Resurrecting a Vox AC10C1

Recently I managed to score a real bargain. A non-working Vox AC10C1, certainly out of my available cash price range in full working order, but the cabinet looked good to revisit my intention to build an original AC4 style amplifier complete with EF86.

It arrived missing the power switch and having suffered a little bending to the metalwork having not been reassembled in a way that supported the L-shaped chassis, but after a quick dab around the chassis with a test meter it appeared intact bar the 'eco' board. Without getting into all the political debate going on about the EU, this is one of those well-meaning attempts to make the world a better place that doesn't quite work out. The function of this board is to power the amp down after 2 hours of no signal - which is kind of pointless really... yes on a TV, yes on a laptop, on a guitar amp? Nah...

So out it came.

A new set of valves went in from Ampvalves - (if the AC10C1 is not added yet you can use the Orange Tiny Terror 2xECC83, 2xEL84 set) - and the sound was there, although very quick to distort and surprisingly quiet. A second dab around with a test meter revealed 6V on the anode of one of the EL84's and a whopping 21K resistance on that connection on the output transformer.

The AC10C1's transformer is a tiny shielded generic unit which isn't really obtainable at a sensible price, so looking for a suitable alternative I ordered a Marshall 18W OT from AmpMaker. Being open it does pick up a little hum, and with its higher rating doesn't break up on the power section as early but wow, what a sound now its up and running!

The adventures of building a Tweed Princeton (5F2A)

I've spent my life being told how much things were better in the good old days of the sixties, the music, real guitars, valve amplifiers and so on. Well, I finally caved and took up the challenge on the issues of valve amplifiers. As for the music being better well... I think theres a certain amount of selective memory going on!

I've never bought into the folklore of valves - well not the fairy tales that say your sound will suddenly become golden, I'm a believer that something well designed engineered will do more for your sound than whichever technology is used. I had planned to set out to prove this by building a recording amp in the vein of the Tech21 Trademark 30, a good sounding transistor amp aiming to offer both vintage and modern tones but its there I hit the first hurdle - there seems to be little interest in the guitar community to investigate the development of such a beast, and little in the way of an active community discussing it. The fact that the MusicMan guitar amplifiers and LAB L4 exist to great acclaim proves that good sounds can be had from solid state.

So why not try the valve thing starting with the two 'families' I see as being the great grandparents of pretty much everything that followed, the Fender tweeds, and the Vox AC - specifically the Princeton and AC4. I bit off more than I could chew initially with the AC4 using this circuit which i just haven't been able to get right at all but I learnt a lot about thinking about your layout and some of the tricks like winding your heater wires with a drill so you can avoid hum.

A little disheartened I decided to aim for something that really is as simple as you can get - the Fender Champ, specifically the 5F1. With such a popular small amplifier theres a huge amount of available parts and kit, as well as knowledge and modifications. Its one of those modifications that really caught my eye, specifically the 5F2-A - aka the Princeton - the addition of a tone control! Figuring the Champ was so simple that even I could wire in an extra couple of wires, pot and a capacitor I started hunting for parts.

First up, a case - scored from eBay I found a rather sad looking tweed-alike case - I'd hazard a guess its one of the Juketone cabinets thats seen better days (if you don't fancy building your own 5f2-a take a look at there Juketone Royal Blood), but it has that used/aged/trashed look to it that feels right for such a build - more mojo apparently (ha!). It appears its been lacquered but the effect seems more like wallpaper paste to me! What they seller neglected to mention was that it was a Champ size panel originally but had been enlarged at some point and the tweed re-fitted. That meant custom metalwork - boo!

Next, a speaker, again eBay to the rescue - the good thing about guitarists is they always want the latest cool mod, I scored a perfectly good 10inch speaker from a Fender Frontman 25R that had been replaced with something that costs more than the amp. Of course being a modern speaker, its 8 Ohm.

Transformers I decided to buy new after misadventure with an undersized power transformer (which actually turned out to be fine - more later), opting for a toroidal from ampmaker.com for power, and the output transformer and miscellaneous sockets and fittings from modulusamplification.com - the benefit here being it offers both 4 and 8 Ohm speakers to be used.

The chassis was a struggle, for a first build I really wanted to stay 'real' but the case meant the fabrication of a wider champ style chassis, all of this was great for revisiting my CAD skills but ultimately was out of my skill level. Thankfully I got bounced out of my brain rut by my father into a simple L shaped base plate, with wooden pillars to form the sides. The control panel then sits on top of these pillars in a recess. The control panel is acrylic - this is definitely not a good material for a panel, its brittle to work with and it bends a little too much. Its also not conductive, so was coated with copper tape on the inside to help shield and ground the components. The acrylic sheet was then covered in neoprene (thats right... diving suit material!).

Ultimately I cheated on the circuit board. It worked out more to buy the board and components from Jellyfish Audio so I bought it pre-made. Its well made, and even with the 'cheap' components its perfectly good. The solid state rectifier had a bad joint which brought the undersized transformer back to life, it really does 'squeeze' at higher volumes adding back in a little bit of what a valve rectifier might have done. The LED light went as I found it just created two much hum and used a traditional bulb. Removing the LED and fitting a 'humdinger' to the 6V6 cathode made a big difference to the amount of hum, its not silent, but its 'vintage correct'.

So how did it turn out? Well, with the undersized power transformer I got a good feel for a clean tone that sounded very rich and warm, quite compressed. In comparison to say a pure solid state amplifier such as my Yamaha JX30 it sounds very dark and muffled, but theres a difference to the sound where it removes something that can get a little nasty. This makes positions 2 and 4 on a Start inherently much more usable which really does seem to be something valves bring out.

Testing against that 'modern evil' - the modelling amp, in this case a Vox Valvetronix AD30VT - I found I could dial in quickly a very similar sound using the Black 2x12 model with a pronounced mid EQ. Again though, the sound is a lot brighter and at least to my ear gives a best of both worlds sound.

Am I a convert? Well... maybe... theres something in the folklore after all.