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Old 6th October 2012, 08:51 PM   #11
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The use of the diode was made popular by John Broskie. Its purpose is mainly to protect the second tube if the first tube loses conduction (pulled from socket or shook loose etc.) in an already hot amplifier. A little redundant if using a double triode like in the op schematic. The need for it to deal with warmup issues would only be when using a current source load on the first triode since just a resistive load would self balance if the grid began to draw current the resistor would then begin to drop voltage and reduce current where a ccs would let the grid reach the full available current before it began dropping voltage.
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Old 6th October 2012, 09:12 PM   #12
kevinkr is offline kevinkr  United States
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Star Trek reference: That diode is a dunsel..
Dunsel - Memory Alpha, the Star Trek Wiki
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Old 8th October 2012, 03:56 AM   #13
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I'm not convinced it is a "dunsel" but then I'm not convinced it is'nt either!!!! (fence sitters unite - yah!)

If you are at all concerned about the capacitance of the diode then use 1N914, 1N4148 etc. small signal diode in preference to power diodes (*as suggested above) and for serious paranoia use 2 or 3 of them in series to divide the total capacitance.

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Ian
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Old 8th October 2012, 10:30 AM   #14
DF96 is offline DF96  England
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Quote:
Originally Posted by gingertube
for serious paranoia use 2 or 3 of them in series to divide the total capacitance.
That could be less successful than you might expect, as the reduced reverse voltage across each diode would increase its capacitance so the net capacitance would not reduce by so much.

Whether this diode/neon is needed could depend on what the valve is. A high gain valve with fine grid very near the cathode (e.g. ECC83, ECC88) is more likely to benefit from it than a low gain valve with a coarser grid further away from the cathode (e.g. ECC82, 6CG7).
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Old 8th October 2012, 05:10 PM   #15
disco is offline disco  Netherlands
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How about using a sting of LEDs under the cathode. Follows the same practise I'd guess?
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Old 8th October 2012, 05:13 PM   #16
DF96 is offline DF96  England
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If you can find LEDs designed for low capacitance rather than high light output.
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Old 8th October 2012, 05:13 PM   #17
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by jerluwoo View Post
The use of the diode was made popular by John Broskie. Its purpose is mainly to protect the second tube if the first tube loses conduction (pulled from socket or shook loose etc.) in an already hot amplifier. A little redundant if using a double triode like in the op schematic.
No, the diode protects against start up too, when the cathodes are still cold and therefore the grid of the CF is pulled up to the supply voltage while the cathode is still stuck at ground. I have killed a few valves this way! (arcing between grid and cathode). This is not just a Broskie technique- it's been around for decades in analog computers.
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