PL81 safe cathode/heater voltage

[boater sam]

Hi, can anyone advise be what the safe cathode to heater volage is for this valve please? I am hoping that it is in excess of 300v, if not is there a suitable valve ?

[John M0GLN]

The only ref' I have is for a PL81A and I don't know how that differs from a PL81, it's in the Mullard Technical Handbook 1975 and it gives the V h - k max rating as 200V for either cathode + or -.

Just noticed this when typing from the book, the cathode when shown in the base drawing and when referred to in the ratings is marked 'k' but in the text it's cathode with a 'c'. ?

John

[boater sam]

Thanks for that John, its a bit lower than I had hoped. I need a pentode or tetrode or even a triode with at least 300v ht/c rating @ about 8w

I have seen some cathodes as 'k' as well, must be standardisation! k for confusing perhaps.

Sam.

[Ian E G7OLT]

I wondered about the PL509. Apart from it being a bit too big for your requirements, maximum Heater-Cathode voltage is 250V.

See http://www.datasheetarchive.com/dl/Datas...178529.pdf

[ppppenguin]

Standard solution for HK voltage problems is to run the heaters from a separate winding. Either floating or tied, loosely or otherwise, to cathode voltage.

The 6080 dual power triode can take 300V HK. http://www.r-type.org/pdfs/6080.pdf They were widely used as HT series stabilisers where the cathodes would be at a full HT. I suspect they will be expensive.

Another approach is to stress test the HK insulation on some PL81s and see what happens. They're cheap enough. They are also inherently pretty tough to survive their normal job and typically live close to the top of heater chains where the HK insulation matters.

[boater sam]

Thanks Jeffrey et al.
6080 prices for new are not bad, I'll try that if giving a PL81 full voltage is a failure.
I can't  tie the cathode due to the voltmeter module needing a common neg supply wire, it won't stand the 300v from pos to ground. The module has to float. the cathode will be @ 300v. in cathode follower mode.

[ppppenguin]

Looking at the circuits of a couple of Solarton valve HT PSUs (SRS151, SRS153) they both use series stabilisers which are effectively cathode followers. They both use a separate heater winding and elevate the heaters to the output voltage so that there is no stress on the HK insulation. FWIW, the 153 uses a pair of something similar to EL81 for the series pass while the 151 uses no fewer than 6 EL37 in parallel.

I've also looked at the circuit of a Tek 547 scope which uses a couple of 6080 as series pass regulators. Thinga are a bit more complex in this design but again the designers have used separate heater windings to minimise HK stress.

If I were deisgning something like this I would look at what some undoubtedly competent designers did before me and budget for a separate heater supply.