Ameco"s Duo-Grid AM Modulation

[WB6LSI]

How does Ameco's Duo-Grid Modulation work? It was use in the TX-62 VHF and TX-86 HF CW & Am transmitter. Thanks.
Henry

[W7TFO]

It looks like a combination of metered control- and screen-grid modulation of the final amp to me.

73DG

[kb3ouk]

Except for the TX-86. On the schematic I found, that transmitter appears to use pure screen modulation.

[WB6LSI]

Thank you for the replies. The operating instructions states: "The TX-86 uses the new, low distortion DUOGRID MODULATION. It is NOT clamp tube or controlled carrier modulation but a far superior system with quality that cannot be distinguished from push pull plate modulation, even with an oscilloscope." DUOGRID may just be what Ameco calls it. I checked the ARRL site but didn't find anything. I was thinking it was something like AM modulating a transistorized transmitter by modulation both the driver and the final.
Henry

[K9PNP]

Dug out my TX-62's manual.  Here is what it says in addition to Henry's quote from the TX-86 manual which is also in the TX-62 manual:

"The second tube, α 6GK6, is the modulator . It
is connected to the 7984 final screen as α Heising modulator.
Α proportion of the audio signal is also fed into
the control grid of the 7984 . With screen modulation
alone, distortioninc increases sharply above approximately
65% modulation. When the grid is also modulated, as in
the Ameco system, this distortion is reduced to α very
low value permitting clean, 100%, modulation with α very
simple circuit . When the transmitter is operated on CW
the modulator tube is reconnected as α clamp tube to
protect the final in case of loss of excitation. On phone,
the reduced screen voltage provides some protection for
the final."

[WB6LSI]

Thanks Mitch. I have a copy of the schematic of the TX-86, which isn't very easy to read, but it does appears that the grid is also being modulated.
Henry