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In Reply to: RE: Technical question on 6sn7 cathode posted by hifipaul on August 15, 2015 at 09:08:58
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...
Follow Ups:
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.
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