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Registered Member #4107
Joined: Sun Sept 25 2011, 07:30PM
Location: London
Posts: 53
Hello, I'm returning here because I deeply value the depth of knowledge here.
I am a reasonably seasoned coil builder, with two electron-tube based (572B + another) and a couple of MOSFET based builds behind me, over 15 years with breaks.
But I'm far from knowledgeable in the area of this Q.
Please see attached photo. My dilemma is this: I need to produce both HT+ and valve heater supplies from the same 2 x mots. It means I will need to switch on the heaters, allow them to warm up and stabilise, and then power up. I will also control the duty cycle (method not shown here, via centre tap of heater + MOSFET based control circuitry with variable f and pulse width, pretty standard.
What is the best way to power up?
1) Hard switch HT+ at the anode. Tricky, I imagine, using series pass MOSFETs? Possible? Pros and cons, please 2) Hard switch grid only. This means there is a charge between the anode and cathode, tube is standing off the entire HT+ supply voltage. Is this stressful to the tube? 3) Hard switch cathode. This is in the style of an old morse key, but using the standard triac method, I guess, instead. The instantaneous voltage at the cathode as it is switched to connect with ground must be very high. I know it isn't the same as the anode voltage... in fact, how do I calculate it for this electron tube, exactly?
All comments welcome, but please can you reference your answers 1, 2, 3, for clarity.
I can't comment on the stress to the tube, but I can say that I would hard switch the HT at the anode by using an optoisolator and a small isolation transformer (perhaps a DIY on the mot core) to power and control a relay.
Regarding the voltage between a disconnected cathode and ground (Question 3) I believe it is a result of the characteristic tube impedance at the particular grid voltage. In other words, if the cathode were connected you'd have a certain amount of anode current based on grid voltage and tube impedance, as such there's a voltage drop across the tube. The voltage at the cathode would be B+ minus this voltage drop.
Registered Member #4107
Joined: Sun Sept 25 2011, 07:30PM
Location: London
Posts: 53
Thank you Sigurthr, this is very helpful.
I'm not quite sure, though, where to get a relay that is capable of switching Kilovolts in an RF circuit. I was thinking of using MOSFETS in a suitable configuration for that reason, if I can research how to use multiple devices in series in a simple application like this. Or can you point me to a suitable source for such a relay, please? I do have one mercury relay, is that suitable?
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Best way to hard switch HT + here? GU81M transmitting pentode design
