Quote:
Originally Posted by patrick-turner
I will look at schems. Some older ideas can always be revisited with a fresh approach to see what happens. But age old discipline is still required to see if anything can be done to justify the visitation.
Patrick Turner.
I just had a look at schem with paralleled 6SN7 driving 6F6 with Ia at 13.65mA and Ea at +250V. Maybe not much class A PO?
Sure, trioded pentode with DC drive to g1 connected to g2 does give tube more gain than usual, and high THD, and much FB must be used and I could not see any.
Patrick Turner.
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This exercise is meant to explore a concept. I had no intention to build a complete amplifier with all its warts & otherwise. Read on & you will realize anyone with an amp using SE or PP output such as 6K6, 6V6, 6F6 & so on needs simply to plug 6N6 into the same socket, whether it be UL or otherwise. The only wiring change would be to remove the bias network. Under ordinary conditions the hookup described is self-biased.
I did get some useful information, see below. One needs to go well beyond RDH4 on a regular basis. Otherwise, missing quite a bit!!
Look again at the table showing the test results. As the plate of triode one is taken from the B+ lead & moved to the OPT CT the Rp goes down. That is NFB. And further still, as one would expect when the plates of triodes one & two are strapped together. The change would be even more pronounced had I used a lower mu triode as the driver.
A while back on RAT there was some discussion of pentodes wherein G1 & G2 are strapped together. That forms a zero bias triode useable in a PP power amp. These were for the most part modulators in HAM transmitters. Right after WW2 there were quite a few built using 807s which were plentiful on the surplus market. Later TV sweep tubes such as the EAR example were used.
As a rule Class B PP has a problem with crossover distortion, in particular at low levels. A simple fix applies a little +ve bias to the driven grids. But most folks don’t bother. Oddly one shows up in a Ham modulator schematic in the 60’s. A pair of Class B 807’s are direct driven by the cathodes of a 6SN7. I would think that would work quite well. But the standing current goes up somewhat, something the mobile guys were allergic to!
My early copy of Electronic Workbench CAD has modeling for triodes & includes some examples. There are no pentodes, so the usefulness of the software is somewhat limited as far as vacuum tube applications are concerned. And I have yet to master SPICE.
But there are both voltage & current controlled voltage & current sources. So quite a bit of AC modeling can be accomplished. Still no way of connecting feedback such as UL or otherwise to a screen connexion.
That is when I thought ‘why not try the composite triode hookup’ such as the 6B5/6N6G, Etc. It turns out a very good AC model of things like 6V6, 6K6, 6L6, Etc can be constructed with two triodes in that connexion. That way another terminal becomes available to simulate the screen in a pentode. All works in the circuit very well.
The simulation uses a somewhat low mu triode for the input side, say 6 to 12. For the output side a mu of 50 to 120. If you look at the characteristics of any common pentode or beam tube, the triode mu is most likely in the range 6 to 12. Sweep tubes come in even lower.
If the G1/G2 strapped characteristics are used, then the resulting triode mu tends to be in the range of 50 or more. So the model simulates all very well, indeed.
Cheers to all, including the doubters!!
John L Stewart