DIY Vox AC30 Anyone?

robkay

I fancy building a Vox AC30 or Matchless DC30 head, anyone built anything along these lines.....must admit building a true layout AC30 looks a real pita.

Looks like someone makes a chassis......

http://www.ebay.co.uk/itm/Replacement-Vox-JMI-AC30-Chassis-repro-faceplate-Steel-Ali-Parts-New-/261496724659

Rob

Misterg

I researched making an AC30, and it looked to be quite an undertaking - that chassis would help a huge amount, but there are still a hell of a lot of bits in it. The parts wouldn't be cheap, either.

Good luck if you decide to do it - I'll be watching with interest

ecc83

If I had the time and energy to build a guitar amp from scratch I would not go "store bought" and re-invent the wheel.

I would make something new based on the ideas and circuits of Merlin Blencowe, especially the vast tonal possibilities of the EF86!

I would also go for very low noise ,( something I am a bit anal about!) using either parallel triodes or a cascode front end.

Other amp techs might like to chip in? (tho' there would be no chips in the audio path!).

Dave.

jpfamps

Most of the circuit in an AC30 is the tremolo/ vibrato function.

If you don't want this, then the layout is not so daunting.

The DR Z Z Wreck is essentially an AC30 top boost channel, and that's a fairy simple layout:

http://ampgarage.com/forum/files/thumbs/t_drz__zwreck_109.jpg

jpfamps

Misterg said:

I researched making an AC30, and it looked to be quite an undertaking - that chassis would help a huge amount, but there are still a hell of a lot of bits in it. The parts wouldn't be cheap, either.

Good luck if you decide to do it - I'll be watching with interest

EF86 and low noise.......

The major source of intrinsic noise in most guitar amps is Johnson noise from the grid stop on the first valve stage; reducing the value of this will have the greatest impact on noise, but may require an additional cap to stop RF ingress.

If you get the grounding right, noise isn't really an issue in a low gain amp like an AC30.

Regardless, noise rather becomes academic when you plug a Tele into the amp.

robkay

Thought about a version without the vibrato, that DR Z does look a lot less trouble......anymore info about on that one?

Rob

jpfamps

robkay said:

Thought about a version without the vibrato, that DR Z does look a lot less trouble......anymore info about on that one?

Rob

I'm fairly certain there's info on this on the web; I've seen Z wreck builds on other internet fora.

robkay

I'm looking.....

Thanks

Rob

Misterg

jpfamps said:

EF86 and low noise.......

Oddly, it was something along those lines that cured me of the AC30 GAS:

http://i255.photobucket.com/albums/hh143/werdnayarg/DSC_0237.jpg

http://i255.photobucket.com/albums/hh143/werdnayarg/DSC_0734.jpg

[ECC83] -> [EF86] -> [MOSFET Follower] -> [Single Ended EL84] -> [10" Greenback]
     +
[ECC83 Tremolo to EL84 grid]

My own design, incorporating Merlin's 'triode/pentode morph control', honed with many, many iterations.

Isn't the Z-Wreck a version of the Trainwreck Liverpool? (If so, the font of almost all knowledge is The Amp Garage - TAG [1] )

If I were to build an AC30, I think it should have the tremolo and vibrato circuits in, too.

[1] I may just have broken the first rule of TAG...

NervousJohn

Trainwreck Liverpool = AC-30 if I remember correctly.

I can also recommend Steven Grosvenor's "service engineer's guide to the AC-30" if you want schematics and layouts.

http://www.thevoxac30guide.com

Easily worth twice the £10 he asks for it. And a nice bloke too

jpfamps

I'm pretty certain the Z Wreck is single channel version of the AC30 Top Boost; the circuit layout would suggest this, although of course the component values are obscured.

The Liverpool from what I've seen is very different circuit, more like a Fender pre-amp with a 4 x EL84 cathode biased output stage. The Mid control is a bit of a give away.

|'ve used MOSFET followers to save a valve.

I've also used a MOSFET osillator for bias mod tremolo, developed from RG Keen's circuit, and this works well too and is cheap to implement.

I've not experimented with the EF86 "morph" control. 

We've had a few Selmers in the shop that didn't sound great when pushed into distortion and we have speculated that this is due to the EF86 in the second stage of the pre-amp over driving, although of course we have no evidence to back this up......

How useful/ good do you find this feature?

Misterg

jpfamps said:

The Liverpool from what I've seen is very different circuit, more like a Fender pre-amp with a 4 x EL84 cathode biased output stage. The Mid control is a bit of a give away.

