DR DEVELOPEMENTS PL 802/T VINTAGE RADIO VALVE

[Crackle]

I was interested to see this being listed on ebay recently.
Is this a MOSFET equivalent for a PL802 valve.

Mike

[ppppenguin]

Yes. I don't know if they were any good. The big resistor substitutes for the heater. They were used as video output valves in colour TVs.

http://www.r-type.org/exhib/aai0092.htm

[boater sam]

I have a new one but no set to try it in.
As I understood it, the PL802 was a problematical valve and stocks quickly disappeared, so the semiconductor /T was marketed.

[Mike Watterson]

It will work without the heater connected :D Since it's for video, it's possible it would work for audio and HF PA. However it's likely based on a common (cheap now) Mosfet. I wonder how they adapted the grid voltage, because the depletion type FETs (that work with negative to 0V bias) are not usually power types.

[ppppenguin]

It will indeed work without its heater connected but in its intended application it was part of a series heater chain. Hence the big resistor.

[Geordie McBoyne]

There's a couple of those things in the shop. Now that I have some limited freedom I'll go there tomorrow and take a look at them. I kinda remember it wasn't a FET but nothing more than a general purpose video transistor of the BF458 type. A firm called LEDCO made the ones I used. The other trick to replace the PL802 was to cut off a certain pin of an EF184 frame grid pentode. Not a long lasting solution but it got the set going for the customer. LEDCO made a complete all transistor module to replace the CDA board fitted in the Pye CT70/1 and 691/3/7 hybrid CTVs.
From the Radiomuseum: https://www.radiomuseum.org/tubes/tube_pl802t.html

Geordie McBoyne.

[Mike Watterson]

Thanks, Geordie. An interesting circuit. Two transistors (BJTs) so there is a "working" g2 pin. The first transistor driving g2 and also acting as a buffer/emitter follower to the o/p stage otherwise the input impedance would be too low. Anyone like to explain why it works with a negative bias?

One valve I'd be tempted to simulate is an 1L6. A DK92 will work at MW and lower frequency SW as a direct plug in despite possibly different construction and different names on the pins. The capacitances are wrong for the higher frequency bands though it does have the performance. The DK40 was used in Europe / UK before the DK92 as the DK91 (1R5) isn't great. The DK96 is also inferior to the DK40, DK92 and 1L6 for HF. People have hoarded the 1L6 so they are stupidly expensive. The 3rd party FET based replacements are also expensive.

I've not been able to find B7G plugs and my homemade attempts are poor.

[valvekits]

The BF337 had already been in use as a video output transistor for some time, so it wasn’t so unusual for someone to come up with a drop-in replacement for the PL802.

I remember reading the MOSFET Follies article a few years ago and never looking back. I’m usually happy to let solid state devices and tubes work together, particularly for the B+ supplies.

There was a time when some of the field service guys had got used to giving a soft crt  "a tickle” by removing the crt anode cap and giving the tube base a squirt of EHT. This wasn’t too bad when say a PFL200 was employed as the video output device, but was instant death to the BF337.

[Raditechman]

I still have a PL802T in stock.  Back in the day when we were using them I thought they  did not give as good a picture as a genuine PL802  so stopped using them , which is probably why I  still have one. I did not know the EF184 trick,  nice to learn something new.  Would it have been pin 3 ?

John

[ppppenguin]

PL802: http://www.r-type.org/exhib/aai0092.htm
EF184: http://www.r-type.org/exhib/aaa0229.htm

I'm surprised the EF184 worked well. Obviously it's overrun and will have a short life but its gm is less than half that of the PL802 which has the exceptionally high gm of 40mA/V.