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What About Beam Deflection Tubes?


2006-11-01 12:59 am
I have a caddy fully of old television tubes that I came into a couple of years ago. While I can not say that I have any interest in old televisions (I do old radios and homebrew hi-fi), I am contemplating some homebrew radio and transmitter projects. I have noticed that TV tubes come in many more different varieties than the usual triode, pentode, and converter types used in radio. I've already breadboarded an AM transmitter using a 6GH8 to screen modulate the signal- and it sounded good.
In my caddy there are some Beam Deflection Tubes, such as 6ME8 and 6JH8. My queries:
1. How do these things work? What are the variables? For example, we all know that a pentode can vary its output current by either G1, or to a lesser extent by G2.
2. Do you think that these tubes could be used create an audio- variable reactance- treat G1, G2 as a normal pentode and use the beams as the "capacitor" output to control the oscillator in a FM transmitter? Just tinkering.
Some related posts:





They are modulator tubes, ie, can multiply signals or a gain control level times a signal. If you put it into some type of gyrator circuit it could probably control reactance for frequency control.

I thought about using these as the variable gain stage in a compressor/limiter, sort of like the so-called "variable mu" Fairchild 660/670, UA175, etc. One would need to limit the p-p voltage of the input signal. I haven't really thought it through. It might be easier for the sidechain to drive than a bunch of remote cutoff tube grids.

There might be a nice 10db of dynamic range available from looking at the curves, and some of them look quite linear.

Another project, another day...

I see I have the 6ME8 page flagged in my RC-30.

The 6ME8 would allow a nice big input swing of maybe +- 40V or more; plenty for pro audio limiters where the signal might be coming in at 30dbu or more on the peaks.

At a reasonable starting quiescent point (7-8 mA/200-250Va) could get 5mA per anode current swing at 0db Gain Reduction. I'd use a pair of CCS or maybe a P-P interstage transformer and load it about 8K anode-anode for a 1:1. 8K sounds low... I'd try some different op points... find the sweet spot if there is one... Maybe I could get more than 10db gain reduction after all.

The sidechain would be driving a low capacitance grid from around -6 volts to -10 or -12?? should not be difficult to do.

OK, use a CCS in the cathode and a p-p transformer on the anodes, with a bipolar supply with the deflection electrodes at ground.
How to balance the anode currents? Series resistors in the anode circuits?

On second thought, the CV from the sidechain comes in between grid and cathode and CCS would not allow the gain to vary, DUH...

So the topology would need to be a little different at the audio and sidechain connections, with a grid/cathode scheme similar to the ones shown and probably an input transformer on the deflection plates. Obviously needing more thought...

Transformer output goes to line out and the input of a simple sidechain amp constructed from an opamp "perfect rectifier" and various filters and attenuators aligned like a Fairchild 660/670.

This sounds like a good thing to investigate further.


Just bought a sleeve of 6ME8s for $10 on ebay