The PL81 TV line output valve.

[FERNSEH]

The original developer of the PL81 was Philips. The Mullard Maintenance Manual describes the PL81 as pentode. This is something I have never given thought to but if one examines the assembly of the Telefunken version of the valve it can be clearly seen that the valve has beam plates. Likewise the Philips/Mullard PL81. Very early made in Holland Mullard PL81s made at the time of the introduction of the valve in 1950 did have a slightly different electrode structure. Perhaps this was a real pentode with a suppressor grid.
The attachment shows the Mullard PL81 in it's most familiar form.

From the Radiomuseum: http://www.radiomuseum.org/tubes/tube_pl81.html

Also the 21A6: http://www.radiomuseum.org/tubes/tube_21a6.html

DFWB.

[Synchrodyne]

As mentioned in post #213 in this thread: https://www.vintage-radio.net/forum/...=38395&page=11, determining the exact nature of the various members of the Philips line output valve sequence looks to be a can of worms.

Philips Book IIIC, which covers the original TV valve world series, gives no reason to infer that the PL81 is other than a regular pentode with a suppressor grid pinout. The same is true for the Philips datasheet with final date 19990803. The latter shows the suppressor pinout as #9 only, whereas the rmorg diagram shows it as pin #9 duplicated by pin #6.

I guess it is possible that the PL81 started out as a true pentode, but was reworked as a beam tetrode. Given that line output service involves highish screen dissipations, the beam tetrode, which all else being equal, is said to have a lower screen dissipation than an a pentode, would make some sense. But if it had been reworked, then one might expect there to have been some change in its published characteristics, particularly typical screen current.

Also, had it been a beam tetrode, then it would have been unusual at the time in having a separate pinout for the beam confining plates, which were normally internally connected to the cathode.

Back when the PL81 was released, Philips might have been a bit coy about describing a beam tetrode as such, given its investment in the pentode concept. But it appears to have “loosened up” somewhat by the time that PL500 was released.

Thinking outside of the box, and as maybe a wild idea, would there have been any advantage in having a line output valve designed essentially as a beam tetrode, not only with the customary aligned grids, correct electrode spacing and beam confining plates, but also with a supplementary coarse-wound suppressor grid? Given the unusual machinations with respect to screen and anode voltage relativity that happen in a line output valve, could there be occasions in the cycle when the electron cloud virtual suppressor grid loses some of its effectiveness, such that an actual suppressor grid becomes worthwhile? It is known that Philips did use aligned grids on some of its output pentodes, as evidenced by the UL84/PL84/EL86, although that is not mentioned in the standard data sheets.

Very puzzled,

[Mr Moose]

Hello,

The Tesla (Czech) PL81 is shown as a beam tetrode in its data sheet http://frank.pocnet.net/sheets/183/p/PL81.pdf

Yours, Richard

[bobbyball]

I have a Philips (surprise surprise) radiogram from around 1955 which, apart from having full coverage of the FM/VHF band, has two PL81 valves in a curious output stage driving an 800 ohm speaker for the bass frequencies. The treble signals are handled by a single EL84 but again driving a high impedence speaker. It was the "top of the range" model of it's day, Model FX995A. Not seen the PL81 in any other audio circuit that I can find.

[Synchrodyne]

Philips Book IIIC does mention the audio amplifier application for the PL81, namely push-pull Class B. Although no circuit is shown, the valve operating conditions for this case are tabulated.

Cheers,

[Synchrodyne]

And Mullard Valves, Tubes and Circuits, #8, describes an unusual DC-AC amplifier using an EF86, an ECL80, a pair of PL81 and a pair of PY82. (I guess that the PF86 had not been released back in 1954.)

Cheers,

[Diabolical Artificer]

Here's a picture of a Mazda PL81A with the beam plates as mentioned, next to a Mazda Pl81. The PL81A is stated as made in England, the PL81 shown as foreign made.

Next to them is a 12DQ6B which is described as a beam pentode for horizontal deflection and other TV applications on the data sheet. It looks more like a pentode; can't see any deflection plates.

On the PL81A data sheet it says "The beam current drawn from the EHT supply should be 300uA". Not sure if this refers to the valve being a beam pentode/tetrode or the beam of the CRT or just the usual electron beam in a valve.

Andy.

[Mike Phelan]

Originally Posted by Dr Wobble

On the PL81A data sheet it says "The beam current drawn from the EHT supply should be 300uA". Not sure if this refers to the valve being a beam pentode/tetrode or the beam of the CRT or just the usual electron beam in a valve.

That must refer to the CRT beam current as the line output valve beam plates or suppressor grid have nothing to do with the EHT supply.

