So, how does one get started making their own audio equipment? For me, I first became interested when I was researching new gear for the family room, and stumbled across AudioAsylum. I was there looking for reviews of some Modwright-modified Jolida equipment (as an aside, I ended up buying this exact amplifier at a deep discount once it was returned to Response Audio, probably something to do with the fact that Scott must smoke a carton a day), and stumbled across the TubeDIY and Bottlehead forums -- I had no idea there was such a thing, but it certainly looked interesting. I then talked my wife into getting me a K-502 11BM8 pushpull integrated amp kit from AES for Christmas. You can get a similar amp (was K-12M, now 12G) from S5 Electronics. In fact, you'll frequently see the two talked about pretty much interchangeably in most discussions online.
I picked up a good soldering station, a decent multimeter, a couple tools, some solder from Welborne Labs, and was off. Even for a first build, it's pretty easy. I don't guess it took me more than three or four hours over a couple of days to finish it (though there was a miscue or two -- all operator headspace errors). I mounted what I could on the underside of the PCB, thinking that at some point in the indeterminate future I would build a box to put the amp in. From the get-go, the little amp didn't sound too bad. One of the upsides of getting this amp over other kits, though, is that a bunch of others have already built it, made some modifications, and posted notes in one online forum or another, making it very amenable for the first-time builder to modify and get in some time with a soldering iron. Try here, here, here, and here for discussions in various forums. Also, here, here, here, and here are some individuals' webpages on their experience with the amp.
I have found that no matter the kit, builders always swap out coupling caps as the first modification to any 'stock' amp and I was no different. I pulled out whatever they were that were in there and put in .22μF 400V Auricaps. I also swapped out the C's and the R in the CRC power supply filter. For the resistor I put in a 470Ω 5W Mills wirewound. Voltsecond recommends adding some B+ beef, so I replaced the PS caps with Nichicon 330μF 250V jobbies from Digi-Key. Also, I pulled out the stock rectification and put in my own bridge using four IXYS FREDs. Obviously, these didn't fit on the PCB and I mounted them on a piece of perfboard and ran some wire from the PC board. It worked, and at that moment that was all that mattered. I also ditched the DC blocking caps on the inputs and converted to shunt volume control. The stock potentiometer came out, 100kΩ 1W tantalum resistors from PartsConnexion went in the holes vacated by the blocking caps, and I hooked up a RadioShack 100kΩ pot.
Next, as the stock output transformers are junk, I popped for a pair of Hammond 1609 trannies. I listened to the amp as-is with the Hammonds for a while, then decided to try out ultralinear. I cut the etches to the pentode screens (pin 7) from the power supply, then hooked up the ultralinear taps from the output transformers to the screens. Of the modifications I did, this was probably the one with the most impact.
Lastly, I swapped out the cathode bias resistors on the output tubes for a CCS, converting to differential output. Voltsecond has the mod for the K-12M here and the changes required to work with the K-502 are here. I wired both CCSs on the same board, tied together the left- and right-channel R3s together to the one tap going to input tube/ phase splitter/ screen B+ (if anyone wants the BOM w/ DigiKey part #'s, I can provide that). Another nice sonic step up. Note: As I'm writing this up I noticed I hooked up the CCS not to the front end/ screen B+ but rather to the B+ going to the CT of the output transformer. Yeah, that's not right, but nothing seems to be the worse for the wear so I'll probably leave it be for now.
All in all, a pretty decent first project -- it sounds better than you might expect for a $140 amp + another $80 in aftermarket parts, I got to mess around modifying the circuit & no one got hurt, and I learned a little bit. Mind you, I didn't learn a whole lot about circuit design; it was long after I put aside working on this amp before I felt somewhat comfortable with looking over the schematic and actually having at least a clue about what was going on. To that end, I strongly encourage those new to this hobby to purchase Morgan Jones' Valve Amplifiers and also RDH4. I got them both about the same time as the kit showed up under my Christmas tree. At first, I understood V=IR and not much else. But, over time and through countless repeated readings it all started to sink in -- to some degree, anyway.
[18 Dec 07] I had been reading about current source/ transconductance amplifiers and how they are reported to work well with single-driver speakers -- the place to start is Nelson Pass's article. Also, John Swenson's resulting sonic journey from said missive is worth a read. His first post on the topic is here, while following chapters can be found in both the TubeDIY and Magnequest forums at AudioAsylum: check here, here, here, here, here, here, and here. Since I already have the differential part of JS's PP pentode amp, I figuered I'd ditch the feedback resistors & capacitors, unhook the UL taps, and bam! I'd have a transconductance amp. The 11BM8's 20kΩ Rp translates into ~40Ω output impedence with the Hammond 1608, so it should be a pretty good approximation of a current source amplifier. Yanking out the caps and resistors were easy enough, but I had cut the screen traces on the PCB for the UL hookup, so I had to jumper them. And, while I was at it, I moved the B+ tap for the CCS to the proper place. Finally, I switched the seconday to the 16Ω taps for voltage compliance's sake (several people have had better luck running their speakers on the higher impedence taps). A Halloween miracle... no smoke and the little amp fired right up. In terms of its merits, I would say that, first off, it's way, WAY better in every respect than it was before the mod; descriptors that spring to mind are 'focused' and 'articulate'. But, not so unbelievably great that it can hope to displace my 27->56->45 playback chain -- which is as you might expect since the 11BM8 amp costs a fraction of the SE amps. The IDHTs and DHT simply sound more real to me; the 11BM8 is more hifi. I do like it and this experiement is prompting me to take a swing at putting together a PP 45 amp sometime in the future. Mike at Magnequest just had a sale on 5kΩ PP outputs, so most likely in 2008 I'll cobble something together.