A very merry Christmas and a happy New Year! Okay, so you could be reading this in the middle of Summer, but don't smirk, the shops will soon have Christmas puddings on their shelves.
It seems the PW Editorial team have kindly corrected an error that didn't exist. So now the text really is wrong. The 'correction' is near the end of the first column on page 51. I'm not going to describe it; here's what the paragraphs should say:
While I was preparing the circuits for this Valve and Vintage column, I realised that I hadn't any new PCL83 valves. Deciding to get hold of some, I took the opportunity to replenish part of my valve collection. Most of the valves I subsequently received were fine, but there were a couple of surprises. Firstly, most of the PCL83s I received weren't actually PCL83s at all! Although they were clearly (but recently) labelled PCL83, the valve code was 'kU', and not code '28', the code I was expecting. After checking, I found that 'kU' was the code for the HN309, a triode-pentode made by Mullard for M-OV. At first I thought there was a misprint in the codes, as the LN309 is indeed the M-OV equivalent of the Mullard PCL83. But the list was correct! The HN309 is similar to the PCL83 but the triode section is not the same; mu is four times greater.
Yup, the second 'HN309' should have read 'LN309'.
I've received a request for information concerning the Hear All Continents (H.A.C.) model 'DX' Mark II: What valve did the set use? It's an Octal-based valve with a top cap. I have a model 'DX', but mine is the Mark I version which uses a British 4-pin, 2V-battery triode. Does anyone know what valve the Mark II used? Better still, does anyone still have a complete 'DX' Mark II?
Well, if the Linux lot can have YAST, then I can have YARR - Yet Another Regenerative Receiver. Last time I mentioned why valves designed for use in television receivers are well worth investigating. There are still lots of them around and they're often sold quite cheaply. Their 300mA heaters shouldn't put anyone off; a low-voltage transformer plus maybe a series resistor, is all you need to get them glowing.
The PCL83 is useful TV valve. It has a pentode output section, plus a triode section whose characteristics are identical to the 6C4/EC90. I know that the 6C4 works extremely well as a regenerative detector, so this month I'm featuring a one-valve set which uses a PCL83 as both regenerative detector and audio amplifier. An ideal Christmas project.
The PCL83 has a 300mA heater, and the nominal heater voltage is 12.6V. But I've found some 300mA-valves deviate significantly from the figure you find in the valve data. So I find it best to put the chosen valve on a d.c. supply, wind up the voltage (slooooowly!) until the valve heater is passing a steady 300mA, and then measure the voltage across the heater.
There are other TV-type triode-pentode valves which should work. There's the PCL86 and the PCL82, and even the PCL84. Actually, the pentode section of the PCL84 is of frame grid construction and has very high gain. It could be the best overall choice, if the triode section works well too. By the way, for the PCL83, the reflected loudspeaker load should be about 5kohm. The other valves may need a slightly different load; check the relevant data. I've included a volume control - R4 - because I'm an optimist. Hopefully, the set will be sensitive enough for it not to be permanently parked at maximum volume.
There is one thing I'm slightly concerned about: with R2 at 18kohm, the anode of V1a ought to hover around 45V, give or take, but the (relatively) high anode load may have a detrimental effect on the reaction. If that happens, one thing to try would be to replace R2 with a 3.3kohm resistor and feed it from a stabilised - and decoupled - 60V supply. Unfortunately, the audio output from V1a will be much reduced if you do this. A better method of coupling would be to use an intervalve transformer - if you can get hold of one!
The receiver uses a 12-0-12V (or 0-24V) transformer followed by a voltage quadrupler to generate the h.t. supply. With the 50V and 100V capacitors shown, the input to the quadrupler should not exceed 35V. A 12-0-12V or 0-24V transformer will be fine, but a 15-0-15V (or 0-30V) transformer may stress the capacitors when the power supply is not delivering any current. This is because the voltage regulation of small transformers is notoriously poor, and the off-load voltage can rise alarmingly.
Another valve you could try is the PCL85. In fact, I have a couple of queries about this valve. TV manufacturers sometimes used a valve for other than its intended purpose. The 'video output' PCL84 is a case in point; several manufacturers used it as an audio output valve. But the PCL85, which was designed as a field oscillator/output valve, was never (as far as I know) used for anything other than its intended purpose. I would have expected it to have been used in the audio stages of some sets, as well as in the field timebase, but it never was. Does anyone know why?
The PCL85 was superseded by the PCL805/85 (or PCL85/805 - take your pick). Does anyone know what the difference is between the two versions? And did the valve fail often in use? TV repair people seemed to keep lots of them around. Maybe the set designers pushed the valve too hard, anyone know?
I recently bought a load of valves, some of which were PCL83s. But.... most of the PCL83s I received weren't PCL83s at all. They had been recently labelled PCL83, but the valve code was 'kU'. I was expecting code '28'. I found that 'kU' was the code for the HN309, a triode-pentode made by Mullard for M-OV.
I informed the supplier, and I hope action was taken. But I am concerned. How often are valves being mislabelled? From now on, I shall check the valve code on all the valves I receive - providing the valve has a code - and I suggest you do the same. There is a recent list of codes here.
Sometimes valve suppliers send ex-military (CV) versions of commercial types. This is okay; the military version is usually better than the commercial version. But lately, I've received some ex-military valves that have not been up to the standard of earlier valves. I think they've come from Eastern Europe and were made after 1970. When did the major valve manufacturers stop producing valves in the UK? Must have been earlier than I thought. From now on, I'm keeping every valve I have - or can get my hands on - that was made in the UK or Western Europe. New or used!
Want a Christmas stocking-filler? The third edition of Morgan Jones's book Valve Amplifiers is now available (ISBN 0-7506-5694-8). At 617 pages, it's 60% bigger than the first edition, and it's well worth a look if you're interested in valve audio amplifiers.
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