[Diabolical Artificer]

I've nearly finished fettling the chassis or rack unit to accept the two amp "modules" and have temporarily wired in the PSU. I managed to fit it all in though this has taken a lot of measuring, jiggling, trial and error. I've had to make several compromises because I didn't really think everything through. To be fair to myself though unless I'd actually built this beforehand, I couldn't have thought about everything before hand. Still, if I'd made the amp module 1/2" smaller on the length and width, life would have been easier.

Starting with the PSU, the mains enters the rack unit through an IEC kettle doodah, goes into a big 32mm/1 1/4" NOS fuse holder and is switched both L and N with a NTE thermistor on the big toroid tfmr primary. I tried fitting caps across the SW to surpress arc, but this means the PSU isn't properly off. The thermistor should extend SW life somewhat anyway.

Without the thermistor even with a soft start circuit, there is a hell of an inrush current. At first I used 100 ohm resistor's on each capacitor "bank" , but this didn't reduce the current surge or delay the rise of HT voltage. I'd obviously miscalculated. I found some big vitreous enamel resistors in the shed, that came of the Tek 454A scope that donated the ceramic wafers I used on the amp. So I used two these, a 700 ish ohm on the "bottom" 225v HT PSU, and a 1k5 on the top. this delays full HT by around 8 seconds, these R's are then shorted by a relay, two contacts for the top HT, one for the bottom. See schematic.

The relay delay circuit is powered by a little tfmr, GR etc, bottom left, pic one. You can just see it under the cap board. the delay circuit is on the PCB on the left. A TIP122 Darlington switches on the relay. It's overkill for this, but works well. I tried using a 2SD880 at one point, but it oscillated.

Next, middle on the PCB is the bias/CCS PSU. I had to change values of the first RC filter resistor. There were two 1k5 100u RC filters, but the first dropped the OP voltage too much. I'd also put a 80v zener on the OP but had to take it off. There is a bias adjust pot on the amp, this dropped too much voltage. I experimented with the "fools bias resistor" ( a R to stop bias from being set to 0v) to stop too much voltage being dropped. 1M and a 50k pot gives -16v to -40v and doesn't drop too much voltage.

Last, on the right of the PCB is the 12.6v PSU circuit that powers the htrs of the triode gain stage and PS. I screwed up here and wound the tfmr secondary that powers it with too high a voltage, EG 34v when rectified and smoothed. this means the LM317 got very hot indeed. So I had to retro fit a dropping resistor of around 20 ohm's (two big cement R's on right) to give 19v IP to the LM317. Not happy with this bodge but it would mean re-winding the big toroid . No way jose! : )

I keep writting PCB, but it was made using a dremmel and fine diamond tip to cut the traces. As you see it and the relay is on a hinged panel to make access easier. This hinged panel took about a week to get right!

The fuseboard needs re-doing. I used veroboard but am not happy with the result. It has gaffa tape on the chassis to stop any shorts at present which on the finished amp will need something better. Also there are no covers on the HT fuseholders - an oversight because as the bridge rectifiers have caps across each diode to suppress switching noise, HT is present on the fuse holder.

Lastly for now, as you'll see in the last pic is a panel meter. I managed to get two the same off ebay for £20. They are the same NOS military 50uA but have different scale plates. I'm thinking of using these to make PPM/VU meters. I saw there are cheap Chinese VU meters that come with a driver board, but thought I could do better.

That's about it for now, TFL, Andy.