[AMRadio] Subj change - HB designs


KX5KW ars.kx5kw at gmail.com
Tue Jul 24 12:32:37 EDT 2007


811's make great modulators, don't they?

I've had 2000 volts on 811A's with a 9v transistor radio battery for bias, 
just to see how/if they would work.  They were very sensitive.  Had to reduce 
the audio drive considerably from the previous 1100 volt/0 bias.  No 
oscillation or any weirdness though.  They actually seemed to sound a little 
cleaner, but that could have been anything - reduced drive - better impedance 
match - wishful thinking?

Sounds like a good project Geoff.

73',
Kent/KX5KW


On Tuesday 24 July 2007 11:02:00 am Geoff/W5OMR wrote:
> Jim Wilhite wrote:
> > Geoff I have never seen a tuned circuit in the screen lead. Have you
> > and if you have, I would like to see the diagram for interest's sake.
> >
> >> question... (and I'm asking because I just don't know
> >> pentodes/tetrodes) should the screen circuit be resonant at the
> >> operating frequency, as well?
>
> I just wasn't sure, Jim.  I don't believe I have seen a tuned circuit in
> the screen lead.  Therefore, it's probably just a control voltage.
> Helps me understand better.
>
> I still want to build a 'desktop' AM rig, with a digital VFO like Brett
> has in his rigs (beautiful setup - Brett, where are those pictures,
> again?) and driver stage.
>
> I've still only managed to come up with (1) 4-65 (thanks to Kent/KA5MIR)
> and do have 3 sockets that were located and secured (thanks to
> Marv/KC9VF) and now have a 4-125 on the way, that I purchased from
> e-bay.  I think I'm gonna change the design up a bit, and use the 4-125
> in the final w a 1500v power supply, and modulate it with a pair of
> 811's with about 9v of bias on 'em, or so.  I think 0 bias is required
> on 811's to bring up to Class B, with around 1100VDC.  To that end, I
> have the old buzzard 1:1 RCA modulation transformer that has a
> screen-winding on it.  A small percentage of the modulation from the
> final, will be sent to modulate the screen DC voltage, reducing audio
> and cross-over distortion.
>
> The power supply is built, I have a roller inductor, plenty of
> air-variable capacitors and I believe a socket (around here somewhere)
> for the 4-125 and plenty of 811's.
>
> This thing is starting to sound like a Globe King sized rig, with a
> built-in VFO :-)
>
> Then again, if I can build the final and modulator on the same deck, I
> could put a receiver in the rack as well, and use the RX vfo to act as
> the frequency required for the xmtr exciter circuit.
>
> Oh, the ideas are plentiful, but I'll probably go with a more
> 'conventional' design.
>
> --
> 73



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