Vacuum tube rectifier

[vetmed]

Too high a voltage on the cathode of a directly heated rectifier: what are the consequences? I imagine the tube's life will be shortened. Possible that the B+ will be higher than expected? I'm using an 80 on a winding that is supposed to be 5 volts, 2 amps and I am getting 5.6 volts ac. B+also came in about 30 volts higher than I expected, and I looked at every data-sheet I could find. It would be easy to fix but it has me curious.

Thanks
     Robert Lees

[galyons]

Is voltage loaded or unloaded? For what primary voltage is the power transformer rated?  Looks like typical results from a 115-117VAC tranny on 125VAC, if the secondaries are loaded. If unloaded, then loading will probably give spot on specs.  If the measurements are loaded, yes easy to fix, but  terribly out of whack!  Depends on how close to your desired design sec.

Cheers,
Geary

[vetmed]

The voltage is loaded. The only reason I decided to check the filament voltage was because it seemed to glow brighter than usual. But I am still wondering what effect this over voltage might have

Robert Lees

[Grainger49]

Robert,

How much load does the filament winding have?  If it is a 2A winding with a 0.3A draw it will float high.  If you are drawing 70% or more of the rated current it shouldn't float high.

[vetmed]

The load for this tube is 2 amps. Perhaps the winding is actually 3 amps. Then with a bit of over voltage from the AC supply and a little underloading perhaps can account for this. Still, I remain curious if this is harmful to the tubes longevity, and if it contribute to an increase in B+.


Robert Lees

[Paul Birkeland]

Still, I remain curious if this is harmful to the tubes longevity, and if it contribute to an increase in B+.

Yes and likely.

There's an easy way to find out - drop that extra 0.7V with a pair of 0.15 Ohm resistors and see what happens to the B+.

5.7V is too much for a 5V tube.

[vetmed]

Last night it was 5.5 volts There is a winding on this transformer with 2 color coded taps: 5 volts, which I have been using, and 6.3 volts. So just to rule out any sort of mistake in color coding I connected the 6.3 volt winding and got 6.3 volts So it seems I will have to add some series resistance if I am to get this to work properly. I'll have to check the local electronics surplus store.

Robert Lees

[Jim R.]

Robert,

Another way to drop this small voltage on a filament supply is to get a couple of schottkey diodes that are rated for somehing a bit more than the filament current and solder them back-to-back and place them in one line. This should give you something like.6v drop, and it will be quiet.

HTH,

Jim

[vetmed]

That's a great idea except that since this is a directly heated rectifier you also carry the rectified HV on the winding. So the rectifier would also need to be up to this HV as well, unless I am greatly mistaken . Can you clarify what mean by placing them back to back? Thanks

Robert Lees

[Jim R.]

Well, I'm not sure what the considerations would be for a DHR. Back-to-back just means in parallel with the cathode of one connected to the anode of the other, and the reverse on the other end. It needs to conduct in both directions.

Jim

[vetmed]

In my last post I meant to say that the solid state diode would have to be up to the HV involved. Sometimes it makes perfect sense while you are writing

[chard]

Parts connection sells a schottsky diode rated  600v at 1 amp.

[Paul Birkeland]

I'm not quite sure what the diode will give you that the resistor won't (other than a lighter wallet).

[Jim R.]

PB,

I know what you're saying and it's bothering me too -- more because I can't remember the reasoning behind this, which was told to me many years ago by somebody who definitely knew what he was doing.
Anyway, it is just a suggestion :-)

-- Jim

[Paul Birkeland]

 It might be helpful if you had a 3A winding with too much voltage, and you wanted to be able to use rectifiers that drew 2A or 3A, which wouldn't do that well with fixed resistors.