Telephone rang, a lovely chap called John.
"I've got a Radford, one of the valves has gone, can you take a look?"
Of course. The trouble is, John lives in Scotland, and posting anything with valves in always worries me, but never fear, UPS did a stunning job and all arrived safely.
It's a rather lovely STA-15, made in 1962.
Ah yes, the disclaimer.....
WARNING. Do NOT try this at home. This amplifier calls for some high HT voltages. The main HT rail is some 390 VDC. It will floor you or kill you if you come into contact with it. It will not warn you, blow a fuse, nor pull your safety trip. It may not give you a second chance. RESPECT IT'S AUTHORITY.
There are exposed 240VAC mains connections inside the enclosure.
I will not, under any circumstances, accept any liability if you decide to repair an amplifier for yourself.
Having got that out of the way....
So, why had the Mullard expired? Switching on with the valves removed, showed the offending valve had a positive voltage on it's grid, with respect to the cathode. Not good. That's equivalent to you holding the throttle flat down of your car for hours... The valve's anode current would have risen, and the anode itself probably glowed bright red trying to dissipate all that heat, followed by the glass envelope failing due to the heat. Sadly, it's an all too common a fault. The capacitor which couples the audio to the grid, after the phase splitter had broken down, and was passing DC. Nothing for it , but a re-cap.
In this picture I've nearly finished re-capping the left hand channel...
I checked the other valves for faults, and emission, and all was found to be in good order. I also did some checks on the values of resistors, especially anode, cathode and screen. Everything was good, and always better than 5% tolerance. Nice!
Now to check the biassing....
There's a lot of BS out there about biassing, some of it comes from manufacturers and importers, some of it folklore, most of it wrong or downright stupid. We need to bias out output stage to get our anode current to less than the maximum plate dissipation of our valve. So we have an EL34. It's maximum plate dissipation is 20 watts. Anything more and we're in trouble, and run the risk of premature failure, the anode glowing red etc etc. Nasty. Don't do it.
So we need to know two things. The current flowing into the anode, and the voltage between the anode and the cathode (not, as some suggest between the anode and ground). That's all the valve "sees" and is all we need to do our calculations.
So, first off, install the bias-o-rama, and valves into the amp. Plug in speakers.
Right, switch on and wait for the valves to warm. It's worth keeping an eye on things here, just in case the bias is miles out, or there's a fault, and our valve starts to red-plate. If it does, switch it off quickly.
Now that everything's ok, measure the voltage between the anode and cathode. In this case a very civilised 390 V. Now measure the voltage across the anode resistor on the "Bias-o-rama"... That gives us the current in mA flowing though the Anode. As these valves are from a matched set, both currents should be within a couple of mA of each other....
Yep , they're good (Thanks HotRox!)
So we have 390v and about 50 mA flowing. Simple ohm's law tells us power = voltage x current, so
390 x 0.050 = 19.5 watts. Less than our maximum allowable 20 watts. (I will point out, this was taken after I'd adjusted the bias, which on the Radford involves changing resistors in the grid feed circuit, there's no pot or anything helpful like that)
Biassing too cool (not enough current flowing) will show up as cross-over distortion.
Some biassing voodoo calls for measurement of the cathode current. I'm not sure why, as this also includes any current flowing into the screen too. Less accurate, but probably near enough. I suppose the cathode is always at a lower voltage, so it's safer.
OK, so now we're happy with the bias, leave the amp running for 15 mins and check again. Still good? (It should be, although I once had a set of clear top 6550's that drifted badly, they went back!)
Right, now go to a darkened room, and play the amp loud for 15 mins. Look carefully at the large grey anode. If there's any signs of it glowing red, or little spots of glowing, it's still biassed too hot. Go and do it again!
So there it is, on test in the living room, much to the wife's delight. Playing the excellent CD "I will be a pilgrim" by Arch Garrison (http://www.archgarrison.co.uk/) , a blend of acoustic guitar and philcordia , some nice folky bits with a psychadelic air to it.
A post script...
My apologies for the poor focus, the infra red from the tubes stops the auto-focus working on the camera (it works by infra-red!), and this was a long exposure. I hope you can see the blue glow around the holes in the anode. This is caused by electrons hitting the glass and causing it to fluoresce. This is normal. If a tube gets "gassy" a similar blue glow can be seen, but very much more pronounced and time for a new valve!