SS replacement for EZ2 rectifier

[SirElwood]

Hi!

I have a old Philips RF-generator (model GM2884), and I would like to make a replacement rectifier for it so that I could use a bigger filter caps (later models had different rectifier tube). Can anybody tell me correct size dropping resistor for simulating EZ2 rectifier? http://www.r-type.org/exhib/aaa1052.htm

Thank you!

[HotBluePlates]

I never considered trying to figure this out on my own... so if anyone spots an error, chime in!!

You linked the museum page for the EZ2, so scroll down to the bottom where the pdf is indicated for the data sheet. On the 2nd pdf page (marked p. 152), figure 3 is a graph of what you want to know.

You'll see it is marked for current as a function of applied voltage. In other words, you have current indicated for each of a range of voltages, so the slope of the curve is the plate resistance of the rectifier. R = V/I.

It's a curve, so you would like to pick a mostly-staight portion of the curve (like the dashed portion). I read the ends of the dashed portion as 11v, 30mA and 24v, 90mA. Divide the change of voltage by the change of current to arrive at resistance.

(24v - 11v)/(90mA - 30mA) = 13v / 60mA = 217Ω

Maximum d.c. output is cited as 60mA, so 60mA² * 217Ω = 0.78w maximum dissipation. If it were my generator, I'd use a 220Ω 3w resistor in series with each rectifier diode.

Or... you might not use either of them just to reduce heat in the cabinet, unless you worry the rectified voltage may be too high.

[PRR]

Why does an RF generator need bigger filter caps?

[tubeswell]

Merlin Blencowe has a webpage where he runs through calcs for figuring out limiting resistances for rectifier tubes FWIW http://www.valvewizard.co.uk/fullwave.html

[SirElwood]

Why does an RF generator need bigger filter caps?

I don't think it does... But only 16+16 capacitor I could find is physically too big to fit, but for some reason 47+47 is same size as original cap. This has been a pain in the butt -type of project anyways, so I figured this rectifier replacement is not a big sin.

Thanks for the replys!

[PRR]

> 47+47 is same size as ....16+16 original cap.

Caps have got 3X better since the box was made.

I'd put 500 or 1K ohms between rectifier tube and first cap. That should be enough to prevent damage. Load is probably small, so it may not drop B+ voltage enough to care.