the 470pf cap back (or a new nicer quality one just in case) and then reduce treble elsewhere. Also, you should try making a pseudo cut control. Like on the AC30, there's the treble cut control, you should try making one here. Of course the ac30 (or 15) are Push Pull amps with phase inverters and there has to be a phase inverter to have a real cut control but that's why I said pseudo. Because that is a big part of making the Vox top boosts not so over the top trebly. So to do that, I would say to try a pot about 500k connected to the master. Top of the 500k connected to the wiper of the master, and the wiper of the "cut" control goes through a .0047 to ground. That should get you kind of there, that way your treble won't be too bad, but that could also just be an annoying tone killer, you never know. Or a way you could deal with other troubles is to put a cap in parallel with one of the plate resistors. Try a .001 piggybacking the 220k on the first triode, that should kill some nasty treble. Oh well, you can play around with all the values I've stated, they are pretty flexible ones here, but the input jack values, I'd say to do basically what I said.

Hope some of this helps. I built a pretty true to schematic ac15 top boost clone, and it sounds great. I'm sure most of that mojo is findable here.

 

[PurpleStrat]

I generally agree about the simplicity of vintage vs. The over engineering on newer amps but I have keep just because I wonder if it really needed it. It seems that matching the trebley TB circuit with a single el84 could be problematic as is but they also gave this amp more gain than the AC15 I have tried with TB.

Don't get me wrong this amp can sound great. I do think that they may have tried to make this amp appeal to modern players by giving it more gain but also made it sound small wich also could have been on purpose. It has a decent sized OT but sounds small through a 12". On the other hand it can sound good and have a great vintage charm with the small cab and 10" that goes away as you crank it and start to notice it's limitations. I really think they designed it with living room and bedroom players in mind.

 

[PurpleStrat]

OK I found a 71pf mica so I took out the ceramic 47pf and the decay fizz is still there. I then took out the 22pf on the input but the amp did seem noisier when turn up so it's going back in.

The fizz does seem to respond to the tone stack so I would think it has to be before it right? It also gets worse the more you dig in to the strings.

 

[Ike286]

Was that fizz there before or after one of your previous mods? It could be some mod you did to it and accidentally did something else without knowing it. And you could lower the gain pretty easily by changing the values of the voltage divider after the treble pot. There's a 220k and a 47k, if you raised the 47k to about a 220k, that should give a small raise in gain, if you lower the 47k and make the 220k higher it will give less gain. Also, on the last 12ax7 triode you could take the 22uf cap off of the cathode, that should give you a considerable loss in the gain territory.

 

[PurpleStrat]

Tried removing themfirst cathode cap but no help. I also put back in the 22pf.

Yes the decay was there from the begining but I did not notice if as much. I put back in a 100pf bright cap on the master and it reduced it or at least covered it up more. So it is there but it blends.

 

[PurpleStrat]

I cliped the 22uf of the last triode and it really did not lose much gain but sounds a bit better overall. For lack of a better word it sounds less congested. Still the fizz decay is there.

I raised 220k on the voltage devider to 470k and it made very little diffrence. Also I noticed that when I measure the resistance of the 220k while on the pcb it changes when turning the treble control. Why is that?

Raised the 27k on the input to 35k and it helped a little with the fizz but only on lower gain. It also warmed up the cleans in very pleasing way. I think I will try to get it as close to 68k as I can then disconect the 22pf and make it more old school.

 

[Ike286]

OK, your results sound right. The 220k on the voltage divider, will probably have to go up to a 1M to be very noticeable, in which case you are probably best to try reducing the 47k. That will reduce the gain, I'm not sure how far you could go without the tone becoming undesireable. If it did become weird but you liked the gain level, I could probably help a little bit in finding some other fixes for that. The other thing is that on your treble control, the bottom of the pot goes to the bass control which is connected to ground, so when you turn the treble pot to zero the wiper of the pot should have 1M resistance to ground, so when you turn the treble down, that 220k will be reduced because there is another path to ground, other than the 47k right after it. And this is normal, don't worry about it.

And the fizz decay... Try taking out all the other cathode caps one by one and putting them back in. If you take one out, and it stops the noise, then that's the one. You could also try a new 220k plate resistor on the first stage. You could also try replacing the power supply filter caps, one by one, with a good one. Those would be my guesses. Also check on all the pots for leads of wires/resistors/caps that go through the hole in the pot, and comeout the other side a ways, and haven't been clipped. Like if you unhooked the bright caps only on one side of the pot but you left them soldered in on one side, that could be a problem, same thing with just wires left hanging... Check lead dress in general before doing the cap suggestions though.

 

[PurpleStrat]

It's all pcb. Only the transformer and heaters have wires so my remaovals are clean.