[Heatercathodeshort]

The PL81A was designed for use in low drive line output stages such as the Ferguson 3805 hybrid portable. [rare receiver] Has anyone seen the curious 'wasp wasted' fat PL81that appears at first sight to have overheated and distorted the glass? Only seen a couple back in the 60's. The PL81 was one of the early World Series valves for television application around 1951. It began scanning 9" and 12" 50 degree tubes and ended up scanning 110 degree 21" tubes. Often blamed for having a short life when in fact the screen feed resistor to pin 8, often a poor quality carbon resistor was to blame, reducing in value to just a few ohms.
The PL81 was a good friend. John.

[G6Tanuki]

It also makes a good, cheap small RF PA-stage because with its top-cap anode you can arrange good isolation between input and output tuned-circuits. With forced-air cooling to keep bulb-temperature under control you can drive them quite hard - 35 watts PEP on 14MHz is possible if you don't care too much about valve-life (and have a convenient box of TV-salvaged valves as replacements).

[FERNSEH]

I have a Philips amplifier which employs a pair of EL81s in push-pull. It's upstairs, when I return to shop later today I'll take some pictures of it.
The successor to the PL81 was the PL36, again I'm not certain if this valve is a true pentode with a suppressor grid. The near equivalents, the Mazda 30P4, and the later 30P19 are definitely beam tetrodes.
The really interesting line output valve is the PL500. This valve was I'm certain was developed by Philips. This valve employs what was described by Mullard as the "cavitrap" construction. A Philips name for the beam forming technique perhaps.
From the radiomuseum: http://radiomuseum.org/tubes/tube_pl500.html

There is a description of the cavitrap PL500 but it is auf Deutch.

DFWB.

[Heatercathodeshort]

Yet another version of the PL81 is the PL820. This was used for EHT generation in the last versions of projection receivers. It looks identical and both the PL81 or PL820 work in either position. J.

[Mike Phelan]

Originally Posted by Heatercathodeshort

Has anyone seen the curious 'wasp wasted' fat PL81that appears at first sight to have overheated and distorted the glass? Only seen a couple back in the 60's...

I remember them, John, but cannot remember what set they were in.

[FERNSEH]

From France the 21B6. A sort of beefed up PL81.
The developer was Mazda France.

http://www.radiomuseum.org/tubes/tube_21b6.html

And another variation on the miniature line output valve theme.
This is the Marconi-Osram N339. I remember this valve well. It was employed in certain GEC TV receivers. A PL81 was often used as a substitute but it would have a very short life. It could be seen glowing red hot. The only answer was to fit the correct and pricey N339 valve.

http://www.radiomuseum.org/tubes/tube_n339.html

DFWB.

[FERNSEH]

The Philips amplifier: It employs four EL81s in a parallel push-pull arrangement.
The other valves are two EF86 and two ECC40s.

From the Mullard Maintenance Manual: Info for the PL820. The characteristics are identical to the PL81 and as mentioned by HKS the electrode structure looks the similar.
Described by Mullard as for use in projection TV receivers.
Maybe this diference in the type number could have been for purchase tax reasons. I'm citing the case of the UL41 and the UL46. No electrical differences although the latter was described as having low microphony properties.

DFWB.

[kalee20]

I have seen two specimens of this amplifier, too.

If I recall correctly, the PL81 and the EL81 are identical apart from the heater.

It is a curious valve line-up as I'm sure that beefy output valves were in the Mullard/Philips line-up at that time - EL34 etc. so why use multiple EL81's when single quantities of these would do?

[Maarten]

In a transformerless push-pull output design, the Vh-Vk of the upper valve is quite high and also the output impedance of the valves needs to be low. Before the EL86/UL84/PL84 were developed especially for such applications, various other valves were used, such as the EL81/PL81.

[FERNSEH]

An earlier miniature Philips line output valve was the UL44 and was used in many Continental and UK made Philips sets.
The Pa of this valve is quite modest at 5 watts. The Philips TF584 employed two parallel UL44s in the line output stage. No efficiency diode was employed in this set. Later Philips TVs like the 385U and TF390 have one UL44 and an UY41 serving as the efficiency diode.
The TF384: https://www.vintage-radio.net/forum/...ead.php?t=8105

I will examine an UL44 to determine the presence of beam plates or if the tube is a pentode with a suppressor grid.
http://www.radiomuseum.org/tubes/tube_ul44.html

DFWB.

[Heatercathodeshort]

In this very early example of the PL81, you can just about make out the presence of the beam plates to the right of the electrode assembly. J.

[Peter.N.]

That would be for colour of course.

Peter