[AMRadio] Subj change - HB designs


Geoff/W5OMR ars.w5omr at gmail.com
Tue Jul 24 12:02:00 EDT 2007


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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