Reviving an old full wave rectifying valve

[SurreyNick]

In another (wanted) thread seeking a full wave rectifying valve to replace a duff 1821 ppppenguin suggested hiding a couple of 1N4007 and a limiting resistor in the base of the duff valve so it could still be used. I have been giving this some thought and I'd like to have a go. Someone even suggested hiding a tiny 6v bulb in the base too.

Although I have acquired a U10 I don't want to just pop it into the Mullard PM Unit and hope for the best, because it would reduce me to tears if it went 'Pop'!

I have never attempted anything like this and so would appreciate some instruction.

Do I remove the valve base from the balloon of the 1821, and if so how do I do this without breaking the glass? They seem well attached to each other.
I'm assuming I will then have bare wires projecting from the valve. Do I snip these off?
How do I then wire up the diodes and resistor to the pins of the B4 base? I am assuming I'll need 4 diodes because the valve was a full-wave rectifier and that's also what the HT Unit takes.

I am very conscious all I ever seem to do is ask for help on this forum and contribute very little but I promise I did have a look on Google first to see if I could find an instructable but turned up nothing.

Nick

[pwdrive]

Nick, do the 1821 heaters work or are they open circuit?

Only two diodes will be needed, four diodes are usually only used for a full wave bridge.

Lawrence.

[Brianc]

The bases are normally held on by shellac and a filler of some sort - sawdust or similar. This can be softened with thinners. The wires need to be un-soldered by heating the tips of the pins and using a sucker or braid. Once the wires are free, the base can be pulled off. As laurence says, only two diodes are required and these go between pins 1 and 3 or 4 (the heater pins) and between 2 and the same heater pin as the first. Do NOT join pins 3 and 4 together or you will short out the secondary on the power Xfmr. You must examine the wiring to see which heater pin to use. It must be the pin to which the reservoir capacitor is connected. I hope I have explained that well enough!

[Yorkie]

You actually need methylated spirit to dissolve the cement which was used to fix the valve base to the bottle. As Brian says, it was a mixture of shellac and wood flour. (It won't have been cellulose shellac so thinners won't dissolve it).

If ever you need to take a base off  to replace with one of a different type,  as I've discovered to my cost, never simply unsolder the wires and remove the base expecting the wires to obediently stay where they are in a nice ring formation. The 'pinch' which is recessed into the base of the bottle is flat - not circular, and once the base is pulled off, the wires take on a life of their own and spring into whichever  random formation they have a mind to. Hence, all you can then identify is the heaters. The anode, cathode, grid, whatever could be any combination.

What I therefore do is make two cuts around the old base with a Dremel & cutting disc to free up a ring from the wall of the valve base, then cut through the ring  and discard it, to expose all of the wires. You can then see which wire goes to which pin of the valve base, tie on a little label to identify it, then you can safely unsolder all the wires and remove the old base, ready to extend the wires to the pins of the new alternative base before fixing it to the bottle and soldering the wires in each pin. I then remove the labels from the wires and slip coloured sleeving over the wires.

To illustrate what I meanI've attached a few pics of an EBL1 side-contact valve being converted into an EBL31 by substituting an octal base. This is all irrelevant in your present case as all you are intending to do is to remove the bottle, snip off wires (unless you intend to keep the heaters connected for appearance sake, if intact), and to fit silicon rectifiers internally in the valve base across the appropriate pins, so you don't need to know which wire was which, and the heater wires can be identified come what may, if the heater is still intact.

You certainly like multiple challenges Nick!

Good luck with it.

[AlanBeckett]

I'd also be inclined to put say 220R in series with each Diode and, assuming it's O/C, 100R across the Filament Pins to avoid having the Transformer winding unloaded.

Alan

[SurreyNick]

In fact I don't have many things on the go at the moment and this seems like a nice distraction.
N,

[SurreyNick]

Do you mean like this?

Brian you said "Do NOT join pins 3 and 4 together", whilst Alan you said "assuming it's O/C, 100R across the Filament Pins". Well it is open circuit so I have shown the 100R resistor. Is this correct?
Brian does it matter which way around the diode goes as long as the polarity direction is the same i.e. as shown in the attached image?
Brian you also mentioned the diodes must be connected to be the heater pin to which the reservoir capacitor is connected i.e. pic A or pic B. Can you tell me how I work out to which pin the reservoir capacitor is connected?

Thanks

Nick
   

[AlanBeckett]

Morning Nick.

It doesn't matter which Diagram you use but your Diodes are backwards. 'Conventional Current' flows from the Anodes to the Filament so they should 'Point' in that direction.
Transformer Windings don't like being left floating so it's best to give them a bit of Load.

Alan

[ppppenguin]

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.

[AlanBeckett]

Maybe.
I've always found it good practice not to leave Windings floating. For the sake of a Resistor it's not worth the risk.
In this case that winding will be elevated to HT Voltage.
Put a Resistor across it.

Alan