Valve Crystal Oscillator Grid Current

[John M0GLN]

I have been reading a few books on the subject of valve crystal oscillators and the importance of preventing the current through the crystal from rising too high and cracking it, in the book 'Amateur Radio, a beginners guide' by J Douglas Fortune he describes a method of judging the current by inserting a incandescent bulb in the control grid circuit and in a copy of the 'Radio Communication Handbook' they also suggest a similar bulb or a 60mA thermocouple meter, in the attached circuit scanned from the former book it shows this small bulb inserted in the control grid circuit, this is a Mazda R48 which is a 2V 60mA bulb, now I can't understand how enough current can flow from the control grid through this bulb to light it up, even dimly, I don't know what the control grid current of the 6F6G will be in this circuit but the maximum for the 6L6 final is only about 6mA, and the maximum anode current of the 6F6G is only in the 30's of mA.

So can anyone please tell me what I'm not understanding?

John

   

[Skywave]

Yes, on the face value of what has been written, it is puzzling, isn't it?

One thought is that that 2v. 60 mA bulb is not there for the purposes of indicating crystal current by the bulb glowing - even faintly - but as a current-limiting protection device. Nominally, 2v. @ 60 mA gives a resistance of approx. 33 Ω. But then that encourages the obvious question: why not fit a 33 Ω resistor?

Moreover, I am puzzled by the book references you have listed. Although I do not have a copy of 'Amateur Radio, a Beginners Guide', a perusal of the RSGB Handbooks, 3rd., 4th. and 6th. editions do not have references to any crystal oscillator circuit which includes a bulb in that manner. So, what edition of the RSGB H/B did you read?
I even wandered through the appropriate section of 'Amateur Radio Techniques, 4th. ed., 1972, (Pat Hawker, G2VA), but to no avail. This publication is usually a good source for cct. ideas, etc., as I sure you are aware.
I looked in the ARRL Amateur Radio H/B, 36th. ed., 1959: again, nothing. Similarly, no mention of it is made in 'Radio Engineering' (1932) by F.E. Terman. (So I did go back quite a few years in terms of Radio Technology ). An inspection of 'The Radio Handbook', 13th. ed., 1951, R.L. Dawley, again was no help. The cct. diag. you have presented looks decidedly of USA origin to me: that is why I checked in those USA-sources.

Al.

[Radio Fixer]

Um! interesting. Beats me too.

I will be pleased to see an answer if someone can up with one.

Gary

[pwdrive]

Here is a basic write up on a similar arrangement:

http://www.rfcafe.com/references/qst/a-s...41-qst.htm

And another one where the bulb was used in series with the Xtal in various osc configurations, the brightness of the bulb being compared to the brightness of a similar bulb fed via a pot from a known voltage source for current determination through the xtal (I think?)

http://www.we0h.us/Amateur_Radio_stuff/B...rticle.pdf

Also some good stuff on xtals here including a description of the bulb in series with the xtal:

http://www.bliley.net/XTAL/docs/eng_bull...lletin.pdf

Bliley made the xtals for the early Hallicrafters comms receivers, my Hallicrafters SX17 Super Skyryder has one.

Lawrence.

[John M0GLN]

Thanks for your replies, regards the RSGB reference I made, there isn't a circuit, or if there is I haven't seen one either but there is this paragraph on page 6.3 of the 5th edition of the Radio Communication Handbook 1991 which mentions the bulb or TC meter, I've also attached a page from the J Douglas Fortune book where he says that the bulb is used to provide a guide to the current.

John

   
   

[pwdrive]

There was also a discussion on this forum:

http://theradioboard.com/rb/viewtopic.php?f=3&t=5656&start=15

Lawrence.

[John M0GLN]

Thank you for those links Lawrence which I've downloaded and saved, they are very interesting, I've not yet fully read the Bliley one but there is certainly a lot of info', but I'm still puzzled as to how a control grid can supply those levels of current.

John

[Skywave]

. . regards the RSGB reference I made . . . there is this paragraph on page 6.3 of the 5th edition of the Radio Communication Handbook 1991
John

And that same para. does indeed appears in the 4th. ed., page 6.3, right-hand column: that's why I failed to find it.
I didn't find it any reference that I listed: pages 5.7 and 5.8 (crystal-controlled oscillators); pages 6.1 to 6.5 (The Crystal Oscillator); and pages 7.1 and 7.2 (Crystal Controlled Oscillators). Moreover, the book's index, under the heading 'Crystal Oscillators), does not list page 6.3 as containing any reference to 'crystal oscillators'.
At least the RSGB's decision to have references about the same topic, to wit, 'Crystal Oscillators', scattered about in several places, (rather than in one coherent section), plus an inaccurate index, is compatible with the many other technical errors that exist in that book and its predecessors. I've always maintained that the RSGB used the word 'Amateur' a bit too literally!

Yes, I do realise that all of that is quite negative, and it gives me no pleasure at all in reporting my experiences of many RSGB publications that I have read over the years, but for me, I expect my technical reference books (especially those which claim a prestigious authority) to be as reliable and as accurate as quality test equipment. Bad & unreliable information or data from measurements is worse than having no information / data at all: at least in that situation, you can always make an inspired guess!

Al.

[Skywave]

. . . but I'm still puzzled as to how a control grid can supply those levels of current.

A bit more research into this topic has revealed that the current flowing through the crystal - and thus through the indicator bulb in series with it - is an r.f. current, not d.c. (Obvious, in retrospect ) Therefore, the d.c. grid current of the oscillator valve is quite a different current altogether. And that explains why it appears that the grid current of the osc. valve appears to be so very and contradictory high when first meeting this technique.
It also seems that this technique was once popular with X-cut crystals operating in series-resonant mode, in which the r.f. current could become dangerously high (for the crystal). Modern crystal oscillators tend to use the AT-cut in the parallel mode, in which the crystal current is much lower and these cuts produce a more robust crystal with a far superior temperature co-efficient too.
So, in conclusion, the technique seems to be one that has now become obsolete. And that may explain some of our (or perhaps just my) puzzlement about it.
Nuff said then: I learn something new every day in this vast field of Radio & Electronics!

Al.

[pwdrive]

Grid current value in relation to control grid voltage for the 6F6 is shown in the link below for both pentode connected and triode connected:

http://tubedata.milbert.com/sheets/021/6/6F6.pdf

Lawrence.