Re-purposing TV valves

[Mark Hennessy]

I never cease to be amazed at the price of stuff you can get from China...

Of course, buck-boost circuits are quite noisy, and very rich in harmonics. I'd expect them to be problematic for RF applications - although at that price it won't cost much to try! For audio use, they'd probably be OK as the switching frequency will be well above 20kHz.

Also, a buck-boost converter will have the advantage of variable output - assuming the modules were fitted with a preset resistor - so you can get the heater voltage exactly right. With the PCL86, it was fortunate that the 300mA requirement equated with what came out of the 50Hz "doubler", but other valves might not be so easy.

If you buy the Chinese modules, it will almost certainly be worth upgrading the electrolytic capacitors

What to build? Well, I do audio. Therefore I'd build a stereo hi-fi amplifier. A pair of PCL86s in push-pull is good for about 10 watts. Apart from repairing Hacker Mayflowers, I've no real experience with valves (born in the '70s - and even as a kid I realised that valve gear was dangerous!). But as time goes by, I can't help becoming increasingly curious about these strange glass bottles

[tiv48]

I used a PCF80 to make a low output audio amp 9 volt heaters and the ht I used was about 120 volt it sounded remarkably good .
A mate of mine used a PFL200 to make a PP amp. I never heard this working.
Rob T

[tiv48]

I have used a buck boost module meant to supply ht for nixie clock to power a vidor valve portable (no trouble with RFI)
Rob T

[Nick]

Don't have to go to China - I did a very popular nixie PSU design that been built many many 100s of times, including by folk who use them for small valve units, like stomp boxes and pre-amps...

http://www.desmith.net/NMdS/Electronics/NixiePSU.html

I designed it as an exercise in efficiency, not economy - not sure that there are any similar supplies out there that are more efficient...

Also, a lot of folk have built VTTCs (Vacuum Tube Tesla Coils) using the PL-series of cheap tubes...

[Paul_RK]

I recall building an amplifier with PL81 output from a published design, I think in Practical Television: I must have been around 12 at the time, which would date the article approximately to 1971-2, but unfortunately my copies of PT from around then were dispersed long ago. Its performance wasn't especially creditable, but my choice of a perspex rather than metal chassis, for ease of cutting and drilling and because I happened to have a perspex implosion screen or two to hand, may not have helped

Paul

[boater sam]

Perspex as a chassis is clear to me, transparent top notes, clearly an innovation worth looking into.

Understood that PL81/EL81 are better as pulse amplifiers but it would appear to be a workable use.
I have seen a few push-pull ECL80 circuits, still got all those EF80 to find a use for though.How about PCC84 and their ilk?

[John M0GLN]

Understood that PL81/EL81 are better as pulse amplifiers but it would appear to be a workable use.

You would think so seeing that PL81's seem to be only used commercially as line output valves, but Philips made the HI-Fi audio amplifier AG 9006 which uses a pair of EL81's as the output valves, it's seems rather strange since Philips could have used any of the valves from their dedicated audio range.
I've never seen this particular amplifier but it is referred to in the book, 'From Microphone to Ear' by 'G Slot' and published by Philips Technical Library.

John

[boater sam]

Thanks for that John. It seems strange indeed when they had proper audio output valves in their range. I wonder if it is as simple as a way to clear surplus stocks of EL81, as the main user, the TV industry used PL81 without exception.
Was this amplifier any good bearing in mind the unusual valves? Anybody know?

[ppppenguin]

The PL81 was used in the UK where series heater chains were the norm. A number of European sets used 6.3V parallel heaters and hence the EL81. It was also used in professional monitors but the numbers here would have been tiny compared to domestic TVs.

[Refugee]

The EL81 is just a PL81 with a different heater so the only change will be to add a different heater so one worker will get a day of fitting different heaters every few weeks and that is it.