Vox AC15-ish amp

HamishR

Well-known member
Just finished this today. It's kind of an AC15, using a Mojo chassis, board and faceplates, some old Matchless transformers from West Labs and a Mercury Magnetics Vox AC15 OT. When built stock it was way too bassy. Maybe the Mojo transformers don't produce as much low end as the trannies I used? I dunno. So I have reduced some of the coupling cap values and used a 1µF cathode cap on V1 - that really helped get the amp into the guitar range.

Playing around with power tube cathode resistors and doing lots of maths got me biased to around 100% plate dissipation, so the driven sounds are glorious. Very Brian May at actually reasonable volumes. It's loud, but I reckon a fair few players wouldn't find it too loud for their gig. Anyway it works beautifully with a decent OD too. :)

The stock chassis has the rectifier hole punched for an EZ81 because the same chassis is used for their Marshall 18W kit. So I found an inexpensive hole punch on Ebay and made the hole big enough for a GZ34. It was easy enough to do, too. A decent GZ34 is easier to find than an EZ81 and is probably more authentic too.

I love making these things.

VoxAC15.JPG


Vox Int.JPG
 
You're making some very very cool looking amps. I've been thinking about building a deluxe reverb for a while but keep being put off by the cost of parts vs just buying a second hand deluxe reverb.
 
Beautiful, just beautiful.

Running the EL84s at 100% of their rating will shorten their life, but if that's what it takes to get the tone, then go for it!

Takes a bit of finesse to use one of those punches to enlarge a hole. You got skills!

In response to SillyOctopus...
Buying a used amp and spiffying it up with some better parts & a few tweaks is definitely the most cost-effective approach, especially if you end up reusing most of the expensive parts: tubes, transformers, speaker and cabinet.
 
Well when the tubes sound this good at 100% then I'm all for it. I really like how cathode biased amps sound. I've cathode biased a few amps which are normally fixed bias - sometimes it makes a huge difference, sometimes not much. But when it works, and the B+ is in a good spot for the tube, it can sound amazing. The JJs seem to take it well.

Did I do a Build Report for my last 5E3? I don't think I did. Stand by!

(edit) Oh, and thanks so much guys! I live in a little hole on the bottom of the world a million miles from anywhere, so it's incredible to get the feedback I get here - much appreciated.
 
Looks great! Not to derail, but I'm curious. I've seen a handful of your threads about building amps here and there. How big of a leap would you say it is to move from pedal-building to putting together an amp - and how does one make that leap, per se?

I've built and modded dozens of pedals (vero, PCB, etc) and modded a few amps as well, but I don't really know much at all by way of electronics "theory," it's more or less been paint-by-numbers if that makes sense. But I've always wanted to take a crack at building an amp, just never really sure where exactly to get started. Would be interested to hear your thoughts/pointers!
 
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I actually got into electronics through building amps. Pedals came later! I have no background in electronics at all. I bought a kit and followed the layout diagrams. Made a lot of mistakes but through being very careful I haven't once been zapped or killed. Not by amps anyway.

Things are a lot easier now than 20 years ago when I got started. There is so much more info it's crazy. I read a lot of books, such as Gerald Weber's books but in hindsight they might not have been as useful as others which are probably available now. Metroamps used to provide excellent instructions with their amps but they don't do kits anymore - maybe Mojo's instructions would be useful? I don't know. The thing is that often there are a few ways of doing anything, each with their strengths and weaknesses. Watch a lot of Uncle Doug and Psionic videos on Youtube. Psionic has taught me a lot about grounding (a very important part of amp building) and just copying the manufacturers is not necessarily a good thing!

I guess I might suggest buying a kit from Mojo to begin. I'd chuck out the orange drop caps and at least use Mallory 150s instead - they're cheap and very good. But they use good switches, sockets and even the transformers are usually good. I like their tagboards and chassis. Everything will fit properly and the hardware is solid. I don't like carbon comp resistors but that's personal preference and resistors are cheap. I prefer to use 1W carbon film for most places and bigger metal films early in the B+ rail - you'll see them marked as 2W or 3W on the layouts.

There's a lot of stuff you learn along the way if you read a lot. Things like don't run plate wires alongside grid wires - it creates noise and interaction. If they must get close to each other try to make them cross at 90 degrees. Keep your filament wiring well away from signal wires and twisted around itself - you'll see what I mean! It might take a few goes to build a really good one but you can always rebuild - and that is satisfying.
 
Hey @HamishR I have a really dumb question. How does the chassis mount in the cabinet? I imagine that the upper back panel on the cabinet provides the cover for the chassis but I don't quite get how the chassis gets bolted to the cab
 
There are no dumb questions only dumb answers. So I'll try not to mess this up! It's pretty straightforward. The chassis bolts to the back cover panel and then you make sure the connection holding the back cover is nice and strong. I used hardwood cleats to bolt the back panel to the cab and 6 decent sized SS screws, three each side. The cleats are the strips of 3/4" x 3/4" timber which are glued and screwed to the cab that you screw the back covers onto. Or 19 x 19 as we call it here.
 
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