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What do you call this? Dual-ctl pentode "improved"...

It's been a long time since I've posted here, not because of a lack of interest in tubes, just because I started playing around with old radios instead.


Anyway, I've recently been playing around with the attached circuit (apologies for the PPT hack schem). Also attached is a very good article from the net on dual-control pentodes in general.


Basically the tube looks like a pentode, more-or-less, to the regular control grid and a kind of wobbly triode to the second control grid. Playing around I found that in a 2-pentode 6BV11, you can cap one pentode with the other, cascode style. Tightens up the behavior of G3 a lot.


Still looking into it as a superhet mixer (not bad so far), but it occurred to me it might have some use as an audio effects mixer or something. It's not really that this is a clever circuit so much as a way to bias the tube into a region where it's quite linear on both grids. It's the tube that's clever after this.
 

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it might have some use as an audio effects mixer or something.

The 6BV11 does do a good job as a voltage variable gain amplifier.....sort of a voltage controlled volume knob. I made a test board for this tube and several others for use in a music synthesizer as the VCA (voltage controlled amplifier). So far my tubes of choice in that design are a pair of semi-remote cutoff pentodes in much the same circuit.

Still looking into it as a superhet mixer

The usual "pentagrid converter" tubes like the 6SA7 and 6BE6 were usually the first choice here, but they were using the tube for the oscillator and mixer at the same time for cost reasons.

It's possible to use one of those tubes for a mixer without oscillation. In this case it performs much like the dual control pentode with extra grids to act as shields to keep the LO signal from getting back out of the antenna.

If cost is no object, a 4 diode ring makes a better mixer, but wasn't really popular in the tube years. It can be done today with a pair of 6AL5's or one 6JU8.

Beam Deflection Tubes and Sheet Beam Tubes were originally designed for color TV demodulators. They can also be used for mixers and modulators, and some have found their way into some high end SSB radios as modulators or demodulators. They work well as superhet mixers too.
 
I'm treating it as kind of a "synthetic hexode", with a separate oscillator.


Two things I've found. Once is that pin 12 on the heater MUST be directly grounded to endure stable operation. The RCA version of the spec sheet calls this out explicitly. This limits the voltage on the top cathode because you can't raise the heater. But 100V is well within spec for this tube.


The other is that with enough tweaking of the screen and "top" voltages you can get g1 and g3 to behave almost identically, which is useful for design. However this requires about a 50/50 split between plate and screen current at quiescence, so it's pretty inefficient power-wise.
 
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