heater cathode leakage

i have a TAB V72 with higher than expected hum in the output noise floor. I put the Tubes which curiously are unlabelled into the tube tester and they showed a heater cathode leakage of the order of 2meg ohm. to prove a point I the subbed in a pair of NOS telefunken EF804S into the V72 and all hum on the output sunk below the white noise floor.
I'm now curious about methods that might reduce the C-H leakage in the original tubes as their other characteristics measure fine. I suspect these tubes might be vanilla EF804. So far i've tried pushing them in the tube tester at a 10volt heater voltage with no HT for a few hours and that has dropped the leakage down to better than 10meg....but feels a bit scary. Anyone got any thoughts on other "rejuvination" processes?

I suspect any 'rejuvenation' will be temporary at best. The only other thing you might try is adding a +ve bias to the heaters in the original circuit as this tends to prevent heater voltage entering the cathode.

Cheers

Ian

yeah, I'd like to avoid modifying the heater circuit of the V72 that these are out of and thus the "best" solution is to simply buy some good NOS EF804S ... but they are getter rarer and quite expensive.
I did end up running these leaky tubes for a couple of hours at 10V heater voltage and have got the H-C leakage down to less than 25Meg now. The proof will be seeing if the hum level out of the V72 is acceptable once they are reinstalled. As you say how long this reconditioning lasts for is another matter.

Hello,

the EF804s has a weaker isolation between heather and cathode than the EF804. Better keep the filament voltage at the level of the cathode.

Best regards
Andreas

an update on this "rejuvination" process. Running the heater at 175% on both EF804 tubes for a couple of hours dropped the leakage to below 30Meg. With the tubes back in the V72 the hum level dropped around 15dB, down into the wide band noise floor. Time will tell how they hold up.

squib said:

an update on this "rejuvination" process. Running the heater at 175% on both EF804 tubes for a couple of hours dropped the leakage to below 30Meg. With the tubes back in the V72 the hum level dropped around 15dB, down into the wide band noise floor. Time will tell how they hold up.

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So, are you saying you ran the heaters on 11V ac for a couple of hours - no HT or anything else attached?

Cheers

Ian

thats correct, just heaters, no HT

squib said:

thats correct, just heaters, no HT

Click to expand...

Interesting. I have a number of tubes that exhibit poor hk leakage. let me know how long the 'cure' lasts for.

Cheers

Ian