Vox AC30 Custom Classic Common Issues

Lyle Caldwell

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When I did the thread on the Class 5 people asked if I could share common issues with other amps.

So I'll be doing a few of these as they come to mind.

I work on a lot of Vox AC30s, from the '60s to now. They all have peculiarities. I'll start off with the Custom Classic series.

First, I'm not slagging this amp in the slightest. It's extremely well made and designed and can sound fantastic. In many respects it is a much better amp than the previous made in UK TBX series (I'll get into that later).

But it has a few drawbacks.

First, there was so much chattering on the net about "jangle" and "sparkle" and "Beatles" that Vox voiced this amp differently from other AC30s, to appeal to those who thought the amp should sound a certain way (even though AC30s don't sound that way at all).

History - the Beatles didn't use AC30s after the first record. The "jangly" Beatles sound was AC50s, AC100, various solid state Voxes, and Fender amps. The Byrds used a combination of DI and Twin Reverbs for their "jangle".

The AC30 has never been a "clean jangly" amp. In their natural state they are a wonderful amp when clean but they have low clean headroom and the real magic is when they are at the edge of clean and overdrive, where they break up and clean up responding to playing dynamics. Don't think Beatles/Byrds - think REM, U2, Yardbirds, Zeppelin. Not too dissimilar from the cleans coming from a Plexi, if the Plexi was being run into an open-backed 2x12.

Anyway, Vox voiced the Custom Classic to have a lot of clean headroom (too much IMO) and to be bright (too bright IMO). This especially makes the stock amp not take drive pedals too well as the overdrive sounds fizzy into such a clean bright preamp.

Here is how to make the amp sound/behave like a "proper" AC30.

1 - R6 and R7 are the plate resistors for V1 (Top Boost and Normal channels). In the Custom Classic they are 100K resistors. Change these to 220K resistors.

2 - there is a 120pf bright cap (C13) across the Top Boost volume knob. The "vintage" spec for this cap was 100pf, but to sound more like a 40 year old AC30 with 40 year old speakers I find that 68pf is a more pleasing value.

3 - Vox went with a 56pf treble cap in the tone stack (C16). Changing this to the "vintage" 47pf value very slightly reduces the treble.

4 - they have made the Normal channel fairly useless in tone (way too dark or way too bright) and limited its max gain. Jumper the 330K resistor ahead of the Normal volume pot (R49), change the 220pf bright cap (C12) to a 100pf cap, and then put a 68pf cap across lugs 1 and 2 of the Normal volume pot. This will allow the Normal channel to have more gain and go from a warmer but not too dark sound to a brighter but not too bright sound.

5 - Vox effed up the effects loop due to a typo. In many Custom Classics there is a drastic loss of low end when the effects loop is on. Many people think "the loop sucks" as a result. It's actually a fantastic effects loop once one capacitor is changed.

The schematic had a typo - they put .22uf where they meant to put 2.2uf, and the factory followed suit. So all the earliest runs of Custom Classics have .22uf caps on C109. If you change this one cap to 2.2uf or larger (4.7uf, 10uf, whatever) all the low end comes back and the effects loop is absolutely great.

I called this cap to the attention of Vox a few years back, and for a while the factory was using the correct larger cap here. Then someone at the factory messed up a memo and the larger cap was being used for C108, not C109, and the low end problem returned. The larger cap is fine for C108, but C109 has to be larger than 1uf for the lows to be unaffected.

So, look at the board on your amp that has the effects loop jacks/switch. Find C109. If it is a yellow box cap, it needs to be changed. If C109 is an electrolytic (looks like a battery, not a box) you're fine.

These are just a part of the mods I perform on these amps, but these are simple, inexpensive, very important mods that many AC30CC owners could perform themselves. So here they are.
 

cochrane

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Intriguing stuff. I wonder how much of this might still apply to the AC15C1 that came out last year?
I've one right now, but all I've really come across in terms of mods is this guy's efforts so I guess it just isn't attracting enough tinkerers yet.
 

