I got these on ebay for an IF strip to replace 6EJ7's which had too much gain and lead to instability. They look nothing like a EF80 internally and don't look ever used. They work perfectly! Thing is when you first power them on you get an incandescent glow from the bottom of the tube which disappears about 2s as the heaters warm up. I've not seen this before is it some sort of heater protection?
https://www.worldradiohistory.com/h...less-IDX/IDX/70s/PW-1972-03-OCR-Page-0090.pdf
https://www.worldradiohistory.com/h...less-IDX/IDX/70s/PW-1972-03-OCR-Page-0090.pdf
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A lot of Mullard tubes were famous for "heater flash" on startup. Maybe your tube was made by them. This was totally normal. Are there seams on the top of the tubes?
jeff
jeff
Yes there's a sort on internal ridge on the top. The flash seems to come from the bottom of the heater where is comes out of the cathode tube. Buts is hard to be sure. Never seen this before.
I guess the exposed bit get hot quicker so its resistance goes up first and so gets most of the power in that bit - a sort of thermal runaway. Anyway its quite an funky looking tube.
pl802 interesting. I assumed the outside was the plate buts its a screen. That's why I get instability if I put my finger near the bottom of the limiter tube where the plate wires are exposed and not the middle where's there a screen. I guess that's normal practise with IF tubes.