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Old 5th January 2009, 03:06 AM   #11
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Quote:
Originally posted by Ty_Bower


I'm specifically thinking about EL34 vs. 6CA7. They're both firmly in the same dissipation and output power class. I'm probably also considering 6BQ5 (pentode) vs. 6AQ5 (beam tube).
I've Xrays of RCA 6BQ5 (not 6AQ5) that were beam tubes.
Grid - Screen Alignment (X-Rays)
I don't know how common this substitution was.

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Old 5th January 2009, 08:57 AM   #12
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Quote:
Originally posted by Eli Duttman
have some RCA 6005/6AQ5Ws. Their anodes are circular in cross section. That suggests that they are power pentodes, not beam power tetrodes. Elliptical construction is what one expects in a "beamie".
6AQ5s are wierd like that, but these really are beam formers with aligned grids and a coaxial internal structure. They do look a lot like 6AU6s though.
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Old 5th January 2009, 09:21 AM   #13
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Quote:
Originally posted by Ty_Bower
So my question is still this - presented with the positively charge screen grid and a relatively large current of electrons, why doesn't the pentode exceed it's G2 dissipation rating? It's got to be some kind of grid spacing thing...
It will exceed its SGd rating if you stray out of the saturation region. Here's a Loadline I did for a new project. It leaves the saturation region at about Vgk= -2.0V. Bad news there since it will still be out of clipping so there won't be any aural indications that something's gonna poof. Everything to the extreme left of that characteristic is red screen territory, and you need to stay away from that. Since these plate characteristics are plotted against Vsgk= 180Vdc, there's plenty of room for dropping the screen voltage.

So you need to either draw a much steeper loadline (more distortion) or bring those plate characteristics down enough to hit the Vgk= 0 line. You do that by reducing the screen voltage. Going to the forward transfer charts, getting Vsgk down to ~100Vdc will do this. That also helps with the sonics since these small signal pentodes work best with the screen voltage as low as possible. Of course, the negative grid bias needs to be reduced to compensate for the greatly reduced screen voltage (it drops to ~ -1.0Vdc) to keep the same plate current, and hopefully, a similarly linear Q-Point.

It'll probably require tweeking in order to make certain that it's really linear and not producing lots of h3 nastiness.
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