I am restoring a 1966'ish vintage lab power supply that uses 5x EL34 in pentode mode for series regulation of separate plate and screen supplies up to 500Vdc (using a 620Vdc raw feed). I may well end up using valve alternatives to the EL34. I don't want to modify the existing circuitry, which is likely similar to HP’s 712B from 1954, and Fluke’s 107D from 1964, although I won't have a full schematic for another 2 weeks. I am interested to hear if anyone recalls any effort to make a solid-state equivalent (ie. valve pin compatible) of a power pentode. I think the most well known low-power ss alternative was the Feltron from the early 1970's. At the moment this post is just to reconnoitre the scene in case I've missed someone's effort.
Thanks PFL200, that gave me a good kick start.
Dave Allen also made up a 6V6 equivalent on p316 of Electronics World article https://worldradiohistory.com/UK/Wireless-World/00s/Electronics-World-2001-04-S-OCR.pdf using a power FET, although it was set up as a triode.
Dave Allen also made up a 6V6 equivalent on p316 of Electronics World article https://worldradiohistory.com/UK/Wireless-World/00s/Electronics-World-2001-04-S-OCR.pdf using a power FET, although it was set up as a triode.
EL34 are in current production and is affordable. SS alternatives might be less
forgiving and cost much more , not counting the time and effort needed to get
a stable design.
Use your time better !
Oh this is much better than burying my head in a 'too hard' basket. And is why the first post covered the obvious path of using valves so that off-topic posts could be minimised.
As the pass pentodes in regulated lab bench power supplies usually are operated in triode mode, they could be replaced by MOSFET's of sufficient voltage and dissipation ratings. At least me thinks so 😉.
Btw, here's a link with tons of pics to what PFL200 mentioned in #2. I also own a pocket full of these oddities. Btw, nowadays the high gm frame grid pentode PL802 is available again at moderate prices. Do they grow on trees 🤔?
Best regards!
Btw, here's a link with tons of pics to what PFL200 mentioned in #2. I also own a pocket full of these oddities. Btw, nowadays the high gm frame grid pentode PL802 is available again at moderate prices. Do they grow on trees 🤔?
Best regards!
I think it is a bit like classic car parts. In the old days every main dealer kept a selection of all the probable parts they'd need, so a lot of that became surplus when the cars became obsolete, so those stocks were accumulated by enthusiasts, and hence NOS parts still crop up. Nowadays stock control is all just-in-time, so stocks are at a minimum, and OEM spares dry up instantly when a car reaches it's 10 year limit for the OEM supplying parts.
On the other hand, a lot of hard to shift parts also end up as surplus. Anyone need a Triumph Acclaim/Honda Ballade left hand rear shock absorber? I have one in the garage.
Series regulation using pentode mode is certainly the exception - apart from the two high-end power supplies by HP and Fluke, all others (apart from the BWD 210B I am restoring) use pass valves in triode mode. Which makes the search for any prior art ss power valves just that more exotic it seems, as the ss 6V6 example is triode connected, as was the PL802/T.
I have quite a few valve based options that may well suit as replacements for each EL34 whilst retaining the existing control circuitry that uses the screen voltage - that is an easy path I can manage, as I also have sufficient EL34 to operate the supply in original format for baseline testing. But given this is a grunt power supply where it is likely that someone may buy it in years/decades to come just to pull out the EL34 and ditch the rest, I find the thought of using ss replacements as worthwhile scoping out now.
As there's no 107D to be found, at least not in this context, I think you'v been referring to the 407D instead. I own one of these boatanchors, it features three 807's as pass tubes in, yes, pentode mode, i. e. with constant screen to cathode voltage.
Best regards!
Last edited:
"Valve pin compatible" is pretty stringent, but would it stretch to socket adapters from compactron sweep valves? (Assuming that the G2 limitations could be accommodated). Semi-con stuff is all fine and dandy, but doesn't much like operating at high temperatures, requiring heat radiators, maybe forced air, who knows? From the perspective of something that must dissipate a lot of heat, the great "flaw" of vacuum valves, that they run hot, isn't a bug, it's a feature. Of course, in the future sweep valves might become trendy, so maybe not.
All good fortune,
Chris
All good fortune,
Chris
Attachments
How about something less desirable than an EL34 that did duty in this role, e.g. 6AU5GT?
Could it be a use for a thyratron?
Could it be a use for a thyratron?
I have a couple of HP 711A's that use a pair of 6L6's in pentode mode. G2 supply done the same way, floating winding. Only good for 100mA, but handy.
All good fortune,
Chris
All good fortune,
Chris
Chris, I appreciate that there would be effort needed to make any form of adaptor, whether that be for a valve like a compactron or eg. an 807, and that comes in to practical considerations - including whether the original circuit can allow 'balancing' even between EL34's let alone disparate valves, as well as heater current differences. At the moment there is physical 'headroom' to leave such options open. And yes, moving to any ss solution is very dependant on die temperature management, and that has a wide range of practical outcomes for adequate thermal management (which I luckily have had decades experience with). And yes I looked at the earlier HP's, with the 712B being the most advanced, although I need to wait for the schematic of my power supply to see what similarities are relevant.
Thanks Mona for that schem - I recall looking at another Heathkit example which was triode mode - so the SP2717A adds another nice pentode mode reference for comparisons.
OldHector, there may well be many valve options, with many associated practical issues to consider, but for the moment I would prefer to leave them for another thread.
Thanks Mona for that schem - I recall looking at another Heathkit example which was triode mode - so the SP2717A adds another nice pentode mode reference for comparisons.
OldHector, there may well be many valve options, with many associated practical issues to consider, but for the moment I would prefer to leave them for another thread.
JJ EL34 is what ive used for my bench supply, why ponder over 60 quid worth of valves?
You could use E235L with no change in socket connection, its slightly superior in therms of Ri too, abeit at lower 22W plate dissipation.
v4lve lover, this thread is not aimed at pondering 'valve' options, and I am only describing that topic to give some context rather than to spend countless posts on it. Maybe you missed my responses to posts #5, #10 and #16.