Rectifier valves and compatibility

bandmaster188

just sorting out some boxes of valves and have a few rectifiers kicking around. I seem to remember reading that I can plug in any rectifier as long  as the amp is  rebiased to suit. Is this correct? And does this apply if the same amp can run 6v6 or 6l6? Amp currently uses a gz34. So could I try a 5u4,5y3 or SS plug in for example?
one of the rectifiers I’ve just dug out is a National Electronics gz34. This only has 4 pins whereas all the other gz34’s I have , have 5. The national is missing pin 1. Should I bin it or is this normal ?
cheers!

WiresDreamDisasters

I don't think that is true.

ICBM

No, it is not true.

There are three important specifications:

Voltage rating
Current rating
Filament current draw

If the substitute valve has too low a voltage or current rating it can fail - which will usually just blow the amp’s primary or sometimes HT fuse. If it has too high a filament current draw it can overload the power transformer - this is quite an insidious problem and you won’t notice initially, but the transformer can eventually fail.

For example a 5U4 has higher voltage and the same current ratings as a GZ34 so you may think it’s OK, but it draws much higher filament current.

For voltage and current ratings you really need to measure the amp in use - including current draw at full power. For example a 5Y3 should have too low ratings to be substituted for a GZ34, but in some amps it can be since the GZ34 was really unnecessary. To be sure about the current rating you also need to know what the amp’s first filter cap value is, since rectifier valves have a rated maximum.

Rectifier valves actually only use four pins for the connections, the fifth is unused - I think it was fitted to prevent them being confused with older 4-pin valves - and assuming it’s that one which is missing the valve will work fine.

A solid-state plug-in (plain diodes, not one with a resistor to simulate a valve) can almost always be used since they have no filament current and the voltage and current ratings are usually far higher than a valve, but they will need the bias checked, and with some vintage amps it’s possible the filter cap voltage ratings could be exceeded since the HT voltage may be substantially higher.

Does that all make sense?

bandmaster188

Thanks guys, yep all fully understood. Think will be best to leave well alone and stick with the gz34.