The AM Forum

Has anyone tried using a pair of 8000's in AB1 in a modulator? Or heard of anyone who has?

It's more "normal" to use the similar 810's in a class B modulator.

But the much lower amplification factor of the 8000 should make it a better AB1 tube than the 810.

For example, at 1600VDC on the plate, and zero on the grid, the RCA curves say the 8000 will sink 400ma and the 810 will sink only 200ma.

You should be able to get a lot more peak power out of and 8000 than an 810 running in AB1.

And this would get around the problem of providing high grid drive.

Dave

I never tried them in AB1, but for a while I  ran a pair of 8000s in the final. They take about 25% less grid driving power in class C than did the 810s I had used previously. Lower mu triodes do work better in AB1 than high mu ones.

The HF-300s work in class-C almost identically to 8000s.  I use the same grid leak resistors and grid drive with HF-300s as I used with the 8000s.  With the 810s I ran a lot more grid current and used lower values of grid leak resistance.

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There is someone on this BB uses them in a modulator. I think I sold him one or both the 8000's I had. I used pair on the CB band in push pull and they worked better than 810's i.e. less drive & more output. I wuz running SSB a few watts above what was considered legal in those days. (I'm Sorry)

W1AJW used them to mod his famous 304TL rig. He ran that set up about ten years. Now someone else owns it in Boston area and I don't believe it's ever been heard again. It sounded great. Drop AJW an email. I'll bet he'll be happy to tell you all about it.

This brings up a couple questions:

How do you set the bias point for AB1 operation?

You could set it at a point where the idle current results in the tube dissipating 40% of max.

Or is 50% or 60% better?

And once you have the bias point set for AB1, how can you estimate the maximum output power of a push pull circuit?

Dave