[AMRadio] Subj change - HB designs

Brett gazdzinski brett.gazdzinski at verizonbusiness.com
Tue Jul 24 15:23:56 EDT 2007

> 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. 

> http://wa5bxo.shacknet.nu/N2DTS/

The receivers have the digital readouts.
For a VFO, I would need to do something different as the digital readouts
are really freq counters with a 455KHz offset, you don't want the
vfo running on receive, and if its not running, you get no readout....

The 3X4d32 rig started out as a desk top rig believe it or not,
but to get any power out of it, and to be reliable, it had to be large.

I am still toying with the idea of building a desk top transceiver
in a DX 100 size box, with maybe a separate remote power supply.

A single 4d32 modulated by a pair of KT90 hifi tubes might do.

> 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 4-125 likes 2500 volts.
At 2000 volts, you can get about 250 watts out of one.
At 2500 volts, about 300.

The globe king 500 was 811 modulators and a 4-250 running at 1800
volts. 540 watts in for 300 out, you can do lots better at higher voltages
on a 4- type tube.

For low voltage, 812A's, 811A's, 813's come to mind.
I run 811A's as modulators at 1750 volts and no bias.
The plates show no color.

The 4D32 is also cheap and available, and you can run a pair,
one 4D32 is like a pair of 6146's, and I normally run the 3x4D32 rig at
1500 volts on the plates. that gives about 320 watts out for three 4D32
tubes, and lots of peak modulation, and the tubes run cool.

> 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.

Forget the roller inductor, what bands will you operate on?
For something like 80 and 40, a coil made out of copper
tubing or heavy wire is likely better and simpler.

> This thing is starting to sound like a Globe King sized rig, with a 
> built-in VFO :-)

And a globe king is very small for its power input.

> 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.

I don't think so.
Once you gather the parts for a 200+ watt AM rig, you will
find the RF deck wants a lot of space.
Big coils and caps take room.

One space saver is to run one power supply for both
the RF and mod deck, which means tubes that like roughly
the same voltages.
4D32-KT90, 600 to 1000 volts,
812A-811A, 1200 to 1500 volts,
813-813, 2000 volts,
4-125/250/400 - 4-125/250/400, 2000 to 4000 volts.

Just some ideas, its fun planning homebrew projects.


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