• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Tube Rectifier protection

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I'm sure this has been covered before but I could'nt find it.

I have a 5R4 => 2uF polypropylene => 8H => 100uF power supply in a 6V6 Push Pull Amp. That is, it is almost a choke input supply.

At switch on I occassionally see arcing in the 5R4. I seem to recall someone suggesting protecting of tube rectifiers against start up arcs by adding solid state diodes.
Should there be a diode in series with each tube recifier anode, a single diode in the tube rectifier output or what?

Any advise gratefully received.

I don't know if this is relevant, but some of the 5R4 data sheets suggest limits on the current and voltage prior to the rectifier filament heating up; http://www.tubezone.net/pdf/5r4.pdf

Try modeling your supply in PSUD. You may find that the initial conditions (first few cycles at the rectifier) exceed the limits. I don't know if protection diodes would help here, as I would guess this is a forward conduction issue (but I'd be curious to hear a learned opinion). Maybe a current limiting thermistor on the primary, to keep the initial current peaks below the threshold?

Joined 2004
You don't say anything about your power transformer. According to data I've seen, the 5R4 needs at least 250 ohms in series with each plate. If your OPT has less resistance than that, in each half of the secondary, you could try adding a resistor in series with each plate to make up the difference.


Disabled Account
Joined 2006
G.Kleinschmidt said:
These are the diodes you want:


Just put one with an appropiate clamping voltage in parallel with your rectifier.
It will clamp the voltage to a safe maximum and conduct the excess transient current.

Sorry, my brain wasn't engaged here. You need to connect the transil diode in series, back to back with a p-n diode for the protection of a tube rectifier.
I was thinking of protecting series pass tubes in power supplies.
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