VOX: Vox AC4 C1-12 Guitar Amplifier

I picked up a Vox AC4C1with a 12" speaker recently, as it was selling really cheap second-hand, and it looked like it was straight out of the box.  I have a sneaking suspicion that the previous owner hated it - he seemed glad to be rid of it.

This post started short, and then became quite long as I was researching upgrade/modding options and jotted down notes along the way.  Maybe this will be useful for someone else too.

Note: comments below relate to the stock amp - no mods.   I wrote this post just after buying the amp.  

VOX AC4C1-12 GUITAR AMPLIFIER

How does it sound?

It sounds truly awful running at low levels - I think this is what has earned its bad reputation on several forums.  However, this little amp sounds pretty damn good when you crank it.  Turn the volume up, and set it on the edge of breakup (or further) with the gain, and the amp comes alive.  

I was running the volume at full for a decent amount of time - I measured 110dbA at one metre on the leq setting or my meter (that's an average, not peak).   I don't think the previous owner gave it much use, so I thought I should crank it to break the speaker in a bit, and it was a fun way to get to know the amp.  

I think some people assumed that the AC4 would be good at low levels because of the low wattage, and the spiel from Vox refers to it as "Famous for its ability to achieve classic VOX tone at low volume levels" which should perhaps include the disclaimer, when compared to how loud a cranked AC30 is.  If you want a small Vox amplifier that can sound good at low levels, buy a Pathfinder 15R or something along those lines.  

VOX AC4C1-12 SPECS

•         Gain, bass, treble and volume 
•         2 x 12AX7, 1 x EL84 (solid state rectifier)
•         4-watt RMS into 16ohms
•         Weighs 10.6 kilograms
•         Vox VX12 12-inch speaker. Which is a Celestion Seventy 80 more on that below.

VOX AC4C1-12 MODS

Warning number 1.  Valve amps contain lethal voltages.  Don't go poking around in there if you are not sure of what you're doing, as you might just end up dead.

Warning number 2.   Upgrading cheap valve amps can be an expensive exercise. 

Bright Caps

OK, so if you have read anything about Vox AC4 mods, you’ll read a lot about bright caps.  People buy the amp, decide they don't like it, and then try and mod their way out of it.  I think these are the same people running it at low levels and regret their purchase and then desperately try to fix it (huge assumption on my part, but there it is).  

The caps in question are C6, the 120pf cap across pins 3 and 2 on the gain pot, and C20, the 100pf cap across the same pins of the volume pot.   If the description I just gave and the schematic don't mean anything to you, the small capacitors are similar to a treble bleed on a guitar volume pot - when the gain or volume are low, the caps let high frequencies through.  The effect reduces as the pots are turned up.  

C6 is present on the gain pot of a regular Vox Top Boost channel.  As far as I'm aware, C20 on the volume pot is not used on any of the traditional line up of Vox amps - in fact the opposite occurs with the AC30, with a treble cut following the master volume in the power section.   

The Cab

After bright caps, the next "fix"' for the AC4 is taking the back off the cabinet, or enlarging the bass port (which is tiny).   I tried it - taking the back off did make a difference, but not as much as I was led to believe it would on the forums.  It was more open sounding - there's a surprise.  I haven’t had the amp for very long, I’ll need to test this in more acoustic environments before I make a final decision on this.  

The Speaker

The VX12 is actually a 16ohm rebranded Celestion Seventy 80 which is described by Celestion as detailed and crisply defined with a tightly controlled low-end and punchy aggressive upper mid-range. The top-end response is also subtly increased. 

The Seventy-80 is a much-maligned speaker, with ample hate online compared to Vintage 30s and Greenbacks etc, although they do have their fans, and a fair number of people thinking they don't deserve the poor reputation they have.  

If you have just bought this amp or have purchased one second hand with very little use, the speaker will not be broken in.  Get some solid hours on it at a decent level to loosen things up before deciding you need to replace the speaker (over 20 hours).   It's not the high end that changes, it's the mids and lows that start to breathe - the balance will shift, which may help with one of the complaints about the sound of the amp.  

Don't take my word for it, check this from Ignazio Vagnone from Jensen speakers.  This advice is not specific to Jensens, according to Ignazio, all speakers, all brands need break in.

If you still don't like it after breaking it in and decide to try a different speaker, it's important to note that there's very little space between the output transformer and the speaker magnet - we're talking about 10mm...  a lot of common speakers will not fit without moving the output transformer.  There is room to do this, however the leads connecting the transformer don't allow much movement, at least on mine anyway, I'm assuming others will be the same. 

As a reference, the Seventy 80 magnet is134mm wide and the speaker is 119mm in depth. 

The Output Transformer 

The AC4 has an unbranded output transformer.  Some say that upgrading to a Hammond or similar transformer will remove the fizz that can be heard at times. 

Hammond 125BSE (rated 5 watt)

Hammond 125CSE (rated 8 watt) 

The internet says the stock transformer has a 10k primary running the 16 ohm speaker (forum post, not an official Vox spec).  Others have mentioned that an EL84 likes to run with 5k on the primary, as per their datasheet.  Either way, the Hammonds provide a lot of options.

Valves

Like any amp, swapping valves may or may not make much difference.  I think despite the stock valves just being labelled as CHINA, there's nothing inherently wrong with them.  For me, if I'm swapping out or replacing valves, I buy NOS or vintage ones that test well and are reasonably priced.  Why - cause I like reusing old things.  I never spend more than the price of a new valve on an old one.

Most modern valves come from just a handful of remaining factories.  Popping Mullard on a new valve and selling in a box with vintage style artwork is just marketing.  

So what’s a stock AC4 good for?

I think it would make a great little recording amp, if you’re chasing the sound of a Vox working hard.  Possibly OK for small gigs if you only have variations of one tone, with that tone being a juiced Vox.

Pairing with another larger AC for a stereo rig.  Paired with my AC15 with a Alnico blue it sounds great.  The larger AC lifted the smaller amp, and the AC4 was in no way detrimental to the AC15.

What is it bad at?

It does not sound good at low levels, the clean tones are especially bad..

How does it compare to the 10" model

The 12" version is noticeably better - do not buy the 10" AC4 unless you plan on plugging it into an extension cab.  The 10" is really boxy sounding.   They're tempting cause they're often going cheap and they look cool.

What mods have I done?

Note:  all comments above relate to the un-modded AC4. 

Mods that I've done so far;

• replaced the stock speaker with a MSP vintage alnico speaker
• cut the C20 bright cap on the volume control
• swapped a pre-amp valve for something vintage I had laying around
• replaced the output transformer - details here

It's a much better sounding and more versatile amp now - still very much a Top Boost Vox.  The bright cap on the volume was only a minor change, as I don't run the amp quietly, and it's bright enough as it is for me.  Preamp valve change was noticeable, but not major.  

Collectively the biggest result was the combination of the output transformer and the vintage alnico speaker.  Apart from the cab size which is inescapable, the combination of an EL84, the speaker and output transformer really have a lot to do with the voice of the amp.  

For me now, it sounds how a low wattage amp should.  There's no fizz, it sounds bigger and more open.  The cleans are better and the breakup is smoother.