30-10-2015, 04:58 PM
(29-10-2015, 06:17 PM)SurreyNick Wrote: 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
Apologies in advance that I simply don't have time to go through all the responses to this thread (this public place closes at 17.45. So, given time is short, I'll just add a few ideas about rectifiers.
Firstly I'm currently working with a U10 so am familiar with the tube itself. I spent some time setting up silicon rectifier alternatives so here's how..
O.K. here goes and very precisely. For some this may be a bit simple:
It's always a very good idea to remove all your tubes first. You stated you fear something catastrophic could be upsetting which is why by removing all the tubes you can test your rectified D.C. in peace and quiet. Output tubes can be stubborn to remove but the rest are much easier. I put my fingers dead on the base of the tube while pushing down with my knuckles on the chassis. You have to ratchet up the force with some care and out it will come.
A U10 has 4 pins. At any rate, it's always wise to be absolutely clear where are your transformer anode wires and where your cathode wire is. The rectifier tube cathode wire is easy to locate - it leads straight from a tube pin to the big reservoir can As a point of interest, if you should test the cathode wire with a digital multimeter when a rectifier tube is working you should get 0 volts A.C. but around 380 or around D.C. (I did this test last week).
Confirm where the transformer anode wires are (the cathode wire leads to the HT can). Both anode wires from the transformer will go to the silicon diode anodes. There are two diodes so for full wave rectification the two transformer wires will lead to both of the diode anodes.
Output will be now at the joined cathodes (bridged to make one output) and you can measure voltage if you wish. It ought to be 1.414 times the A.C. voltage of the transformer.
At this point I put in parallel with each diode a 10 Nanofarad capacitor between anode and cathode of the diode each side. That is also 0.01 Microfarad if you wish. This reduces the high voltage at the cathode somewhat through capacitive and inductive reactance.
Finally the choice of series resistor. No harm starting with 1 K to see what the voltage will be. It must be at least 5 watts. If voltage is too low, reduce the Ohms. I found 220 Ohms worked just right.
Check also the grid bias resistor to avoid any excess positive bias on the grid.
Locate correct output valve anode voltage in schematic and use that as a guide. When all seems well you can connect the H.T. to the H.T. can. Then put tubes back in.
Output will be at the cathode. These leads are indicated by a stripe.
CAUTION: All of this work involves very high voltages and none of it should be undertaken till absolutely certain. I only outline the mechanics of it all but how you approach it requires a personal understanding. Matters of load on the system I left out of it.
