15-09-2014, 12:03 PM
(15-09-2014, 07:50 AM)John M0GLN Wrote: 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.
