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Vintage threads from the first ten years

Valve rectifier Q

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12/8/1999 1:46 PM
Lars	Valve rectifier Q How can I secure the amp if the Valve rectifier break down and let AC in to the amp ? Lars

12/8/1999 2:00 PM
Dave Stork	As a safety measure, some builders like to install silicon rectifiers in series between the transformer secondary and the tube rectifier plates. That ensures that the B+ will continue to be rectified even if the tube rectifier happens to short.

12/8/1999 3:53 PM
lars	2 questions If I do so must I rebias the amp ? Will the amp sound be the same ? Lars

12/8/1999 4:46 PM
keith	The sources I have read lead me to belive that no rebiasing should be necessary and the tone should be unaffected. Anyway, the diodes will only be active in the event of rectifier failure, so replacing the rectifier tube should fix any percieved tonal change. Keith

12/8/1999 4:47 PM

Inserting SS rectifiers in series with the existing tube rectifier will give an added voltage drop less than 2 volts, hardly anything to worry about compared to 300 or more volts B+. You won't have to re-bias for normal operation. The amp will sound the same, as long as the tube rectifier is active.

If the tube rectifier shorts (that is what you intend to guard against happening?), the plate supply voltages to the rest of the amplifier will go up, maybe 20 volts or so. Under these conditions, the output tubes could be running in an overcurrent condition and you wouldn't know it has occurred before it's too late.

You may want to consider a fuse in series with the high voltage supply. Marshall amps have this feature. The SS diode addition will prevent AC from being applied to your filter capacitors in the event of a shorted tube rectifier, but the fuse may be necessary to prevent excessive current draw by output tubes not having enough negative throttling voltage on the control grid compared to what's needed when the plate & screen voltages have been raised above normal during the upset condition.

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