P-Series Valves, what to do with them?

[davegsm82]

Dave, have you been able to identify some of your 'mystery' stock yet by decoding the etched codes?

I've not tried yet Terry, I was aware of the Etched codes but didn't realise there was such a comprehensive identification guide available, I certainly will give it a try in the future.

The reason I was thinking of this method of valve identification was to create my own standalone automated valve tester, which would also be able to identify the valve series from their heater resistance, then perform a full test after identifying pins from a likely pin layout stored in memory.

Anyway, that's a plan for the future, not something that will come to fruition any time soon.

Dave.

[Kalee20]

A bag of worms then!!!
From what I've read the PL84 characteristics were taken as the HT available is less in AC/DC TV sets and that is the raison d'être for designing them in the first place. I'd suggest cutting open both types and do a comparison perhaps something will be gleaned that way. Thing is there are a number of references to say they are the same bar the heater.

My highlighting!

It's a bit drastic. One comparison tip (not my idea regrettably), is compare inter-electrode capacitances. If they are the same, then internal structure is likely to be the same. If the internal structure is different - closer grid/cathode spacing, finer pitch screen-grid, etc - then capacitances will change too.

My understanding is that UL84 = PL84 (apart from heater) and they really were designed to work well with a lower voltage HT so are genuinely different from EL84. nThe 6.3V counterpart is EL86

Differing heater 'letter' but same rest-of-the-type-number is an unreliable guide really. Counter-examples are abundant. DF91 and EF91. Or how about DM71 and EM71 where they are not even on the same base!

[pwdrive]

A simple DC anode current test would show what's what for sure, EL84 test at anode/screen and grid voltage given in the PL84's valve data, If anode current is substantially different then they are different animals.

Lawrence.

[ppppenguin]

Even if they are different, they're not wildly so. A useful question is: You need to replace an EL84 but you can't get one easily. But you have a PL84 and can arrange the heater power. Will it work OK? Replacement the other way round is less likely.

[Murphyv310]

Of course it will work. (see post 36)
Remember the 3-3 is DC coupled so any major changes in biassing or characteristics would bring in distortion at lower levels. So I believe this amp and it's design is a good amp for subjective tests. A more conventional Class A amp would more likely be even more forgiving than the 3-3.
David Boynes mentioned to me last week a particular TV with push pull audio using a PCL85 & PCL86, work that one then!!!!

[pwdrive]

A proper test for any characteristics difference is to do a comparison measurement in like for like DC terms, the results whatever they may be won't lie providing both valves are within spec.

Check out the data in the Avo valve data book, something stands out to me.

Lawrence.

[Murphyv310]

Yes Lawrence I do know this, my comment was clearly said it was a quick and dirty test.
I can post up all the DC conditions between the three valves in due course.

I don't have the Avo valve book.

[pwdrive]

Copy of the 17th edition Avo valve data manual here:

https://frank.pocnet.net/instruments/AVO...VDM_17.pdf

Lawrence.

[Murphyv310]

Cheers Lawrence.

[Diabolical Artificer]

Never having seen a P type valve used in the wild, I've always been uncertain what the 300mA means as regards a P type valve - 300mA max? And how the Dickens would you regulate a series chain? A bit higher mains and current goes up no? Light dawn's, use a big resistor. A Mullard data book I have gives Vh too, for P types, some are 15v ish, others 21v and so on, so 15v "types" could be run all together connected in parallel. Or am I just talking out of my hat?

Just to confuse things more on the subject of E/PL84's , am sure I read somewhere that the PL84 is a beam tetrode (even though it's called an output pentode by Mullard), not a true pentode like the EL34.

After you've found a use for P types perhaps someone could come up with a use for EF80's.

Andy.