do tubes ever leak b+ into heaters?

[ryuji]

i was rolling through tubes and found a tube i liked better in my subwoofer. while checking all voltages etc later after some unrelated adjustments i noticed my heaters were 130v. cut the wire leading to the divider network raising my heaters, the divider network puts out a flat 70v which was intended, heater voltage increased to 200v.

sat down and one by one pulled tubes watching heater voltage... pulled the subwoofer pre-amp tube and voltage shot down and is now slowly drifting towards zero again... bad tube?

[infernal]

First check for short circuit whithin soldered wires on the socket. Be sure that they are not touching each other.
Which tube?
If you have the same one for which you are sure that is ok, place her in the same place. And watch. Voila, the problem is gone.
(my opinion: bad tube)

[ryuji]

two tubes are doing it, one of them, somewhat convieniently leaks 50~70v into the filament. i didnt notice probably because the divider was already there. is any kind of such leakage bad and should be replaced, or is 'convienient' leakage ok

these are 12au7/12ax7 tubes btw

[DF96]

Are there any cathode followers? If so, it could be heater-cathode leakage.

[ryuji]

yea there cathode followers in both cases. i've decided that i have tubes that are leaking into the heater, and thus just one step away from a complete short. so they have been replaced

[kevinkr]

Originally Posted by ryuji

yea there cathode followers in both cases. i've decided that i have tubes that are leaking into the heater, and thus just one step away from a complete short. so they have been replaced

DF96 is spot on, and you are on the right path in replacing the culprit(s) - the sooner the better IMHO.

[DF96]

I'm curious: what make are the bad tubes? H-k leaks can happen to any, but to get two in one amp seems quite bad luck or a bad batch?

[ryuji]

Seems to be bad luck, two out of 9 tubes. 14 if you count output tubes. Completely different brands and designs but same type tubes I use 5814a's in my au7 sockets

[DualTriode]

Before you shoot the guilty tubes a dawn test your assumptions and do a differential diagnosis. You say that it is not just one tube acting oddly in your estimation. It could be environmental.
The tube data sheet will list the maximum voltage delta between the heater and cathode. Circuits are designed to control the cathode to heater voltage. Could be a voltage divider between B+ and ground tied to the heater. Could be a capacitor tied between pin 9 and ground then letting the heater voltage settle due to capacitance between heater and cathode. (Note that there is capacitance between H and K but not listed on the data sheet). If you let the heater float without referencing it to something the observed voltages will leave you scratching your head trying to sort out the differences between “good” tubes.
DT
All just for fun!

[ryuji]

i have a voltage divider+filter for elevating heater voltage, i only temporarily disconnected it to see if the divider was the problem(it puts out 70v) 200v or thereabouts makes sense as that's my cathode resistor voltage reading
heres a schematic of my preamp if your wondering. listed voltage test/read points are incorrect, my amplifier runs at 400v b+ and my anode resistor is 33k yielding about 170v for the first half and 215 or so for cathode follower