Valve theremin - pentode substitution for heptode?

[patricks]

Hi all, I’m hoping to build a valve theremin at some point in the not too distant future, I have a schematic and a good number of components, including some old valves that I’m hoping to substitute in.

What I’m wondering at this point is whether I can substitute pentodes in for the 6BE6 heptodes. It almost looks from the schematic that the heptodes are strapped as pentodes, with the first screen connected to the cathode, which is grounded via the coil, and the second and fourth screens tied together.
I haven’t worked with heptodes before, though, so I may be reading it wrong.

Any advice most appreciated :)

[sluckey]

What I’m wondering at this point is whether I can substitute pentodes in for the 6BE6 heptodes.

Why? 6BE6 is plentiful and cheap.

https://www.ebay.com/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=6BE6&_sacat=0

[patricks]

Crumbs, they really are cheap!
Well that solves that problem 

I guess I’m still interested from an academic point of view, since I’ve got a few pentodes in hand :)

[PRR]

So why would anybody bother to invent and build a bottle with more parts, when an existing bottle with less parts would do the same thing? (And now: cheaper!)

First "screen" (grid?) connected to cathode? Grounded via the coil? The coil is not a useless part, it has real impedance at working frequency.

I completely do not get a "tube theremin". For Toan? Most of it isn't working at audio.

6BE6 is too new-fangled. Try 6A8GT 6L7G 6SA7

[patricks]

Good point!
Your first two answers just reflect the fact that I don’t know much at all about this design or heptodes, thanks for humouring me :)

As to the “why build one”, cos I only have two theremins so far 
Partly because the sound clips I’ve listened to sounded different to my Etherwave and I liked it (but was probably to do with whatever amplifier it was being played through) and partly because I’m hoping this one would be a bit more linear in the pitch response. But then I could probably mod the Etherwave to fix that for less overall cost and effort

[PRR]

On first thought: The pitch non-linearity is all in the core math and the geometry.

At an extreme that Leon could not easily do (the man was brilliant): measure hand distance some other way (Ultrasound? Tape measure?) and correct it to get a control voltage.

[patricks]

Cool, thanks.
I think the easiest approach for me might be to look into ways of changing the timbre on the theremin I've got. It's a Moog Etherwave Standard, and I'm planning on adding the Etherwave Pro mods so that'd be the time to do it :)