Lyle Caldwell

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I haven't had any of the new Custom series come through yet.

Looking at that guy's mods, I don't much care for the sound of the TB channel with no bright cap - 68pf sounds like the old ones. The rest is pretty standard. If the new Custom series has the 100K plate resistors those would need to be changed to 220K.

When one comes through I'll post.
 

WhippingPost

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I've seen all of this info posted by you elsewhere, but thanks for bringing it here. :thumb:
 

nedhogan

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Hi. These mods have made my vox sound pretty darn good. I liked it before but now basically I can get more gain out of it. I will say though, replacing r6 and r7, and then replacing the bright cap C13 with 100pf does not change the brightness of the amp therefore an off the shelf ac30cc basically is a copy of the old jmi's, just with less gain on tap. I put 100pf in because I wanted it accurate to the originals. Doing these 3 mods has really made both channels come alive. If you think these amps are too bright 68pf or something around 80pf might be best but when they are cranked the power section loses top end anyway. The normal channel is now useable! r7 is the only mod I done to the normal cannel.
Word of warning though. I chopped and changed r6 and r7 between 220k and 100k once or twice and it really sounds lifless with the 100k in comparison BUT the cheap circuit board has suffered for it!! so be carefull. the copper track has became detatched around the mods! the 1w 220k cc resistor has thick wires and can catch the track when you put them in, and god those circuit board screws!!!!!! lost a washer off one inside there somewhere!!?!!!
If anyone has any advice re fixing the detatched track I'd like to hear it.
 

Lyle Caldwell

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Glad you like them. 100pf leaves the TB channel still too bright IMO - 68pf sounds more like the JMIs.

The traces, well, there's a reason I use a variable temp soldering station. It takes experience to know what temp to use for different things, and with a fixed voltage iron sometimes a trace will lift.

The repair for a lifted trace is to run good quality insulated wire from point to point below the board.

Go ahead and do the other mods to the Normal channel I listed - really makes a great difference.
 

Liam

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First, there was so much chattering on the net about "jangle" and "sparkle" and "Beatles" that Vox voiced this amp differently from other AC30s, to appeal to those who thought the amp should sound a certain way (even though AC30s don't sound that way at all).
Having owned a couple of mid to late 60's Top Boost AC30s, and with all due respect, I'd beg to differ. The harmonic overtones of these amps reached the point where they just plain pissed me off, jangly wasn't the word, more like cut your head off. Great little amp, but a non top boosted cherry panel made me realise why the top boost circuit had been invented. Before the top boost the ECC83 amps just sounded dead at the top end. I think Vox went a little overboard with the top boost circuit, and I probably could have lived with a cherry panel and a Rangemaster.

I've not tried a CC yet, and would be interested to do so. I've had a few AC30s over the years, but there's probably a reason wy I don't have one now.

Liam
 

Lyle Caldwell

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Liam,

I know what you're talking about but the bright jangly sound doesn't happen at stage volume - by the time the amp is really loud enough to sit with drums there's a lot of breakup and the amp isn't as bright. Which I love.
 

nedhogan

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When I think of the Vox sound, I think of the very first footage of the Rolling Stones, very swampy overdriven dirty sounding. Very early Stooges too.
Im not keen on what U2 do with them.
Thats just me though. They are very different styles.

The thing is its harder to replicate the early stones sound at home, whereas I imagine the U2 sound is'nt.

At home its almost too bright but at some especially loud gigs I cant get enough top end. Thats tube amps for ya tough. A Dan CTO-2 helps though.

Also the accutronics reverb upgrade is an improvement. Type4 600in 2250out although 2500out would be louder.

As for the subject of od pedals. Some people think ac30s dont like em but mine loves them. The Lead/TB channel of any amp loves OD pedals. I think its impossible to go from super clean to overdriven on any amp though, especially on a single tube channel. Maybe those mods to the normal channel will change that but I doubt it, not without multiple gain stages.
 

