Valve Crystal Oscillator Grid Current

[Skywave]

Lawrence - I see that in that article, the author refers to a crystal triode oscillator producing output powers in the range 50 → 100 watts. In Radio Amateur circles, a valve that would do that would be the 'final' r.f. amp. that feeds the aerial!
It's my understanding that trying to get r.f. powers of that magnitude from a crystal oscillator was a common technique in the 1930s when valves were expensive items - so, in a given transmitter, the fewer the valves, the better. But as we all know, it wasn't long before more efficient (and cheaper) valves were developed - including the tetrode and pentode - and that practice of "lots of watts" from the oscillator vanished into obscurity - which, in retrospect, was just as well.

So that author's remark effectively confirms what I have maintained along - this 'bulb in series with the crystal' technique is simply archaic and has no relevance to any crystal oscillator designed and built since the 1950s - at the very latest.

Al.

[pwdrive]

While looking for clues about this subject on the web I came across an article for a 100w plus transmitter using one valve for the osc/pa it was a then new RCA valve, If I can find the article again I'll post up a link, yes I agree bulbs in Xtal circuit is old hat.

Lawrence.

[pwdrive]

The 100w CW Tx using the RCA valve:

http://n4trb.com/AmateurRadio/RCA_Ham_Ti...ps0104.pdf

Lawrence.

[John M0GLN]

I'm with you now regards it being r.f current and probably a different cut of crystal in your test rig Al, now perhaps its my limited understanding of the intricacies of the thermionic valve, but 40mA even if it is r.f still seems a lot to pass through the control grid, for the size of valve used, a 6F6, 40mA of r.f from the anode if it were used as a small output valve would seem to be about correct.

John

[pwdrive]

I've received a reply from the same author regarding the last question I posted on another forum:


Posted by me: "Just another question, if say 40 or even 30 ma flows through the xtal then if it's not the grid cathode diode action that passes the above mentioned current am I right in assuming that the current flow circuit is completed by the inter electrode capacitances within the tube along with any external stray capacitance routes?"


Reply: "In the case of this circuit, which I've gathered from your description is a tuned-plate, crystal-grid oscillator (in the manner of a tuned-plate, tuned-grid oscillator), the RF current flow circuit that endangers (or does not endanger) the crystal is through the grid-plate capacitance of the tube--from, and therefore through, the plate circuit of the tube back down through Cgp in series with the crystal (and any capacitances in parallel with it, including strays and the grid-cathode capacitance of the tube in series with impedance of the cathode's RF return).

The situation would be different if the circuit was arranged to be one of the variations (tri-tet, grid-plate, modified Pierce) in which that triode formed by a tetrode or pentode's screen-grid-cathode triode does the oscillating and the tube plate serves as an output port by means of electron coupling (mainly) and capacitive coupling (partially)."

Lawrence.

[John M0GLN]

Thanks for posting that explanation Lawrence although it's getting a bit too advanced for me, I'm still just a little surprised that as 40mA is flowing through the lamp and crystal that the control grid can either source or sink a current level which is the more or less the rated anode current of that particular valve. I think I'll just accept that it happens and that's that.

Thanks

John

[pwdrive]

In the case of capacitance between the grid and other electrode(s) you have to view the grid as one plate of a capacitor, only the R component of the capacitor formed between the grid and other electrodes will dissipate any power, the R component of any capacitor formed between the grid and other electrode(s) is likely to be a low value...I think.

The capacitor formed between the grid to other electrode(s) is only one part of a capacitor in which current flows, the others parts being formed by external stray capacitances in the physical wiring of the circuit, valve socket etc.

EDIT: Altered bits to describe cga/cgk etc as a capacitor rather than capacitance so as to validate R.

Lawrence.

[AlanBeckett]

I fear I still don't understand it. While not disputing the 40mA figure I just don't know where it comes from.

As I see it there are two sources:-

First there's the Electron stream. But for Electron Current - Cathode to Anode - that implies that the Grid is the source of the Electrons. More sensible and plausible is an Electron flow from Cathode to Grid, ie a Conventional Current in the usual direction.

Second there's the Inter-electrode Capacitance. However, the 6F6 Data Sheet quotes 0.26pF for Cgk. That will be Pentode connected. So what about a Triode? ECC81 Data quotes 1.6pF. For a 'Modernish' Pentode, like EF91 it's quoted as <0.008pF. None of those figures seem to me to be capable of passing 40mA unless the Voltage and/or the Frequency is very high.

Alan

[pwdrive]

I fear I still don't understand it. While not disputing the 40mA figure I just don't know where it comes from.

As I see it there are two sources:-

First there's the Electron stream. But for Electrom Current - Cathode to Anode - that implies that the Grid is the source of the Electrons. More sensible and plausible is an Electron flow from Cathode to Grid, ie a Conventional Current in the usual direction.

Second there's the Inter-electrode Capacitance. However, the 6F6 Data Sheet quotes 0.26pF for Cgk. That will be Pentode connected. So what about a Triode? ECC81 Data quotes 1.6pF. For a 'Modernish' Pentode, like EF91 it's quoted as <0.008pF. None of those figures seem to me to be capable of passing 40mA unless the Voltage and/or the Frequency is very high.

Alan

In all of this thread the only thing to go on as far as I can see for the current is that a 60 ma bulb is in series with the xtal, it could be that only 20 ma (or less?) flows through it to give any kind of illumination?

The 0.26 pf is the Cga capacitance, Cg to the other electrodes including the cathode is 6.5 pf, the largest contributor to the latter I imagine would be Cgk as the control grid will be nearest to the cathode.

Just to ponder, if the xtal was operating in the parallel zone (xtal now inductive) I wonder what the branch current would be through the xtal?

Lawrence.

[AlanBeckett]

My figures above should have said they were for Cga, which is where the Current is supposed to be flowing - sorry

The Cgk Figures are:-
6F6 6.5pF
ECC81 2.3pF
EF91 7pF

Still tiny.

I'll have to ponder on Inductve effects, but I'm still confused.

What we need is a really old Crystal so Al can do his experiment again.

Alan