Seeking Valves

[Nowhere-Man]

In Chas Miller's Radio And Audio handbook, he devotes a few paragraphs to Silicon rectifiers. He basically recommends starting with a 1 K series resistor or even up to 2 K. The major point he makes is that the DC Ripple voltage that emerges at the silicon diode cathodes will be 1.414 the A.C. 260 volts. That would be 360 D.C. roughly.

Yesterday, after doing that little job, I decided to retest the voltage at the cathodes of the silicon diodes. I got 290 volts. The other side of my 220 Ohm smoothing resistor I got 280 odd volts. The output tube anode voltage ought to be about 280 so I was happy with the tests.

When you did this test you had a 10 volt drop across your 220 ohm resistor, so there is something on the circuit drawing 45mA of current, probably the valves and the rest of the circuits.
This is why the HT voltage is not the full 1.414 x 260v or 360 volts. In addition your volt meter will probably not measure the peak ripple voltage.
If you disconnected the rectified and smoothed DC from the rest of the radio circuits then all you should have on the rectified voltage are the smoothing capacitors. The purpose of these capacitors is to smooth out the "ripple" to a more constant voltage, you should then see something more like 360v. (assuming there is no other current drain on the circuit)
Be careful.


Mike

It seems to me it's probably the shunted caps. I tested voltage right at the cathode output and it's 290. I don't know what it would be with just the diodes on their own but that then ought to be way higher. The capacitors will be out of phase with the mains supply voltage because they shunt the diodes. By rights, the capacitor current will lead the capacitor voltage. Chas never mentions using capacitors in his book so seems to be thinking of a large series resistor and just the diodes. That's my guess.

I also note in the ARA book dated 1960, people had no real worry over using silicon rectifiers. Many people think silicon rectifiers are better than valve rectifiers....

There is indeed a 10 volt drop across the 220 Ohm smoothing resistor but that appears fairly normal and it's not in any way over-heated. I changed the AF coupling and GB resistor some weeks ago.

The output anode voltage on this GEC is high and is meant to be high. The manual gives out about 285 volts D.C. On the other hand, for the HMV the output anode voltage is stated as much lower, maybe only 220.

Maybe one day I'll test the voltage with just diodes maybe when I do my battery radio in the future.