Tube Rectifer Plate Voltage Question

[David Root]

Joe, where did you get 50 ohms for Rt? The Philips GZ34 datasheet gives 75 ohms at 300VAC, the GE datasheet is 80 ohms.

Half of 98 is 49 ohms, plus (300/120) sqd. * maybe 2 ohms (PT) = 12.5 ohms, so you are still shy a little of 75 ohms. I wouldn't worry too much about that difference though. A lot of circuits don't add any additional resistance at all and have much bigger deficits.

The GZ34 will not be stressed DC current wise at all, as you note, and unless you insist on an NOS Mullard it won't cost you an arm and a leg!

[JoeTele]

I got the 50ohms at the link below, but should have scrolled down a page as I see this is an earlier edition of the datasheet, with a later version containing the 75 you mention.

http://www.r-type.org/pdfs/gz34.pdf

As for the winding, I measured the 290-0-290 (unloaded) tap to have about 78ohms from end to center tap, and so plugged the 290v, the 78ohms, the 3.5ohms on the primary and the 120v wall supply into the PSUD source impedance calculator to get 98.44 ohms of "resistance" but I see now that that is actually source *impedance*.

[Aside: Is it correct to have plugged the end to center tap values into that calculator, or should I have gone end to end?]

So, with the 78 (end to center tap) + 350/120-squared, or 8.5, I end up with about 86 ohms per side.

I had also assumed, based on Merlin's article (http://www.valvewizard.co.uk/bridge.html) where he states ...

"If the transformer alone doesn't have enough resistance to meet this requirement then you need to make up the deficit by adding resistors in series with each anode ... Alternatively, you could use one resistor (with twice the power rating) in series with the cathode"

...and an earlier post on this thread, that the 330ohm resistor I'd be running right off of the GZ34 cathode to drop voltage would also effectively double as a limiting resistor. False assumption?

Thanks again for your help, David (and all)!

Joe

[martin manning]

...and an earlier post on this thread, that the 330ohm resistor I'd be running right off of the GZ34 cathode to drop voltage would also effectively double as a limiting resistor. False assumption?

You can place current limiting resistors on each leg of the secondary or immediately after the rectifier cathodes. Adding resistance in either location will limit current and reduce the output voltage.

[JoeTele]

I'm nearing first power-up of this build and have been reviewing calculations, etc, and I'm just trying to clear up limiting resistance for tube rectifiers. The way I've understood this from the discussion up to now is that the internal resistance of the PT is the end-to-center tap secondary winding resistance plus the square of the PT voltage ratio. Having had occasion to more carefully study Merlin's article on this (http://www.valvewizard.co.uk/bridge.html) I see that he has the formula as:

Rlim = Rpri + Rsec × (Vsec/Vpri)^2 + any extra resistance

Where:
Rpri is the DC resistance of the transformer's primary winding;
Rsec is the DC resistance of one half of the transformer's secondary winding, i.e measured from one end to centre tap;
Vpri is the primary (i.e. mains) voltage;
Vsec is one half of the secondary voltage, i.e. measured from one end to centre tap.

----------
The difference of addition vs multiplication by the square of the voltage ratio is obviously significant. Can someone clarify?

Thanks!

Joe

[martin manning]

I'm nearing first power-up of this build and have been reviewing calculations, etc, and I'm just trying to clear up limiting resistance for tube rectifiers...

Here's the correct formula:

Rlim = Rsec + Rpri × (Vsec/Vpri)^2 + Radd

Where:
Rpri is the DC resistance of the transformer's primary winding;
Rsec is the DC resistance of one half of the transformer's secondary winding, i.e measured from one end to centre tap;
Radd is additional resistance between the transformer and each rectifier section;
Vpri is the primary (i.e. mains) voltage;
Vsec is one half of the secondary voltage, i.e. measured from one end to centre tap.

[JoeTele]

Does the red indicate that those numbers are added together and the sum multiplied by (...)^2?

Thanks as always Martin!

Joe

[martin manning]

No, it's Rsec + (Rpri*(Vsec/Vpri)^2) + Radd

Rsec and Rpri are swapped on Merlin's page. I sent him a note already.

[Kagliostro]

I don't know if is too late to add an answer to this thread .....

The solution is simply

if you have a rectifier that can afford the current but not the voltage, simply add two diodes in series with the diodes of the tube

placing it between the transformer and the tube and all is done

the inverse voltage will be the sum of the inverse voltage of the tube plus the inverse voltage of the diode you use (the same you see in FullWave rectify when two or three diodes are connected in series)

this arrangement didn't affect the effect of the tube rectifier and you'll have the voltage drop and SAG effect

Franco

[Kagliostro]

Please delete this post

it was doubled by the program

Franco