Changing a vacuum tube rectifier with silicon diodes.

Hi
I recently received a radio from the 30s which wasnt working at all. I came across the schematic and checked all tubes individually. The only one that wasn't in a good state was a the rectifier, a GZ32, which had their filaments broken. Because I can't afford a new valve, I inmediately thought of replacing it with diodes. Only problem is, am really new when it comes to vacuum tubes (I have studied manly the theory behind their opperation and stuff like that) and I am not sure about what diode. So, I was wondering, is a 1N4007 good enough for this application, do I need differents components or this is just a terrible idea? I have been searching in the Internet and I have came up with two conclusions: many companies sell wallets with the diodes (which I cannot afford) or that I need a resistance in series in order to emulate the internal resistance of the tube, but I have not been able to find it. I leave here the datasheet that I have found.
Any help is appreciated!
Datasheet: http://www.r-type.org/pdfs/gz32.pdf

Looks like you need something (a diode) which can safely handle 700V @ less than 1/2 an Amp.
A 1N4007 (1000V @1A, I believe) should do nicely.
Where you might have a problem is if the filament voltages are derived loading them in serial with the line, instead of in parallel with one or more 'filament transformer(s)' (sometimes done to save expense of additional transformers). then you will need a load in the filament line to simulate the drop of the rectifier tube (valve). do ALL the other tubes light up now with the rectifier blown? if so, you are good to go with just the 1N4007.

Hi,

There are two major considerations:

  • Is this a transformer-based power supply?? The 5V 2A rating of the tube suggests it is. In "Series-operated" radios the tube filaments are in series and must be replaced with simuilar type or special work done to compensate for the series resistance of a tube filament.

  • The silicon rectifiers are "instant on" and the internal B+ voltage will rise very quickly. Some circuits depend of the slow heating of the rectifier tube to allow capacitors to charge and other series B+ circuits to settle. Hard to say.

Lots is know about this; Google "replacing a vacuum tube rectifier with silicon diodes"

1n4007 is a good choice IF it's workable.

So, in conclusion, I have to check if the valves light up without the rectifier. And if does, just put the diodes. If not, I have to add some resistance to compensate. The question is, how do I calc it, should I build a delay circuit. Tomorrow I will take the necessary 220-125 transformer and I will try it.
Thanks for al the help so far!

I'm old enough to remember a little about tubes, but it's been a long time.

I recently received a radio from the 30s which wasnt working at all. I came across the schematic and checked all tubes individually. The only one that wasn't in a good state was a the rectifier, a GZ32, which had their filaments broken.

Did you find a tube-tester or did you just check the filaments?

You'll need two diodes, but silicon diodes will probably work and series resistance probably isn't necessary. The resistance is actually an undesired characteristic of the vacuum tube rectifier. (We can't be 100% sure that the circuit will work properly without the "normal" resistance, or without the normal voltage-drop, but a silicon diode is closer to an ideal diode.)

Once you've replaced the tube with diodes, check the power supply without connecting-up the rest of the circuitry. (I assume there is no PC board and you can just un-solder a couple of wires.) Or if that's not possible, at least remove the other tubes. Make sure the polarity is correct and make sure there's no AC.

Capacitors can go bad too...

as wondering, is a 1N4007

You're tube data shows it's rated for 2A, and the 1N4007 is rated for only for 1A. And, you can't measure the actual current without a working power supply. I'd look for something with a higher current rating.

Have you measured the AC voltage (without the tube)? Remember that the peak is ~1.4 times the RMS voltage, and you should allow for unexpected voltage spikes, so maybe get a diode rated for at least twice the AC voltage.

If it was me, I'd find a way to plug-in the silicon diodes... You might be able to find a matching plug, or you can break the glass and use the old tube base-plug, or find another old tube you can break and use.

Hi!
I know its broken because inside is a piece of filament which is visually broken ( its dancing inside the tube). Unsoldering seems pretty difficult por even impossible BTW the connections are made. But of course I will take out al the tubes. About the diode, would it be possible to use 2 1N4007 in parallel?
Sadly I am in holidays in a quite remote location, so I al afraid new components is difficult.

Also, it is a transformer based PS, and the heaters dont have his own transformer. Luckily, connecting the diodes isnt a problem: I will use the plug from the tube.

Atomillo:
Hi!
I know its broken because inside is a piece of filament which is visually broken ( its dancing inside the tube). Unsoldering seems pretty difficult por even impossible BTW the connections are made. But of course I will take out al the tubes. About the diode, would it be possible to use 2 1N4007 in parallel?
Sadly I am in holidays in a quite remote location, so I al afraid new components is difficult.

Don't put two diodes in parallel and expect to double the current capacity. One diode will always take the full load because of variations in internal resistance. One diode will be plenty for a radio receiver.

Another suggestion. Break the glass envelope on the dead rectifier. Clean the wires out of all pins using a soldering iron. Then solder the two diodes to the socket pins to replace the old tube functions.

The plug in your modified tube base.

Paul

Thats a very good idea. After I have checked that voltage is where is supposed to be, how do I make sure that the tubes are not damaged?

The CZ32 is rated a 5v 2.3 amps for it's heater, but only 3-400mA anode - cathode current ( using both anodes) with 500v HT.

Hence 1N4007's should be OK.

But

The slow heating of the GZ32 cathode implies a slow rise of HT, by which time all the other valves will be conducting, and will reduce the voltage.

So there is a danger that the HT will over - volt the reservoir capacitors until the other valves heat up.

Allan

Can I create some sort of delay in order to deal with this?

Inrush currrent limiting resistors can be useful here - see eg

http://www.mouser.com/ds/2/168/920-325D-LR-245176.pdf

to choose a suitable one it would be useful to know the current taken - what other valves does the set use?

Allan

Allan

As son as I wake tomorrow I will post the entire list of valves, but now I have to go to bed.
PS: im on Spain.
Thanks for all your help so far!!

It would also be useful to know if there's an HT choke - could you post a picture?

Allan

Hi!
Luckily for me, at the back of the radio there is a schematic with al the tunes and parte. I proceed to put it here.

A historic document! The heater wiring seems to be implicit in an otherwise comprehensive circuit diagram.

Its got a set of fixed stations with individual bandpass filters, rather than a tuning dial by the look of it...

I suspect any current limiting resistor can go between the rectifier and CH1 (choke 1).

While you are at it move whichever of the fuse and on-off switch is on the neutral to the live, its not
great to have live potential on something that's off!

How can I calc the necessary resistance and wattage of the resistor?

I have another problem: the input AC wires are connected directly to the rectifier, and not to the select wheel. There is also no fuse and the principal pin of the swich is not connected anywhere. Do I plug directly or what do I do?

By understanding the assumptions in the rest of the circuit, not so easy I think. If you know the characteristics
of the failed rect. valve you can figure out its voltage drop at load, and add an R to drop that, so long as you
know the load of the rest of the circuit. The rapid risetime issue is not fixed by just a resistor, but it might
not be an issue at all of course. Valve circuitry is pretty robust against overload (apart from the heater
filaments), but a good understanding of that kind of circuit is needed - I'm too young for that(!).

300Vdc bench supplies are few and far between these days too - a specialist would have the right tools for this
of course.

wattage of the resistor is easy once you know the load current - I-squared-R.

So its not like the diagram, but live-chassis? I think you need an isolation transformer and variac to work on
this really, and do take care with the lethal voltages...