....

I've not experimented with the EF86 "morph" control. 

We've had a few Selmers in the shop that didn't sound great when pushed into distortion and we have speculated that this is due to the EF86 in the second stage of the pre-amp over driving, although of course we have no evidence to back this up......

How useful/ good do you find this feature?

I was mistaken - it is the TW Rocket that is supposed to be close to the AC30 TB (TAG is still the place).

I tend to leave the morph control at the same setting (about 2/3rds of the way towards pentode), and have speculated about hard-wiring it and using the pot for a mid control instead. If I was building it again, I wouldn't bother with a front panel control.

IME, the EF86 sounds really bad if it is driven to cut-off (which is easy to do in the 2nd stage), and quite a bit of my tweaking was related to biasing it and the previous stage to stop this happening. It also sounds bad if the grid is driven positive, but thankfully one can get away with a huge grid stopper to tame this (no Miller effect).

Misterg

jpfamps said:

I've also used a MOSFET osillator for bias mod tremolo, developed from RG Keen's circuit, and this works well too and is cheap to implement.

I tried, but I couldn't get it to work without 'thumping'.

ICBM

I'm not really a fan of these "half-done AC30s". To me they don't have the sound of the proper Vox circuit, overcomplex and expensive Vib/Trem channel and all. Neither quite do modern AC30s that don't have it, for that matter.

I don't really like EF86s either despite their current fashionability. I don't think they sound at all good overdriven, and even as a clean first gain stage they seem to lack anything interesting, or can have a slightly metallic tone - I by far prefer a cascaded 12AX7. The one in my old Centurion amp is probably only OK because all it does it hit the EL84 very hard and all the tone comes from there…

Just my opinions of course.

It's also not true that an EF86 is capable of more gain than a 12AX7, as seems to be commonly said. It is capable of more gain than *half* a 12AX7, but that's not the point - you get two halves of a 12AX7 in the same socket as one EF86, and a cascaded 12AX7 has far more gain, as well as more possibilities for layout and function.

ecc83

The EF86 will not worsen noise if used as a later stage at a higher level and lower gain. It is a basic principle of all low noise design that  noise is determined almost wholly by the front unless you make a complete pig's ear of subsequent stages!

Noise determined by RF stopper: Agreed, replace mostly by an inductor (a la Self)  and, as said, an extra (to Miller) shunt capacitor. This would definitely be needed if a cascode were used.

Noise not a problem with AC30? Said I would not build an AC30! I did say I had a bit of thing about noise and I would love to find the time to experiment.

There are quieter guitars than Teles! In any case, guitars are not the only ways guitar amps are used. My interest is mostly in a recording, "studio" amplifier.

Dave.

jpfamps

Misterg said:

jpfamps said:

The Liverpool from what I've seen is very different circuit, more like a Fender pre-amp with a 4 x EL84 cathode biased output stage. The Mid control is a bit of a give away.

....

I've not experimented with the EF86 "morph" control. 

We've had a few Selmers in the shop that didn't sound great when pushed into distortion and we have speculated that this is due to the EF86 in the second stage of the pre-amp over driving, although of course we have no evidence to back this up......

How useful/ good do you find this feature?

I was mistaken - it is the TW Rocket that is supposed to be close to the AC30 TB (TAG is still the place).

I tend to leave the morph control at the same setting (about 2/3rds of the way towards pentode), and have speculated about hard-wiring it and using the pot for a mid control instead. If I was building it again, I wouldn't bother with a front panel control.

IME, the EF86 sounds really bad if it is driven to cut-off (which is easy to do in the 2nd stage), and quite a bit of my tweaking was related to biasing it and the previous stage to stop this happening. It also sounds bad if the grid is driven positive, but thankfully one can get away with a huge grid stopper to tame this (no Miller effect).

That confirms my (our) suspicions that the EF86 doesn't sound great when overdriven hard in a pre-amp.

DaveTutt

Hi

I have an AC30 that was badly damaged when it arrived at my workshop and was going to cost around 400 UK to fix. Since then I have stripped the bottom half of the cabinet as it was too twisted to use. The speakers too were far too damaged to be worth saving. So I have a doner unit with all the 1960's original parts but in need of a redesign and repair to make it more user friendly. 