I have changed the 47k up to a 220k and all th3 way down to a 220r (yeah I grabed the wrong one). Raising it increased gain and bass but added noise. Lowering it took out some gain but tightend the amp up to much.

II am going to try a carbon film 220k on the plate next and I will take you advice and try taking out the cathodes one at a time. The power caps will be trickier since I have nothing that size to replace, them with. I may have to order some. I'm thinking I wouldmraise them to 22uf.

 

[Wally]

Fizz decay...have you tried another EL-84? They are infamous for this trailing 'fizz' thing.....or sub in some 12A_7's. That could be the source of that fizz, too. Tube subbing is much easier than circuit changes, and I always make sure the problem is not in a tubebefore I go off into a flimsy PCB amp.
And can you explain to me how removing a 100pfd and a 120pfd cap can increase bass?

 

[Ike286]

That's true, somehow I completely forgot tubes in all of this.............

And Wally, I found with bright caps, they don't increase bass, but when you remove the bright caps, since the treble has decreased, the amp seems warmer and the perceived bass is bigger. I think that's what he's trying to say, or at least something like that based off of an uninformed blogger, who doesn't know anything...

Putting a 220k in for the 47k will increase gain for sure. Lowering is what you need. Did you try replacing 47k with a 15k, or a 25k. Those values would get you closer to what you are looking for I think.

And Wally, in case you missed it, some of these other suggestions are just for gain level and tone, not for fizz.... Although once again, now that I think about it, we should probably go for no fizz, then tone hints... And also, I found that in many newer amps, that PCB's aren't "too" fragile. It's not like working on my SD1 which is horrible for burning traces while modding...... But I haven't worked on one of these, although I did work on the AC15 version of this amp. The handwired one, with the nice beige grillcloth. Which I wasn't impressed with, as many of the soldering joints were BAD... Oh well. Anyway, hope tubes might help you out, and that you might read this whole thing and understand any weird dyslexia/typos that could be there.....

And please if anyone thinks I may have said something that doesn't make sense, please correct me... hate to give bad advice.

Anyway, yea the El84 fizz thing... I only had that once, out of a lot of 84 amps so I always forget about it... That's a good chance. And I wouldn't raise the filter caps, if you did somehow discover that one is the problem, stick with the original values.

 

[PurpleStrat]

About the bass it is just precived I'm sure but when you put a Big Muff through the bright caps and then without them whooo! So a better way to say it is you can hear the bass more.

I have tried numerous 12ax7s and el84s. Tone changes dramatically but the fizz remains.

Not sure if you saw my edited post above about the input but I raised it and got some warmth that the amp needs so bad. Warmth is what the amp really lacks. I know Vox has a lot of treble and can be shrill but besides the decay the amp has just seemed cold. Even just increasing the 27k to 35k has made the cleans sound awesome.Bright but warm. It's the best mod so far besides losing the bright caps.

I went with a 22k in place of the 47k for couple days but went back as it just felt to tight sounding.

Right now what I want to do is find whats causing the fizzy decay first and foremost. Then increase the input to good place and try to get the overdrive sound as warm as the cleans do now. At one point I wanted more gain but now I just want it sound good even if I lose gain.

 

[PurpleStrat]

So I will just go a cap at a time and see if thats the issue and change the 220k plate resistor to carbon film. I'll keep the caps at 10uf unless anyone thinks I really should change them.

BTW sorry for any typos or weird typing. I only have a tablet right now and I am not good at typing on.

 

[Ike286]

The input jack 27k I assume? That's good. If you use a 68k it will drop some more highs and will warm a bit more. But I wouldn't go any higher. It could start getting dull. But anywhere between 27k and 68k is a nice place to be with that resistor.

For the fizz, look at EVERY ground in the amp. Make sure that all the soldering connections are good. Make sure that the input jack and pot nuts are tight.

And for warmth, you could change the 5k6 on the EL84 for a 8k2, that will help a bit. The other thing, is you should try your amp through a full size 1x12 extension cabinet or similar. That will probably make your amp sound WAY bigger/fuller. There are three other things Ill mention now that you can do for more warmth/bass. Replace the 220uf cathode cap on the el84 with a 500uf/50v. and there are two caps that go to the bass control. both of them are .022's. The one that goes to the treble and the bass control, the one on top. Change that to a .1 for more bass. It will warm up a little bit. As I mentioned before, you can put a 1000pf (.001uf) across any of the plate resistors to get rid of some of the highs. And even though this will change the character, you could also change the 220k plate resistor for a 100k. I would leave it for this especially since it is a vox, 220k plate resistors add some of the voxiness in amps.

But still save all this for after the fizz is fixed.