Liam

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Liam,

I know what you're talking about but the bright jangly sound doesn't happen at stage volume - by the time the amp is really loud enough to sit with drums there's a lot of breakup and the amp isn't as bright. Which I love.
Thinking back, yes, I know what you mean. But they are still to bright for me with the TB circuit, even when cranked. Definitely a unique old amp. I'd really like to try an EF86 version to see if I got on with it any better.

Liam
 

D_Strasse

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Lyle,

I really appreciate these threads and your input. Keep it coming and thanks.

(Also looking forward to your thoughts on an AC15C1 if you come across one.)
 

nedhogan

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I've always thought the op amps in the ac30cc signal path are something that shouldnt be there, one mixes the channels and one seems to serve no purpose at all as the reverb is mixed just before it. Can anybody suggest another method? Would removing it and its feedback resistors alter the volume at those points? If so exactly what could go in thier places. Would the two channels be out of phase?
The Deluxe reverbs channels just connect together with a 0.1uf cap.
I'll try and upload a schematic image of the opamp.
 

Lyle Caldwell

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Don't do that!!!

First, that is a really really good opamp run at DC voltage rail to rail. It's pretty much "invisible" to the circuit. It's a good implementation.

What the opamp sections do is allow both the Normal and Top Boost channels to be mixed in phase with each other (in old AC30s the two channels are out of phase with each other) and then to allow the loop and reverb circuits.

I have done comparisons with hard-wired switches taking the opamps/loop/reverb out of the circuit and there is no tonal improvement. If you don't want the loop or the reverb, just don't use them.

But if you want to turn the CC into a "more pure" AC30 without the opamps, don't mod the opamp sections. Just bypass them. Take the outputs of each channel and bring them to either side of the PI inputs (if you want the channels in phase) or to one side (out of phase, just like on the older amps). Easily done at the coupling caps on the board.

If you do that you will give up the loop and the reverb (loop's great, reverb sucks) but also the ability to use both channels at once in phase with each other (if you really go "old school").
 

Liam

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What the opamp sections do is allow both the Normal and Top Boost channels to be mixed in phase with each other (in old AC30s the two channels are out of phase with each other)
Didn't know they'd inverted one of them. Seriously good idea, as when you hear two channels that need mixing to get a great sound, and find they're out of phase, it can make you hit yourself on the forehead pretty hard and shout DOH! Been there a long while back...

Liam
 

Thundergod

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Lyle, thanks for posting this in here as well (I had read it somewhere else before).

Just one question, is there a particular place where we should buy the parts required for all the mods? Or can we just go to the neares radioshack and get whatever they have in there?

If you know of a place that sells online and is willing to ship internationally I'll be grateful for life!:thumb:
 

Lyle Caldwell

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Lyle, thanks for posting this in here as well (I had read it somewhere else before).

Just one question, is there a particular place where we should buy the parts required for all the mods? Or can we just go to the neares radioshack and get whatever they have in there?

If you know of a place that sells online and is willing to ship internationally I'll be grateful for life!:thumb:
mouser.com, tubedepot.com, tubesandmore.com
 

Thundergod

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...so I'm looking at caps and resistors online and I get lots of "models" and materials :shock:

What should I look for? Metal, carbon, metal film, paper, and other stuff that I think are models and/or brands... sorry I don't really know a thing about this so be patient :laugh2:

Should I go with whatever I find first or are there any brands/models/materials that work best for guitar amps?

Thanks in advance.
 

D_Strasse

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...so I'm looking at caps and resistors online and I get lots of "models" and materials :shock:

What should I look for? Metal, carbon, metal film, paper, and other stuff that I think are models and/or brands... sorry I don't really know a thing about this so be patient :laugh2:

Should I go with whatever I find first or are there any brands/models/materials that work best for guitar amps?

Thanks in advance.
With resistors, I believe specs/contruction/tolerance matters much more than manufacturer.

Maybe someone with more experience (or Lyle) can post as to what would be best along those lines for this project.
 


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