Valve Crystal Oscillator Grid Current

[pwdrive]

Alan wrote: "What we need is a really old Crystal so Al can do his experiment again"

Yes, a rock that can walk the walk.

I read somewhere that the tuning of the anode tank has an effect on the xtal current.

Lawrence.

[John M0GLN]

I've read through the book by 'J Douglas Fortune' again in hope of finding some crystal details but all he says is 'select for frequency', no mention of type or of holder, I have attached 2 more scans, one in which he talks about crystal current and the other showing the circuit of the valve oscillator he's referring to, this oscillator has 6L6 valve with a 150mA bulb in the grid / crystal circuit, I ran a 300mA bulb, the smallest I have from a variable power supply and it was barely glowing red at 150mA so I assume that the 150mA bulb in the 6L6 grid circuit is going to be passing at least 60mA to show some visible indication, now for me it's not so much where this current is coming from, it's obviously there in his circuit, but how can the control grid pass this current? The permissible control grid current for an 807 which is virtually just a 6L6 with a topcap is generally listed at about 5mA or less.
The photo in the scan does show the crystal holder he's using.

John



   

   

[pwdrive]

My take on it rightly or wrongly is what I suggested in my earlier post, the grid forms a capacitor, AC current will flow via that, the reactive part of that capacitor will not dissipate any heat, only the R component of that capacitor will, we have to differentiate between any DC grid current and the AC current flowing through the grids capacitance.

Lawrence.

[John M0GLN]

My take on it rightly or wrongly is what I suggested in my earlier post, the grid forms a capacitor, AC current will flow via that, the reactive part of that capacitor will not dissipate any heat, only the R component of that capacitor will, we have to differentiate between any DC grid current and the AC current flowing through the grids capacitance.

Lawrence.

Thanks Lawrence, I think I'm up and running now with that explanation.

Thanks

John

[Kalee20]

I've read through the book by 'J Douglas Fortune' again in hope of finding some crystal details... I have attached 2 more scans, one in which he talks about crystal current and the other showing the circuit of the valve oscillator he's referring to... now for me it's not so much where this current is coming from, it's obviously there in his circuit, but how can the control grid pass this current? The permissible control grid current for an 807 which is virtually just a 6L6 with a topcap is generally listed at about 5mA or less.

The circuit is a TATG oscillator (tuned anode tuned grid), with the tuned grid circuit replaced by the crystal. If the valve is a 6L6 then it's going to be running at a fairly high level, with a chunky crystal. A few hundred volts of oscillation on the anode are not unreasonable.

I confess I've not read every post in this thread, but putting in some typical (?) figures (400V rms amplitude on the anode, 5MHz frequency, 2pF grid-anode capacitance including strays, 50V grid voltage) gives 450V across the Cga (the grid and anode voltage will be nearly in antiphase for oscillation to occur) and this results in 28mA rms flowing through Cga.

Now, yes this has to flow through the grid wires, but at least this current is not (mostly) the result of the wires being continually bombarded with electrons landing on them. So grid heating isn't what it might be if you were to put enough positive volts on the grid to draw 28mA DC.

If, as a thought experiment, you think of the case of a perfect crystal and a lossless anode tank, then you could power-up your oscillator, get it oscillating, and then switch off the heater. The oscillations would continue because the circuit is perfect - and the 28mA rms reactive current would persist. Now if you quickly inserted a low-voltage bulb, then it would briefly light, though oscillations would die as energy is being removed. So switch on the heater again and the valve would do its job of making good the losses and sustaining oscillation, and in principle it could do this with a few mA of HT being drawn even though the reactive current flowing between anode and grid is several times more.

[pwdrive]

I was a site chippy building the room for an MRI scanner, Philips Medical I think it was, the coils were super cooled, I watched them filling bits of it up with the magic liquid and also watched them ramp up the system, fascinating stuff.

Lawrence.