Valve Crystal Oscillator Grid Current

[Skywave]

My understanding, by what I have read, is "yes". Having said that, if a chosen crystal does have a -ve temp. co., then the fitting of a bulb with a +ve temp. co. can only be a benefit.

You're becoming nocturnal young Al

Err, no: not "becoming": I've always been an owl. I've long since held the somewhat tenuous theory that if a child is born in the 'small hours', that starts his / her body clock at that time. And that applies to me.


Al.

[pwdrive]

Yes Al, but what was the Lamp for?
Nowhere have I seen an explanation, or am I missing something?

Alan

I would say it's there for a warning eg: xtal destruction imminent, the increased filament resistance causing the bulb to illuminate would probably give enough time to take evasive action.

As to the reply I got from the question I posted on another forum, the reply was very good but I'm still not sure of the path of the current flow if we are talking of say 40 ma for full illumination, this is a pentode we are talking about with (I think) less of a Miller effect than using a triode.

I might contact the chap again at sometime to ask a bit more, the chap concerned is usually quite knowledgeable, at least I think so.

Lawrence.

[Skywave]

Yes Al, but what was the Lamp for?
Nowhere have I seen an explanation, or am I missing something?

I would say it's there for a warning e.g.: xtal destruction imminent.

Yes - exactly. Every such oscillator should carry a clear warning message . . . .

   

Al.

[John M0GLN]

Thanks for all your comments, especially to Al & Lawrence for the explanations, while I understand most of what has been said , there are still a couple of points that I'm not clear on..

First, why does the test rig that Al built only take 1mA of grid current although more or less the same circuit from my book take 40mA? The only real differece I can see is that there is some cathode bias in the former.

And how does a valve like a 6F6 supply 40mA from its control grid? Just looking at the RCA page showing 'Typical Operation and Charcteristics' it lists a 'maximum signal plate current' of 40mA.

Thanks

John

[AlanBeckett]

Al actually measured it with modern Gear and a different Crystal maybe.

Alan

[pwdrive]

The xtal frequency in Al's test rig is probably a lot lower than the minimum frequency envisaged using the design that John posted, inter electrode reactance values will be a lot lower at say 3.5 mhz or 7 mhz than at 500khz.

Lawrence.

[Skywave]

First, why does the test rig that Al built only take 1mA of grid current although more or less the same circuit from my book take 40mA?

From my earlier post:
I do not know the cut of the crystal used in my experiment. If I had had a known X-cut type, I would have used that.

But that current is not being supplied by the grid! It isn't d.c. It is an r.f. current flowing through the crystal by virtue of the feedback capacitance from the anode of the valve. No d.c. current can flow through the crystal: as far as d.c. is concerned, the crystal looks like a low-value capacitor. And that is the salient point of the cct. that uses this bulb technique. With the triode crystal oscillator of yesteryear, (which this design is) it was common for that oscillator to supply at least 5-watts to the next stage. And that level of output of r.f. resulted in relatively large currents - r.f. currents - flowing through the crystal. So, try to draw too much r.f. power from the oscillator - the crystal current rises - and suddenly, poof! Exit one crystal.

Al.

[Skywave]

Al actually measured it with modern Gear and a different Crystal maybe.

The test gear that I used was specified in that post of mine . . . . .
Quote: All r.f. waveforms were measured with a 300 MHz Tektronix 2465 'scope plus low capacitance X10 probe (7 pF); all d.c. voltage were made with a Fluke 77 D.M.M.
As for the crystal, the freq. was stated as 500 kHz, but the type of cut was unknown - but it was an ex-W.D. type - probably dating from early 1940s - so could easily have been an X-cut type.

Al.

[pwdrive]

I've posted the question about the current again on another forum, I'll see what turns up.

In my own mind I'm still not sure what's what, 40 ma (RF) flowing through the xtal means that it has to flow through the valve with maybe a certain percentage of that current flowing via external stray capacitance, valve inter electrode capacitances are comparatively small considering the probable frequency of operation in John's schematic, 0.26pf g1 to anode, 6.5pf g1 to the other electrodes.

Lawrence.

[pwdrive]

There is a good description of a very similar xtal osc to that in John's schematic in the link below, it starts at the bottom of book page 64.

http://books.google.co.uk/books?id=f9QXAAAAYAAJ&pg=PA66&dq=%22crystal+current%22&hl=en&sa=X&ei=5aYIVNDsKorH7AapgoHwDQ&ved=0CDcQ6AEwBQ#v=onepage&q=%22crystal%20current%22&f=false

Lawrence.