Brian Goldsmith wrote:

snip however the original poster stated he wanted a
replacement for a 5R4.This tube has a maximum plate Voltage of 750 RMS
per plate for which a 1N4007 will suffice.


The 5R4 has a maximum piv voltage of 2800 volts,
so you would still need 3 1N4007's in series
in each leg for equivalent safety factor.

The rated max supply voltage of 750V rms per plate means that
you should not use a transformer higher than
1500V rms center tapped in a full wave single phase
rectifier circuit using a 5R4.

The designers expected switching transients
when the current is interrupted in the HV transformer
and HV choke, and therefore included a max
supply voltage for guidance.

73,
Ed K4PF

In article , Edward Knobloch
writes:

The 5R4 has a maximum piv voltage of 2800 volts,
so you would still need 3 1N4007's in series
in each leg for equivalent safety factor.

Absolutely correct.

The rated max supply voltage of 750V rms per plate means that
you should not use a transformer higher than
1500V rms center tapped in a full wave single phase
rectifier circuit using a 5R4.

The 750 volt rating is for capacitor input. With choke input you can go a bit
higher.

The designers expected switching transients
when the current is interrupted in the HV transformer
and HV choke, and therefore included a max
supply voltage for guidance.

Even with no transients, the peak voltages that the diodes have to withstand
are quite high, compared to the RMS transformer voltages. Here's why:

First off, the voltages in the transformer catalogs and tube manuals are RMS
values, not peak values. 750 volts RMS equates to 1060 volts peak with sine
waves. So under perfect conditions a 750-0-750 (1500VCT) transformer with
fullwave center tap rectifier and capacitor input filter can have voltage
output of 1060 volts.

But there's more! In our 750-0-750 example, each transformer HV lead goes from
1060 volts (peak) positive to 1060 volts negative with respect to the center
tap. So each diode is subjected to as much as 2120 volts when it is in the
nonconducting (inverse) state. So you need *at least* three 1N4007s to replace
one side of a 5R4. Four diodes won't hurt a thing.

The tube manuals I have show voltages higher than 750 for capacitor input, but
they are for the 5R4GYA.

In any event please use at least three and preferably four 1N4007s or
equivalents in a 5R4 replacement. Plate transformers and filter chokes can be
expensive and hard to replace.

73 de Jim, N2EY

In article , Edward Knobloch
writes:

The 5R4 has a maximum piv voltage of 2800 volts,
so you would still need 3 1N4007's in series
in each leg for equivalent safety factor.

Absolutely correct.

The rated max supply voltage of 750V rms per plate means that
you should not use a transformer higher than
1500V rms center tapped in a full wave single phase
rectifier circuit using a 5R4.

The 750 volt rating is for capacitor input. With choke input you can go a bit
higher.

The designers expected switching transients
when the current is interrupted in the HV transformer
and HV choke, and therefore included a max
supply voltage for guidance.

Even with no transients, the peak voltages that the diodes have to withstand
are quite high, compared to the RMS transformer voltages. Here's why:

First off, the voltages in the transformer catalogs and tube manuals are RMS
values, not peak values. 750 volts RMS equates to 1060 volts peak with sine
waves. So under perfect conditions a 750-0-750 (1500VCT) transformer with
fullwave center tap rectifier and capacitor input filter can have voltage
output of 1060 volts.

But there's more! In our 750-0-750 example, each transformer HV lead goes from
1060 volts (peak) positive to 1060 volts negative with respect to the center
tap. So each diode is subjected to as much as 2120 volts when it is in the
nonconducting (inverse) state. So you need *at least* three 1N4007s to replace
one side of a 5R4. Four diodes won't hurt a thing.

The tube manuals I have show voltages higher than 750 for capacitor input, but
they are for the 5R4GYA.

In any event please use at least three and preferably four 1N4007s or
equivalents in a 5R4 replacement. Plate transformers and filter chokes can be
expensive and hard to replace.

73 de Jim, N2EY