Reviving an old full wave rectifying valve

[pwdrive]

For testing purposes as I suggested in post #16..... the valve base would be open at the top, plenty of air, a 4 ohm 10 watt wouldn't get too hot, it would be for testing the rest of the PSU before Nick plugged what might be a hard won U10.

Lawrence.

[SurreyNick]

I really like the sound of this most recent suggestion i.e. just the valve base open with the two diodes and the 4 ohm 10 watt load resistor open to view, especially if that will emulate the conditions for the U10. This was after all one of the two primary objectives;

a) Demonstrate the Mullard HT Unit is working.
b) Make sure I don't blow the U10 if and when I pop it in.

If we can therefore agree that this proposal at post #21 achieves this then that's what I'll do first. If there is then a better solution having re-attached the glass balloon to the valve base so the adapted 1821 can be used on an ongoing basis we can agree that too.

Can I please just check that the diodes as shown in the pic at post #13 are now the right way around too?

Thanks

Nick

[pwdrive]

Yes, the diodes are shown the right way round in the picture, each anode (AC in) is connected to it's own AC supply from the transformer secondary via a limiting resistor.

Both cathodes connected together (rectified +ve output) and then connected to the reservoir capacitor (or choke if it's a choke input filter) the three types of HT filter circuits commonly encountered are C-R-C (Capacitor-Resistor-Capacitor) C-L-C (Capacitor-Choke-Capacitor) and L-C ) Choke-Capacitor, the latter is encountered to a lesser extent and is commonly referred to as choke input.

As has been said earlier you will need to identify the connection on the valve socket that connects to the reservoir/filter circuit.

If the substitution rectifier is fitted and all checks out ok it would be worth loading up the HT output of the PSU to it's maximum recommended current.

I would replace any capacitors in the unit first though to save any bother later on

Hope this is of some use.

Lawrence.

[SurreyNick]

It is of great use Lawrence, thanks.
I will go quiet on the matter until I have done the testing and will then report back. I think I have some diodes kicking around somewhere.
My thanks to everyone who has contributed thus far with advice, suggestions and technical contributions. It has been both interesting and informative.
Nick

[Yorkie]

I've just realised that the U10 has a directly heated cathode, hence comments concerning connections to the heater.  Maybe Nick knows this already, in which case apologies for mentioning it but in a directly heated rectifier, the heater and cathode are connected together. Hence, on the heater you have the AC heater Voltage - in this case, 4V, and also, the DC HT output from the rectifier, the U10 being a full wave rectifier, 250 - 0 - 250V.

Some small valve rectifiers (EZ81 etc)  are indirectly heated and can be run from the same heater supply as the other valves in the equipment. Most, however, are either directly heated (i.e. the heater and cathode are one and the same) or the heater is connected internally to the cathode. This avoids the need for very thick heater-cathode insulation. If the heater and cathode are connected then the rectifier needs its own separate heater supply.

Had the rectifier been indirectly heated the diodes would need to go to the cathode pin, but as it is, as has been noted, it needs to go to the heater connected to the rectified DC output to the smoothing cap.

A little sketch attached showing the two arrangements of direct/indirect rectifiers.

Nick's already sorted, but it might be of use to others some time.

[Brianc]

For testing purposes as I suggested in post #16..... the valve base would be open at the top, plenty of air, a 4 ohm 10 watt wouldn't get too hot, it would be for testing the rest of the PSU before Nick plugged what might be a hard won U10.

Lawrence.

I'm sorry, Laurence. I missed the point that this was for test purposes only. Has measuring the voltage across the heater winding/resistor when the test fixture is in operation? It should be 4VAC.

[pwdrive]

According to the r-type org site the U10 is described as a directly heated job but it then goes on to say that it has cathode tubes that are connected to one side of the heaters:

http://www.r-type.org/exhib/aaa0643.htm

The layout for the U10 on the radiomuseum site shows no cathode tube:

http://www.radiomuseum.org/tubes/tube_u10.html

The Radio Valve Data book (Iliffe) also describes the U10 as directly heated and as a consequence shows no cathode tube that's internally connected to one side of the heaters, the UU5 however from the same data book is described as indirectly heated and shows the cathode tube internally connected to one side of the heaters.

The r type org site is a very useful resource but it does contain lots of errors eg: read the heater description for the DW2 rectifier then look at the actual Mullard data sheet:

http://www.r-type.org/pdfs/dw2.pdf

I've always assumed that a directly heated cathode is just that, directly heated ie: the current that heats the cathode flows through the cathode itself.

I think as a general indication the heater current can sometimes reveal as to whether or not the valve has a directly heated cathode as opposed to an indirectly heated cathode.

Make of it what you will.

Lawrence.

[AlanBeckett]

I always took 'Directly Heated' to mean no Cathode, ie the Filament is the source of the Electrons.
There can be 'Indirectly Heated'with the Cathode internally connected to one side of the Heater as with many Rectifiers.

Alan

[Kalee20]

Transformer Windings don't like being left floating so it's best to give them a bit of Load.

That can be true of an audio output transformer in a valve amp. Audio transients can excite self resonances to give excessive voltages. These can damage the valve and/or the transformer.

Not true for a mains transformer which is handling a steady sine wave.

Absolutely - a mains transformer operating with a constant voltage won't mind having its windings unloaded. In fact it will like it as it will run slightly cooler.

Being it's less loaded as a whole, it is barely possible that the voltages from the other windings may be slightly higher, maybe 1%. But that won't matter.

[Kalee20]

I always took 'Directly Heated' to mean no Cathode, ie the Filament is the source of the Electrons.
There can be 'Indirectly Heated'with the Cathode internally connected to one side of the Heater as with many Rectifiers.

Alan

I agree - there are several rectifier valves which are directly heated (eg 5Y3, which is 350V 120mA capability with a 5V heater); indirectly heated but with heater internally connected to cathode (eg 5U4, which is 450V 225mA capability with a 5V heater); and indirectly heated with heater isolated from cathode (eg 6X4, which is 325V 70mA capability with a 6.3V heater).

The idea of an indirectly heated cathode, even though it is internally connected to heater, is so that it will take a while to warm up so the HT capacitors won't charge to a high peak voltage due to the rectifier being active before the other valves in the equipment have warmed up. It's something to bear in mind when replacing valve rectifiers with silicon diodes - check your capacitors!