Been a while huh? I’ve been busy tidying up mostly but a few things have been occuring. Tidying ’cause the inorganic is this weekend and it’s forced me to go through all the tube stuff and I thought it’s high time I built a tube amp.
This one is a combination of a radio chassis I picked up from Ben at Radio Specialties at the top of Upper Queen St, who still has the odd treasure stashed away, and the output tubes and output transformers from an ol’ akai reel to reel given to me by Johnny Pain of the now defunct Halleluoya (phonetic see) Picasso’s.
The radio chassis had one 6AV6 tube left in it, all the other tubes gone, and a power transformer that were of use to me. The 6AV6 is basically half a 12AX7 in a 7pin so they are somewhat overlooked and NOS they can be bought cheaply and will sing just like the ol’ vintage 12AX7’s which can be very expensive. The output tube is a 6BM8 and these feature a voltage amp triode, somewhere between a 12AX7 and a 12AT7, and an output pentode, the english is the ECL82 (E for 6volt filaments, C for a triode VA, and L for output pentode) and one worth looking for is the PCL82 which is a TV tube with 16V filaments. Two PCL82’s and you’ve got a stereo amp for almost nothing. Actually two PCL82’s and a suitable class AB1 output transformer and you’ve got a very decent small guitar amp. Add in a few bs109 mosfets, small signal mosfets that’ll take 400V max on the drain, and you’ve got a very cheap way to get a cutting edge amp that’d fool even the purists.
So my little amp will have the 6AV6 doing the first stage then have a IRF820 mosfet as a source follower into the tone stack, which is a very interesting almost parametric style thingy I got at AX84.com, then into the triode of the 6BM8 then output pentode in class A, single ended. The rason for the source follower is that the plate, or anode, of the 6AV6 wants to see a high impedance to get the most of it’s ability to make big voltage swings but the tone stack requires being driven from a fairly low impedance so the obvious thing to do is to chuck in a cathode follower or even better a source follower using a mosfet. A cathode follower using a tube allows a little bit of tube tone shaping as the tube follows the voltage on the grid with extra current from the cathode so you get a little extra harmonic content as the tube hits its edges of available current and voltage but a msfet source follower is cheap to impliment, takes up far less room, and as long as it isn’t hitting it’s rails then the difference is unnoticable. Doing the same thing in a marshall amp would have consequences as the tone stack is after a whole bunch of voltage amps that crunch into the rails. This one won’t and any distortion I do get will be from the OT and the pentode hittings it’s rails.
As well as this I’ve also got the parts and drawn up a PCB to do the Boss slow gear. This is a pedal that allows one to emulate the ol’ volume swell so the initial attack disappears and the note played swells out of nothing. Simpllish type circuit that may need tweakin’ and depends alot on the jfet chosen to effect the signal through. Based on the schematic at BYOC, build your own clone, pedals.
You get all the info to do it in the instruction .pdf
I designed my own PCB though
Funny thing, I performed at Vit S the other night and most of my gear didn’t work but I was playing a guitar normally and through a homemade delay and a small tube amp with 2 x 12″ speaker cab and I finally got some kudos from other gear head Rohan. I’ve used the tube amp and cab before but with oscillators… funny how it may have been the more familiar sounds, wound pickups under nickel wound strings, that had his ears perking up!
Analog, digital, tubes, strings, oscillators, whatever! If it sounds good then it is good. I hadn’t thought about the concept that these prejudices might actually stop people from actually hearing things. Given we’re going into an mp3 world then the prejudices are even sillier.