My intention is to swap around the internals so that it can be sensibly used with a single input. The effective three channels will be merged such that there is a good clean channel in keeping with the original, a dirty channel sorely missing in this amp and then keeping the original (and horrible!) tremelo circuit in place. In the original that I have there is only a set of tone controls for one of the inputs which I think is a shame so the internal routing will allow these to be used on all inputs or turned off with a new foot switch arrangement. 

Anyone interested in buying it when its finished? I don't play but I am a repair engineer so usually know what I'm doing!!!

By the way the EF86 is not a good valve for guitar unless it is used in very low power amps and definitely not overdriven. It seems to work well in triode mode rather than pentode though as I discovered by accident when re-configuring an old Bush extension speaker amp for a tiny 3 watt practice amp with ECL86 outputs.  

ICBM

DaveTutt said:

I have an AC30 that was badly damaged when it arrived at my workshop and was going to cost around 400 UK to fix. Since then I have stripped the bottom half of the cabinet as it was too twisted to use. The speakers too were far too damaged to be worth saving. So I have a doner unit with all the 1960's original parts but in need of a redesign and repair to make it more user friendly. 

My intention is to swap around the internals so that it can be sensibly used with a single input. The effective three channels will be merged such that there is a good clean channel in keeping with the original, a dirty channel sorely missing in this amp and then keeping the original (and horrible!) tremelo circuit in place. In the original that I have there is only a set of tone controls for one of the inputs which I think is a shame so the internal routing will allow these to be used on all inputs or turned off with a new foot switch arrangement. 

Don't do that! If you have an intact original 1960s AC30 chassis, that's worth a lot of money to someone. Many old AC30s got butchered by bad repair work or modifications over the years, and that chassis will be ideal for restoring one.

You can blend all three channels on an AC30 very simply anyway, with two patch cables.

The original Vox Vib/Trem is a thing of wonder and beauty too .

DaveTutt

Actually it was in such a bad condition when it arrived that there was very little of it apart from the chassis and transformers that was really worth saving. I have put it back to working to see if it would do anything with an external speaker set. These old amps are okay but there really isn't much value in a Vox or any other amp for that matter that has stood in a damp garage for best part of 20 years with a rake through a speaker cone, speaker baskets twisted into the shape of the twisted cabinet and the voice coils rusted into the magnetic gaps. Many people love these amps I know but there does come a time when all the agro of trying to use it live means you will move away from the great EL84 tone just to get a more practical box. As I say to put it back as it should be would have cost £400 and as such you can buy another working one on EBay without the trouble of 20 years of rot!  

I know all about the patch cable process but again if you use it live it is less than helpful. The weight alone will put most AC30's for the average guitarist with a bad back into home ornament mode. 

Likewise the vib trem for my customers is something they used once or twice and then never used it again. Its an outdated effect that most of my customers would gladly see replaced by a good long spring-line reverb. Doesn't matter how good it is if its never used its a waste of 2 valves that can be better deployed! 

Of all the guitarists I repair for, only one now has a AC30 most of the rest having had their fling and decided that lugging one about is not what they want to do. Still most of my customers are old, been round the block, and don't want the strain on their hernias/ bad backs! LOL

Same goes for the similarly weighty Mesa Boogie lumps. If your a part time weight lifter then fine. But there aren't that many who are guitarists!

I am only too happy to repair these things but there is a cut off point which exceeds the value of the finished product. 

 

ICBM

Haha... I suppose that makes me a part time weight lifter . My Mesas are even heavier than AC30s . (Although with wheels and better handles, so arguably easier to move.)

I don't doubt the cabinet and speakers were beyond saving, but really think twice before modifying an original 60s AC30 chassis with intact transformers, many people would pay a good amount just for that to rebuild another one.

Seriously, I love the AC30 sound, I love the Vib/Trem - which is very unique to the AC30, not at all the same as tremolo - actually reverb is easier to do externally these days - and really does come into its own when you jumper the channels, since you can blend in just a little and still get the great tone of the other channels. Jumpering is easy and if you use short right-angle cables you can leave them in place permanently, so I don't see it as a hassle. I rarely run an AC30 without it.

Lighter and more practical amps just don't sound the same, which is why you see so many AC30s on pro stages even today. The only reason I sold my last one was because a friend really wanted it - I could easily see myself buying another at some point. Truly one of the great amps.

DannyP

DaveTutt said:

 As I say to put it back as it should be would have cost £400 and as such you can buy another working one on EBay without the trouble of 20 years of rot!  

 

Can we have links to these £400 working original 60's AC30s on eBay? I'll take two...