Bias-Shift Heising Modulator

[W9GT]

Has anyone tried this scheme of high level modulation?  It is outlined in the Bill Orr Handbooks as an alternative scheme to produce high level plate modulation without the accompanied high cost of plate mod iron or the typical low efficiency of Heising modulation.  The example shown utilizes one (or more) 304TLs (in parallel) that run in a biased-off configuration.  They then go into class A range when audio drive is applied through the use of a bias-shift circuit.  The bias is controlled by a bias-shift (triode) tube and the resulting bias variation creates a much more efficient Heising modulator because the modulator tubes are not running "wide open" all of the time.  By playing with the bias settings and the time constants, you supposedly can acheive syllabic response and resulting shift of the bias level.  The "pumping" effect is minimized because the actual carrier level is not going up and down.

I have been talking with a friend about trying this scheme on a transmitter that he recently acquired, however, wondered if anyone has some real world experience with this circuit and could suggest component values, etc.  I was thinking of using a 6SN7 as the control tube with both sections wired in parallel.  Probably will be using a pair of parallel 304TLs as the modulators and some series wired filter chokes as the modulation reactor.  Any suggestions would be appreciated.  Yeah, I know conventional class B or AB1/AB2 push-pull is the classic and perhaps best way to go, however, we want to try something different.

73,  Jack, W9GT

[w4bfs]

had a quick discussion on this in July

..http://amfone.net/Amforum/index.php?topic=16583.0


hope this helps ...73 ...John

[KL7OF]

When you do this.....keep notes and keep us informed.   I am very interested since I saw it in the bill orr handbook......Should make a very cool TX....Steve

[W9GT]

Will do Steve.  A couple of us are going to be working on parallel projects to test out this concept.  Looking forward to seeing how it might work out.  Will keep everyone appraised of results.

73, Jack, W9GT