Vox AC4C1 schematic and mod ideas

[Chris Twang]

With my AC4C1 I had this fizz, too. I snipped C6 and C20 and changed C5 for a Silva Mica (I was in a hurry and snipped C5 instead of C6 first ;-)
I swapped tubes for an NOS Philips 5751, a Tube-Town 12AX7 and a TAD EL84. I also swapped the speaker for a 10" Celestion V30. The fizz is gone. But I didn't do one step after another so I can't tell you what caused that fizzlesizzle...

 

[metamp]

First thank you all for posting all mods
I just bought his amp and it sounds bright indeed so I may remove the bright caps. but what really bothers me on this amp is the lack of gain and no FX loop

I want to add FX loop so I can add delay and EQ after the preamp so I may not need need to remove the bright caps. I'm not sure where I would add the FX loop. my guess is somewhere before 200k volume pot.

Now the gain, I don't know what to do here I tried this amp with emg81 pickup and od 808 in front of it and still doesn't saturate enough on othe low E. I'm trying to achieve some ZZ top or 80's metal distortion. the amp is close, the crunch channel is good but its not there yet.

I would welcome any suggestions regarding the gain and FX loop

 

[Ike286]

You are probably best just to get a different pedal. The 808 may not have enough gain to do it anyhow.

 

[metamp]

I'm trying to boost the output from the pickup not to get the distortion from from the pedal so my setting is drive 0 and output to max sort of an clean boost. I also tried to add second clean boost pedal but it just sounded horrible. so I'm not even sure if boosting the pickup output can get me what I want.

 

[Ike286]

Oh ok. Try turning the gain up on the pedal just a little. Is the amp cranked as high as it'll go?

 

[metamp]

yes my gain on the amp is maxed out there is nothing I can do get more gain out of it as is

there is a mod for ac15 that can be aplied to this amp and it suppose to add a voicing with increased top boost
I'm not sure if it will work

search for BITMO VOXMO15 MOD KIT FOR VOX AC15C1

 

[Ike286]

I'm curious how that mod works, sounds interesting. Read through this whole thread from start to finish. I remember making a lot of suggestions for different things and what stuff could be changed, you might find something.

 

[metamp]

I'm curious too if someone came up with some high gain mod for this amp

 

[Ike286]

I think that on the voltage divider coming off the gain control, it's like a 220k and a 47k. Try replacing the 220k with a 150k or so, and the 47 with anywhere from 220k to 500k. That should get some more gain. I don't know how much you'll notice.

 

[JohnnyCrash]

EL84s seem to have fizz issues a lot (it does seem to be during note decay). I know the guys at 18watt.com deal with it all the time on that old Marshall circuit. I tried chasing the fizz out of too many 18w amps and decided to forget about that circuit entirely. It makes me wonder if that's why AC30s have the Cut control right at the PI...

I love AC30s and AC15s, but I'm considering an AC4C1... maybe not so much after this thread.

 

[Ike286]

I know with really nice transformers (especially the OT) el84's almost don't fizz at all. It's a huge difference. It's all been discussed many times, especially at 18w.com, but I don't know if anyone makes any replacement transformers for the AC4. It's probably a bit expensive to do, but I'm sure could make a big difference.

 

[bparnell57]

I say look for a nice hefty output transformer out of an old reel to reel or console amp on eBay, and then get a filament transformer and add a pentode preamp as another input. That would be pretty sweet.

 

[PurpleStrat]

Just to update here I got rid of the fizzy decay problem I had, It was one of the cabon comps I put on the plates or my solder joint on it. Changed them back to carbon film and I am good to go!

I also put in a Greenback for a huge improvement overall. It still lacks bass even through a 12" and I can't figure out why but it sounds great with all the mods I have done. The feel and the dynamics of this amp could blow away amps that people pay thousands for with just a few tweaks.

As for anyone looking for highgain, forget it. It will sound bad and fizzy if you up the gain much. I went through a time where I was trying to make this in to a Marshall but now I have a Marshall SL5 and I am glad that this thing is a Vox!

Oh and don't waste money on the Bitmo mod thing. All it is switching between different cathode caps.

 

[Ike286]

What you probably could do to make it a marshall, (i tried this on an AC15) change the .022 in the tonestack (not the one on the bass control) to a .001 and change the 10k in the tonestack to a 1k and change the 47pf to a 500pf. Believe me, it roooaaars. Just rips your face off. So it should sound pretty cool on an ac4. i changed some other stuff, but this is the most of it. But I'm waiting to find time to return it to ac15 values, to work on modding it for a vox sound still, instead of angry plexi...

Good to hear that you got rid of the buzz though. Probably part of your high gain buzz is just the single ended El84. Push pull amps are smoother (generally) than SE amps, so that may be a part of it.

One other thing that you might try is a celestion blue speaker. They sound soo good with voxes. The best I've ever heard so far.

 

[PurpleStrat]

Yeah it has a little fizz that comes with a single end but what I had was unnatural!

You could try the tone stack mods but if you have played this amp you'll know it has no bass. It must be the the OT. Plus it is so touch sensitive and ragged around the edges that it will never sound smooth and compressed like a Marshal. I tried quit a few things I listed in the thread and many I didn't and I have found you can make it sound great but it will always want to sound like a Vox.

I really believe that Vox meant this to be a practice or recording amp and may have mismatched the OT to handicap the bass. I can say I am much happier now that I am not trying to fight it. In fact I thought about getting an upgraded OT but decided not to since I am afraid it could ruin it's charm.

 

[Ike286]

You could always put the old OT back in if you don't like it. I have had good like with being able to return things to their good mojo of before the mod, so I usually so go for it..

 

[PurpleStrat]

I just might one day. Right now I have a killer 2 amp set up with this and the Marshall SL5. The one thing I may go for one day if I get a new OT is to try to get that Matchless type sound out of it but I don't know how. They have almost the same parts but Matchless has that velvet like sound compared to jangle of a Vox.

Here is what my mods look like now except I changed the carbon comps to film and am going to adds a speaker in on the back.

 

[pac90]

Many thanks for the tips in this thread. I have an AC4-1-12 and a PRS with 53/10 humbuckers, and was hating the sound on the bridge either HB or coil tap. It was shrill and brittle, far too bright. The neck pickup was great with the amp out of the box.

I took the leap and did the bright cap mod to both capacitors and it made a world of difference to having far too much treble on the bridge pickup. I don't have a lot of hours on the amp yet as its new, but after a time may upgrade the speaker from the VX to the greenback used in the more expensive hand wired one.

Its a bloody loud wee amp, I'm still playing with it, but right now prefer gain 11-12oclock and volume 12oclock +, and cut the guitar volume right down for clean tones.

 

[atomicbot]

I have an AC4C1-TV-MC which has the same "fizz" sound, and like the OP I only notice it with humbuckers (in particular when digging in and playing harder).

 

[Ike286]

Try replacing all the resistors around the first gain stage with the same value, but high quality types. That might help, otherwise contact the OP and see what he did. I can't remember now...