Topband: DX-100 adventure contiunued

Charlie Cunningham charlie-cunningham at
Wed Jan 8 20:52:57 EST 2014

True! - Remember the warm glow of 866 rectifiers?  Ever had  one
"flash-over" from plate to filament? I did once, when, I think in the "heat
of battle" I applied the HV too quickly without enough filament and mercury
warm-up time! Used 'em as rectifiers in a home brew 805 gg amp. First real
amp I ever had. Built it when I was about 15, I think. Worked some good
stuff with it! Heady stuff for a kid and aspiring CW DXer!  :-)


-----Original Message-----
From: Topband [mailto:topband-bounces at] On Behalf Of Tom W8JI
Sent: Wednesday, January 08, 2014 8:28 PM
To: 'topband'
Subject: Re: Topband: DX-100 adventure contiunued

> Well, I agree. And if I follow your reasoning, it doesn't take much of an
> increase in line voltage to push the HV from 825 VDC to over 900 VDC., as
> Bill observe. Might be the choke, but I'd be really surprised at the 
> entire
> winding being shorted. I'd surely want to start by measuring line voltage
> before digging into details like looking for a "shorted" choke!

The DX100 has 30k of bleeder resistance and a published choke inductance of 
~5.5 H. It would be interesting to measure one.

Critical inductance with 30k ohms at 750V is up around 20 H. The more the 
better up to about 30 H. This is why the no load voltage goes way too high.

With the choke it has, assuming it is ~5.5H as stated, the bleeder would 
have to be ~3 to 4 k ohms. There would be around 200 watts bleeder power to 
make the choke work properly. This explains why the HV settles to proper 
voltage range with ~200 mA of PA current.

Another worry is too much filter cap makes the rectifier tubes go way over 
the repetitive current rating. More isn't always better. 

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