Vox AC4 head I built from scraps

[crusty philtrum]

Hi everyone

A couple of years back i built a little combo amp for a mate of mine who didn't have much money. I used stuff i had laying around, the amp was the Vox AC4 circuit, which i have built many of, they sound great. I had old transformers, wood, aluminium for the chasis and a low-power Alnico speaker from an old radiogram. Sockets and components weren't too much trouble as i build and repair amps and have plenty of parts on hand.

The combo turned out quite well, though i was never particularly happy with the amp itself, didn't sound quite right to my ear. Nonetheless, my mate was happy (and why not?....it cost him nothing except he did choose to pay for a couple of brand new tubes, 1 x EL84 and 1 x EF86).

More recently he felt as i did about it's sound and he sent the chasis back for me to tweak a little bit. It sat here neglected whilst i worked on many other projects. Finally, out of guilt, i got to work, and fine-tuned it into a real performer. (This circuit rocks for a 4 watt amplifier !) While i was slowly trying 7 values/tapers for each of the volume and tone pots and experimenting with coupling and tone cap values and types, (very slow and tedious work) i started thinking how nice it would be to build a little head cabinet and send this back to him as a tiny 4 watt tube head. In my spare moments i searched and found almost everything i would need right here in my workroom.

When i was happy with the electronics, i went to work on a cabinet. Please keep in mind i have no workshop or garage or shed for doing woodwork, i just have a large kitchen and some crates and boxes to work on. The only power tool used was an electric drill. Most of the work is done crawling about on my hands and knees. BUT....i got a result, sent it back to him and he has been absolutely thrilled with both the new housing and the finely-tuned performance.

I don't have a camera, so he took some shots for me. To give you an idea of how small this is, it measured approximately 350 x 150 x 150 mm ( 13 3/4" x 6" x 6" ). It's just so cute in real-life, but sounds great and can push a 2 x 12" cabinet happily and is deceptively loud, but always toneful and musical. (The original AC4 had tube rectification and tremolo, but this one is SS rectified and has no trem).

See what you think of the look.....


(Edit : BTW, the control panel appears 'backwards' because in it's original combo configuration, the chasis was up the other way, with the tubes hanging down, Fender-style. I could have kept that happening in the head but i thought this way looked better, and as my buddy likes Marshalls......weeelllllll.......hehe)

 

[andrizzle]

Re: Vox AC4 head I built from scraps

looks sharp man! you're a good friend to do work for free... it's simple and classy looking... just how an amp should be

 

[jeremy]

Re: Vox AC4 head I built from scraps

very cool, id love to hear it. pure simple things like that usually sound awesome when done well

 

[crusty philtrum]

Re: Vox AC4 head I built from scraps

Well i'm just sitting here feeling a bit funny about 'going public' with these pics, so i thought i'd show ya's another one.

Another AC4 rig i built for a friend, an awesome bassist who started to play some guitar, but then at 39 was diagnosed with bone cancer. I had to keep going to Sydney to look after him, and built this amp for him 'on the road'. It was similar to the one above but all black and rather rough.

Within 12 months of his diagnosis, he passed away. He only heard the amp once, i remember him flying into the room on his crutches with a huge excited grin on his face when i first fired it up. I knew no-one in his family would want it and it would most likely end up on the tip when they cleared out his house. So i kept it, and over the years had scavenged parts from it and felt kinda bad, and promised that one day i would rebuild his amp, with a matching cabinet, and make it the best of the best. I stored away bits and pieces as i found them and built an image in my head.

A couple of years back i threw myself into it and did the whole lot. All that remains is to actually build the circuit inside !! To give you an idea of the size, the cabinet holds 1 x 10" Celestion. I just measured the rig, it's 600 mm x 400 mm ( 24" x 16" )

And the 'PP' logo on the front of the head? My mate's name was Phil Pittman.

 

[Rich_S]

Re: Vox AC4 head I built from scraps

Cool amps. I like that little AC-4 head.

After building my ECL-86-based "18-Watter" on a Bogen chassis, I have second, matching Bogen amp on the shelf. I want to turn that into a slightly underpowered, EF86-preamped, "AC-15" - one channel with no trem. That way, I can find out what all the EF86 love is about.

 

[crusty philtrum]

Re: Vox AC4 head I built from scraps

The EF86 is a great valve, somewhat dark but it gives notes a 'bloom', a blossoming effect, which is gorgeous but may not be ideal for metal. It was used as a preamp tube in many european tape recorders in the 1950's and '60's. I've been using it in guitar amps and mic and hi-fi preamps for the last 15 years or so, but it has seen a growth in it's popularity in recent times, mostly by the simple act of more people becoming aware of them (internet, anyone?) hehe.

The ECL86 type tubes can be very good, though their distortion characteristics are not very good, not very useful in any musical sense. The best way to hear things at their very simplest is probably the AC4 circuit, an EF86 preamp and an EL84 or maybe 2 for the output.

 

[Animal]

Re: Vox AC4 head I built from scraps

Looks like neat stuff!

The folks at http://www.guitargear.net.au/discussion would probably love to see your work.

 

[Rich_S]

Re: Vox AC4 head I built from scraps

The ECL86 type tubes can be very good, though their distortion characteristics are not very good, not very useful in any musical sense. The best way to hear things at their very simplest is probably the AC4 circuit, an EF86 preamp and an EL84 or maybe 2 for the output.

They were the tubes that came in my Bogen CHB20A, so I have them running as the PI and output stage of my "13 Watter" - wired up exactly the same as a normal 12AX7 PI - into - push/pull EL84s would be. Of course, it's biased a bit lower to account for the ECL86's lower overall dissipation rating.

I like it's distortion characteristics just fine, though I haven't A/B'd it with a similar EL84-based 18 Watter. Actually, a 20 Watter would be a better comparison, since mine retains Bogens stock voltage-doubler PS with SS rectifiers.

Sorry for hijacking your thread. Your amps look (and probably sound) nicer.

 

[crusty philtrum]

Re: Vox AC4 head I built from scraps

Hey, it's cool, i rebuilt a vingtage 2 x ECL86 Aussie-made combo recently, i was impressed by it's quality, it's cleans were great and it was mostly clean anyway, but what little distortion it had was not as good.

The ECL86 was used in a few budget amps back in the day, and then the 'next level' of similar amps would use 12A*7's, EF86's and similar pre-amp tubes, and EL84's for the power stage. The problem with trying to convert some old bits of ECL86 gear is that there often won't be enough tube sockets for extra preamp or driver tube(s) and also that the 12AX7/ EL84 type configuration will draw a lot more filament current and could exceed the rating of that winding on the power transformer.

I have a tiny two-tone Teisco amp i was given, it originally ran an ECL86, i restored it but soon changed the circuit to the AC4, although i changed *everything* in the amp, transformers, the whole lot to cope. Lots of work for 4 watts, haha !

 

[Philcguitars]

Re: Vox AC4 head I built from scraps

I think your handy work is probably worthy of a job offer from someone!

 

[Rich_S]

Re: Vox AC4 head I built from scraps

The problem with trying to convert some old bits of ECL86 gear is that there often won't be enough tube sockets for extra preamp or driver tube(s)

True dat. My Bogens have only three 9-pin sockets each. Two sockets are taken up by the ECL86s comprising the PI and power stage, leaving only one socket for the preamp. My "Marshall" build has a 12AX7 in there, wired so I can use one triode alone, or both in parallel, but either way, it onlky has a single tone control. Adding a normal TMB tonestack would require another socket and more heater current.

The AC15-ish build I have in mind would have an EF86 preamp, but with only volume and cut controls, no other tone controls.