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This amplifier was designed as a courtesy to a DIY forum member

С101 (1,000 uF) should be rated for at least 200 V.

I've got the capacitor (220V, 220µF) and rectifier (from a computer power supply, just like you said!) and was wondering about the power ratings for the other components. Will 0.5W for example be all right for the resistors that aren't in the B+ path, like R7. I assume R1 would be a couple of watts. What should C2 be rated at? 200V as well or lower as it's only got the signal going through it?
Thank you.

Ok, let's start from the beginning. Everything is stabilized by rock solid LEDs.

Voltage drop on each Ga As diode is about 2.4V, so cathode voltage on the triode is 4.8V

Idle current is about 3 mA, voltage on anode is about 80V (I believe it will be a sweet spot for the triode, judging by curves from it's datasheet).

Looking at the same curves for a triode part of PCL84 grid bias is about -1.5V in respect to cathode for such current. That means 4.8-1,5=3.3V.

DC feedback to the grid goes through 2 voltage dividers: resistors R2 and R3, then R4 and R6.

To get 3.3V on the grid the amp has to set 6.6V on R3 (since R4 and R6 are equal). That means, voltage drop on R2 will be 12 times higher than on R3, i.e. 6.6x12= about 80 Volt.

80 Volt on 120 Ohm causes about 666 milli Amperes. Too much! That means I made a mistake first time calculating mentally.

R2 should be 1.2 KOhm, R3 should be 100 Ohm, in such case an idle current will be about 66.6 mA.
Power dissipated on R2 will be 5.33 W.
I would take 4 resistors 4.7K 2W each in parallel to avoid usage of wirewound resistors capable to dissipate 6W and more (they have an inductance that we don't need).

6.6V x 0.066A = 0.44W on R3, so more than 0.5W rated resistor will be needed.

R1 sees 80V and 3 mA, so dissipates 0.24W
0.5W resistor will be fine, you were right!

Now, about capacitors: all of them except C2 have to be rated for 200V. C2 should be metal film.

Without feedback amplification factor will be about 50, with feedback it is about 20. Input resistance is 100 kilo Ohm.

That's immensely useful - it helps me understand the whys and hows a bit better! I couldn't see what was dropping where. I've got a detailed PCL84 data sheet now so I'll be looking at that. I should be ordering the parts tomorrow, so will let you know how it goes...
Bests,

Good luck!

By the way, you need 30 VA transformer for B+ and 40VA for 15V (if you did not calculate yet)

I've googled for Morgan Jones, it consumes less because it is loaded on a modulated current source instead of on a plain resistor. But it is more critical to tubes used. Also, a current source load is overmodulated, so plate resistor's value should be decreased a lot. I personally would implement a feed-forward modulation of CCS instead of a feedback one like in Morgan Jones, so the thingy would work much better on the real complex load that differs from a plain resistor. If you don't afraid of adding transistors to tube gear I'll show you how.

I'm back again at long last! I've made the amp...but it doesn't do anything. I get very distorted sound when the volume going in is 100%, but otherwise nothing. I wondered if the valves were knackered, so I have some new ones on the way. Assuming they're not and my wiring is all right, what readings should I look for on the amplifier if all is well inside? I thought a poke around with a multimeter couldn't do any harm but I don't know what I'm looking for... I think it's likely to be my wiring, but I can't see anything obviously wrong and neither could someone else checking it. Any help would be much appreciated!
Bests,
J

R2 should be 1.2 KOhm, R3 should be 100 Ohm, in such case an idle current will be about 66.6 mA.
Power dissipated on R2 will be 5.33 W.
I would take 4 resistors 4.7K 2W each in parallel to avoid usage of wirewound resistors capable to dissipate 6W and more (they have an inductance that we don't need).

What voltages do you measure on anode and cathode of the output emitter follower?

Do LEDs light up?

I've done some metering of resistor values and I think I've found a few dodgy ones that meter differently from what the colour codes on them say! A few of the 1M ones are actually 0.5M. I'll go through the rest in case they were the same dodgy batch and I'll post when everything's the right value... The B+ is hovering around 160V and the filaments were on 16V. Is this within acceptable levels?

Yes, 160V B+ is acceptable, but what voltage is on cathodes of tubes?
500K instead of 1Mohm is Ok, especially when measuring you touch both legs of the resistor, then you measure your body in parallel with it.
500K will work well there: less distortions, but less amplification.

On pin 3 there's ~3V and ~45V on pin 7. The LEDs light up then die away immediately, all I get from the headphones is a loud hum, though that may be my wiring (!)

Let's see...
45V on pin 7 means 45/1.3= 34.6 mA current through the output cathode follower.
It creates about 3.4V on R3. That means, 1.7V on the 1'st grid of the triode. Now, let's add 1V between grid and cathode and we have 2.7V. Not enough for 2 LEDs. I've calculated without R6 in mind that I decided to add later.

You are right, one LED is odd! There must be one LED only in cathode. Try to short one and see if the amp comes alive.
Also, since it is a single ended amplifier it have to be powered from a very clean supply.

Well now! That did the trick. Shorting one of the LEDs suddenly made the music play, but there's a very loud hum... I'n going to have a poke around as I'm certain it's just my point to point picking up the mains. I can't say that circuit layout's my strong point! Would you recommend putting the power supply in a separate chassis?

First of all, I would suggest you to make your power supply well filtered. Can you describe what it is made of?

Also, does the filament winding have a grounded center tap?

C2, R6 and R6 must be as close to the tube as possible, with short legs. The wire to R7 from input jack should be coaxial, if the input jack is far from the tube.

It must be grounded in the single point, and the wire from the tube must go to the - leg of the filtering capacitor in your power supply.

It doesn't have a centre tap, which I now see is a slap-my-forehead silly oversight on my part. What resistor value should I use to make my own? The power supply is as we discussed on diyaudio.com - two transformers with 120V primaries and 15V secondaries joined together. I'll check on the ground wiring layout later this afternoon.