Thought I'd start a new thread, as future searches based on 'guess what it is' are pretty unhelpful...
Young Kevin (Tel) was kind enough to pay me a visit a few months back, bringing a gift in the form of an afternoon and evening with his amps. These were built by our own Coco and I think it's fair to say that 'spectacular' is an understatement. They have decent power delivery with masses of subtlety, a wonderful tonal range, mids to die for and a really open and deep sound combined with tightly controlled bass and really lucid treble. My current Jadis are better than good, but the 300Bs are better than better. These are right up in la-la-land in terms of what it would cost to buy, but once heard, never forgotten. That left me with only two options. As I'm unlikely to win the lottery and don't have any mega-rich loving relatives likely to pop their clogs anytime soon, that narrowed it down to one option. Build a pair.
This thread is intended as a help to anyone insane enough to consider something similar. So far there are no pictures, because there isn’t anything to take one of yet. As soon as the soldering iron comes out, I will post stuff.
As I have learned over the years, at the heart of any decent tube amp are the transformers. Output transformers are crucial, but anything else in the signal path is equally important. The basic design of these is a 1J6 first stage, which powers a pair of 46 tubes. These feed into an interstage transformer, into a pair of 300Bs. As a result, the following iron is required for each mono: One output transformer, one interstage transformer, one mains transformer for HT, one HT choke and one mains transformer for filament supplies. The design incorporates a bias supply for the 300Bs and a constant current source for the 1J6.
One thing I found out right at the start is fundamental to the build, and not something I had got my head around before. 300Bs, 46s and similar tubes are directly heated triodes. This means that the filament heater is also the cathode. As a result, the power to the filaments is massively important. It needs to be a constant voltage and current, but also should have an extremely high effective impedance to avoid messing up the audio side. Amongst others, Rod Coleman produces small PCB boards which are perfect regulators to supply the filaments. However, every tube needs its own, separate, floating filament supply. Therefore I already need, for each mono, five filament regulator boards and five DC supplies to them. This in turn means an LT transformer with five windings and with rectifiers and smoothing for five supplies. And, of course, cute little rectifiers are a no go. Schottkys are required,so that’s 20 diodes, five caps and assorted resistors, plus the populated regulator boards, just for the filaments.
The current position:
I am in component-buying mode at the moment. Thanks to Coco,I managed to steal a pair of Hashimoto HW-60-5 output transformers off ebay. When I say steal, that means these were only cripplingly expensive, as opposed to divorce-territory costly. Nobody makes interstage transforms that meet the required specs, so I’ve ordered a pair of hand-wound ones from DaveSlagle in the US. The mains transformers have to be custom wound, or there will be a mass of small ones all over the place. In the end, I specced one pair forthe HT and other stuff, and a second one for all the heaters. HT caps are a nightmare. High voltage ‘lytics are horrible, but the field is narrow. The HT runs at 400V or so, but a fault or breakage could see it run up to 700V plus for a short while, so a lot of care is needed. In the end I plumped for the Clarity Cap TC range. These are polys rated at 700V, so even if the unexpected happens, they should survive. Then, of course, there are all the odds and ends that are crippling unless you think ahead. I have found a decent Chinese source for things like tube sockets and transformer covers, but these bits all add up…
In the hope that nobody else at Rab Towers reads this thread, I’m going to try to be as honest as I can about this, as it will hopefully be an indicator for anyone else starting off on this path. I’ve already sold my beloved Jadis monos to attempt to offset some of the cost, but I fear more may have to go. Anyway, to date:
Filament regulator boards: £100 from Rod Coleman. Need to be populated, but stupidly cheap frankly and a more helpful guy you could not hope to meet. A beer definitely owed in that direction.
Mains transformers: £880 in total for the four.
Interstage transformers: £330 so far, but there may be some duty when they arrive.
Output transformers: £600 give or take a penny or two.
HT caps: £140 (much cheaper than in truth, as I already had a few in the spares pile).
One matched quad of 300Bs: £280 at current exchange rates. EH Gold Grid. Cheap, as I had a discount voucher for Tube Depot. May be some duty on top.
Tube sockets for 46s: £80. May be some duty on top.
Breath needs to be taken here. That’s already nearing two and a half grand, and there are no chassis yet, no PSU chokes, none of the rest of the passive components and none of the other tubes, tube sockets, transformer covers, etc. Plus all the bits we tend to ignore, like all the chassis hardware, wire, connectors, etc. Not to mention 40 PSU diodes and all the rest of the caps.
Frankly, if it lands at less than £3500 the pair, I’ll have done very well. And that’s with a really major coup for the output transformers and a bit of a deal for the 300Bs. And that's with cooking 300Bs at this stage. In short, these will not be cheap amplifiers by any means and it is a salutary warning for anyone who doesn’t understand why some manufacturers charge serious wedge for products. Bear in mind no development or design costs…
The initial build stuff. Once all the basic bits are here I can work out the fundamental layout and spec the chassis. Some pics will start at that time. I’m fortunate in that I have a source for the chassis plinths. I also have a mate whose parents run a light metalwork company. My initial thought is therefore to go a bit nuts. Build the basic chassis on a piece of 3mm aluminium, with all the bolts, screws and mayhem, then top that off with a piece of 1mm copper to cover it. However, I might just go for a proper copper chassis with a bit more depth and live with some bolts etc.
Anyway. I’ll start posting pics when there are some more bits here. So far, I only have the output transformers, a heap of oddments and the filament boards.
Do not expect this to be quick. Expect tears. Expect a visit or two to A&E. Expect unpleasantness with the bank manager and the nearest and dearest. Expect hissy fits. Expect major fuck-ups.
I have only one aim, which is to have them finished, de-bugged, run in and ready for Scalford next year.