[AMRadio] Screen Modulated 813


Brett gazdzinski brett.gazdzinski at verizonbusiness.com
Mon Dec 11 09:09:01 EST 2006


While its fun to play with, I think you should
plan on plate modulating the setup eventually.

You can experiment with screen modulation, it can be tricky 
to get to sound good, wont do loads of power, and depends 
on how the screen works as a modulator, some tubes are better
(lower distortion) than others I think.

I talk with someone who is screen modulating
a 4cx250b to 70 watts out, and it sounds very good.

To screen modulate what you have, you need to run
it in class C, and using a transformer in the screen
circuit, put audio on it.
You adjust the screen voltage for something like
1/4 the full power class C output.

A mike preamp like the berringer with a line output
will do it, or an 8 ohm to single ended output
transformer. 

I think you have to use a separate adjustable screen supply,
(variac?) I think it would be real tricky to use a dropping
resistor from the high voltage supply, and might be dangerous.

But all you need is a 0-400? volt supply and
a small transformer on the screen mod deck, with a
screen voltage and current meter.

You do not have to run 2000 volts on the 813 plates,
the books say about 70% of the rated plate voltage
works best, about 1400 volts.
1200 volts would work ok also.


Since my pair of 813's is adjustable in every respect,
I should tack in a transformer in the screen circuit
and try the screen modulation, but just for fun,
I can get 700 watts of carrier out of the rig as it is,
why run it at only 100 watts or so in screen mod?

Brett
N2DTS
 

> -----Original Message-----
> From: amradio-bounces at mailman.qth.net 
> [mailto:amradio-bounces at mailman.qth.net] On Behalf Of Jack Schmidling
> Sent: Sunday, December 10, 2006 7:49 PM
> To: Discussion of AM Radio in the Amateur Service
> Subject: [AMRadio] Screen Modulated 813
> 
> Taking another look at my recently acquired "treasure", it 
> seems like it 
> is going to take months to gather all the parts and build my "Mantra" 
> (813's modulated by 811's)
> 
> My treasure consists of a nice 4" rack cabinet, a HV power supply 
> needing a transformer and an RF deck which was apparently a 
> linear and a 
> blank panel.
> 
> It occurs to me that after I get the transformer, I will have nearly 
> enough parts to build a screen modulated 813 rig.
> 
> I can't seem to find any info on doing this but it seems like 
> it could 
> be a neat interim rig that I could have on the air in a few weeks.
> 
> I understand the efficiency problems but isn't this sort of academic 
> when talking about a max 300 w carrier?  Won't two 813's produce that 
> much even when run inefficiently?
> 
> Surely it would be an improvement over using the Heath SB200 and the 
> Ranger... no?
> 
> Any thoughts?
> 
> js
> 
> -- 
> PHOTO OF THE WEEK: http://schmidling.com/pow.htm
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> http://schmidling.com
> 
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