KT66 storage precautions

[daviddeakin]

Stick them in the oven at 100 celsius for an hour, that'll get the getter going. (Not hotter or it may spoil the phenolic base)

[Peter.N.]

I used 6L6's for my hobby amplifiers when I was a kid as they were cheaper than KT66's, they worked OK but I believe they were beam tetrodes rather the pentodes.

Peter

[Herald1360]

KT66s are beam tetrodes too. The clue is in the "T". (The "K" is for Kinkless).

EL34s are pentodes, though.

[barrymagrec]

I believe the KT66 was an MOV redesign of the 6L6, the point of the beam tetrode construction in both cases mainly being to change the shape of grid 3 in order to circumvent the Mullard patent on power pentodes.

[Peter.N.]

Thanks for that, I thought KT's were pentodes there were lots of them, know most of the Mullard nomenclature but not so much Marconi. I'm still learning

Peter

[GrimJosef]

Annoyingly at least one KT valve, the KT77, is a pentode https://www.jj-electronic.com/en/kt77. It's a modern design though, not from what we might call 'the valve era'.

Cheers,

GJ

[barrymagrec]

The KT77 is pretty much an EL34 in most respects I think.

[ms660]

GEC valve data I'm looking at describes the KT77 as a beam tetrode, just sayin'

Lawrence.

[turretslug]

A cynic might be tempted to paraphrase Mandy Rice-Davies here.... I recall examining a few Mullard-branded "pentodes" in the past to find that they actually appeared to possess beam-plates as opposed to suppressor grid, and this has been touched on by others here extending to triode-power pentodes.

I suspect that description was frequently swayed by patent-infringement rather than technical accuracy considerations, Mullard being part of the Philips (pentode patent) group, GEC being closer to the RCA beam tetrode camp- indeed, they apparently originated the concept but RCA took it up when GEC struggled with repeatability.

Whether anyone actually dismantled rivals' valves for the sake of a court-room at the time I don't know- but they surely would now!

[Synchrodyne]

This subject (beam power valves that is, not the paraphrasing of MRD) is quite murky. Notwithstanding the conventional wisdom, it is not easy to determine who did what and with which and to whom, so as to speak.

Firstly, the KT77 was described by its maker as a beam tetrode:

KT77.pdf

It dated from c.1961 as best I can determine:

Audio 196108 p.107 Genalex KT77.pdf

Secondly, I have found no hard evidence that supports the notion that Marconi Osram found reliable production of the beam tetrode initially too difficult, and so let RCA undertake that work. RCA had free access to Marconi (and EMI) patents, etc., (and vice versa). So it could be simply that RCA saw a good idea and ran with it. Given that it was already making pentodes, it would not seem to have had a compelling reason to adopt an alternative technology in order to avoid the pentode altogether. Its first “beam power tube”, as it mostly seems to have referred to the type, the 6L6, dated from mid-1936. One of the claims made for it was that it more closely approached the “ideal” pentode characteristics than did the actual pentodes of the time, and so was a better audio output valve. The KT66 et al date from mid-1937, and appear to correspond to Marconi Osram’s initial release of a wider range of valves with the international octal base. The Hivac Harries valve had been in production in the UK since 1935 at least, perhaps 1934, and I’d be surprised if Marconi Osram had not been able to achieve the same level of precision in manufacture. (Consider that Marconi was at that time heavily involved in the development of TV equipment for the Marconi-EMI system, which I think would have been a precision exercise.)

The use of beam plates (variously referred to as beam-confining and beam-forming plates) appears to have allowed a departure from the circular cross-section electrode of the Harries valve. Possibly a denser resultant electron cloud at the Lagrange point, so as to speak, permitted closer spacing of the anode relative to the screen grid. With the Harries valve, the anode appeared to be somewhere out near the orbit of Saturn.

Osram usually referred to the KT66 as being a beam tetrode. When the KT88 was released, GEC described it as a beam pentode. But later data sheets have it as a beam tetrode. Go figure!

As noted, Philips also used the beam tetrode where appropriate, but often referred to such valves as pentodes. Insofar as they were notional pentodes, with virtual suppressor grids, that was not wrong in an absolute sense. That might have been more of a marketing decision than an effort to hide their nature from prying patent litigator eyes. An early (1961) data sheet for the EL500 described it as a beam power tube (i.e. RCA terminology). Its release was accompanied by cutaway drawings to show its Cavitrap feature, but these also showed its beam tetrode nature, so there was no hiding going on there. A later datasheet (1969) described it as an output pentode. Perhaps yet another case where less-well informed marketing folk told the informed engineering folk what to write.


Cheers,

[ukcol]

Originally Posted by Synchrodyne

....Osram usually referred to the KT66 as being a beam tetrode. When the KT88 was released, GEC described it as a beam pentode. But later data sheets have it as a beam tetrode....

I have always been used to valves like the 6V6 and the KT66 being described as beam tetrodes but thinking about it, it could be argued that beam pentode was a more logical description.

A tetrode is a 4 electrode valve and a beam tetrode is a tetrode with beam plates added making it a 5 electrode valve. So perhaps you could argue that the KT66 is indeed a beam pentode, that is a 5 electrode valve but with the name "beam pentode" to distinguish it from a conventional 3 grid pentode.

[John M0GLN]

Is this storage problem peculiar to KT66s or are other valves affected?

John

[turretslug]

It'd seem odd for the problem to be confined or even particularly associated with KT66s- perhaps it's more a case of the latter being a well-known and continuing in popularity power valve, with the additional possibility that the circumstances of circuit and usage lead to a popular wisdom of being "prone" to this problem.

I wouldn't be surprised if the Quad II power amp was one of the best-known and most popular KT66 applications- one can imagine a scenario of many of these being consigned to lofts or garages for a couple of decades, then being "just tried" without too much fretting over gentle re-introduction of power. The Quad II being well-known for its high value grid circuit resistors will be more susceptible than others to prolems resulting from gassy leakage, and the common cathode resistor will also be a weak point for leaving a gassier valve of a pair liable to "running away".

Pehaps it's more a case of many valve types being prone to slight gassiness over long periods, but the KT66s conditions of employment making it more likely to be the subject of discussion. I wouldn't be surprised to find that, say, the PL519 or the QQV06-40 might similarly be prone to a little internal gas but perhaps the circuit conditions make it less troublesome until mopped up by getter action.

[John M0GLN]

I finally made up Colin 'ukcol's circuit and included the grid current measuring facility suggested by GJ 'GrimJosef' , I connected it up to the power supply with an AVO measuring the anode current and a Fluke DMM across the control grid resistor switch, set the KT66 running at 300V with 95mA anode current and the grid current was about 1.6 μA, I let it run for 3 hrs and it had dropped to about 0.7 μA which looks like it should be OK, I should really adjust the cathode resistor as GJ suggests so it can run at 350V and 85mA and see what happens then.

Thanks Colin & GJ for the tester design.

John