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Suicide Varistor Diode

Many times there are vintage pieces of audio gear that have components that are destructive if left as-is. Transistors and diodes are top of the list along with old capacitors.


Here we're looking at the VD-1221/22 varactor diode. It is a double-diode (in series). >|>|. In a bias circuit, the warmer they get, the lower the forward voltage drop of the diodes which tracks the behavior of the base emitter junction of the power output transistors. This keeps the bias current from running away when the output devices heat up.


Commonly called jelly-beans, spider-eggs, etc., here is what they look like:





The above diodes are inherently unreliable and when they fail...they fail HARD. Many times this failure event will take the output transistors with them. The Luxman L-80V, L-85v, L-100 and the ultra-rare Sony VFET amplifiers all have them (TA-3200, TA-4650, TA-5550, TS-5650). Many other amps, receivers and even turntables do, too. If you smoke a VFET, you're out of luck as there are no modern replacements for them in this old gear.


Luckily 2 x 1N4148 diodes in series can backfill for these, easily. If I see them, they are eliminated on sight!

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