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How many volts can the cathode of a cold 6sn7 withstand before problems occur. I have a split differential with -90v under the tail and B- is there for ~30sec before the tube warms up and B+ arrives.
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No need for protection diodes or the like as well.
There is a specification for heater to cathode voltage. I believe it is just insulation, and thus not very sensitive to temperature.
See the GE spec sheet for the best detail - the 6SN7GTB can take +/-200v on peaks, but the heater should not be more than 100v positive relative to the cathode on a steady basis. Since you are only 90v you are technically OK, and if you assume 30 seconds is not steady DC then you have 110v margin.
I believe lower voltage differentials are usually preferred; this specification is often aspirational than realistic. If you need serious heater/cathode insulation your best bet is a Bendix Red Bank 6900...
I think the problem is ...
-90V on the cathode with no B+ applied so
no anode current and ground reference on
the grid for 30 seconds before B+ comes up.
DanL
If that's the problem (original post was not clear to me about that) than a diode between grid(s) and cathode will prevent positive grid greater than the diode forward voltage.
An NE-2 neon bulb can make an effective voltage limiter that is not there until it conducts. It was used often in oscilloscopes, etc., to protect heater-cathode insulation during warm-up.
Spec: Chicago Miniature A1A/NE-2 (standard brightness, wire leads): maximum strike voltage = 90 V DC; minimum and sustain voltages are not specified.
It's an interesting situation. IMO, what you have to watch out for is control grids positive with respect to cathode, if the cathode is emitting. Small signal triodes are most definitely not rated for positive control grid current regimes.
In "El Cheapo", B+ comes on quickly and B- is delayed. Cathode stripping is a complete non-issue below 400 V.
Eli D.
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