Seeking Valves

[ppppenguin]

The point where a problem may arise is that tube radios aren't grounded like normal appliances. Of course, we all know this to be the case.

Some are grounded (earthed), some aren't. If the chassis is designed to be earthed (I prefer the British term) then it should be earthed. This is usually indicated by a 3 core mains flex. On some sets there is only a 2 core flex and the chassis is meant ot be earthed by a separate wire. This then brings up the whole question of safety earthing versus functional earthing.

If the set was not designed to be earthed then one side of the mains will normally be connected to the chassis. If you then earth the chassis you are connecting either live or neutral to earth. Not a good idea.

I imagine if you did ground the chassis to the RCD box, it would simply keep tripping as voltage tends to find its way onto the chassis, even if small.

An RCD is NOT tripped by voltage. A current has to flow. For example if you touch an input socket on a modern double insulated amplifer you will often get a slight tingle. Measure between there and earth with a high impedance voltmeter and you may find up to 110V. Use an ammeter and you'll likely find less than a milliamp which won't trip an RCD. The problem is stray capacitance between the mains and the rest of the equipment. It's safe but could give you a surprise.

[Nowhere-Man]

Here they're called Ground Fault Circuit Interrupters (GFCI or just GFI) and I thought they tripped at 6mA though mains voltage here is of course only 120V.

A fair percentage of heat in a resistor exits through the leads.

Due to the fact there was some concern about my enveloping the series resistor in heat shrink, I changed this yesterday. I didn't want to come across as overly stubborn or dismissive of safety concerns. So, the body of the resistor is now exposed to air. I was playing it last night and some time after I switched off and carefully felt the body of the resistor. It wasn't really all that hot. The GEC is a very well designed, spacious radio and doesn't get hot like my Ultra does as there is more space. Anyway, that job is now taken care of.

I was a bit concerned about the HT feed to the smoothing can. I'd hate for that to ever come off. I wouldn't fancy as 270 volt wire recklessly swinging around at any time. I'll have to tie and anchor it.

Here is a good question you can all have a good think about:

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.

So, I can only assume the 2 caps I shunted across my diodes explain the fact I wasn't getting 360 volts at the cathode. It must be Chas was talking about a rectified ripple just using silicon diodes. For sure, I saw no signs of any high voltage D.C. so I was finally content to just smooth the current out a bit with a 220 Ohm series stopper.

According to theory the supply voltage of 260 A.C. ought to be out of phase with the voltages across the capacitors so that must account for the drop? I guess it would be interesting to do a test using just the diodes (but not on the radio).

[Crackle]

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

[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.