[AMRadio] Subj change - HB designs

[Brett gazdzinski]

Push pull parallel can be hard to do.
I have a push pull parallel 100TH deck, and it
does not sound very good when compared to the
4x150 deck running AB1.
Same drive audio, same mod transformer, much worse
sound.

With 4 tubes to drive, it takes a lot of clean
low impedance power to drive class b triodes.
I also think slight differences between tubes introduces
a fair amount of distortion.

Despite it looking very cool (4 100TH tubes behind glass)
I never use the deck, and should scrap it and build
something different.

Brett
N2DTS



 

> -----Original Message-----
> From: amradio-bounces at mailman.qth.net 
> [mailto:amradio-bounces at mailman.qth.net] On Behalf Of Geoff/W5OMR
> Sent: Tuesday, July 24, 2007 1:03 PM
> To: Discussion of AM Radio in the Amateur Service
> Subject: Re: [AMRadio] Subj change - HB designs
> 
> KX5KW wrote:
> > 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.
> 
> Well, there's a multitude of reasons for going with 811's in the 
> modulator... heck, I might even run 4 of 'em, in push-pull parallel, 
> just to drop the plate-to-plate impedance down a little more, and 
> provide a little more ability for the modulator to handle a naturally 
> peaked asymmetrical voice.  Not that I would -consistently- 
> be running a 
> kW of Audio... just to have the ability to properly modulate 
> the plate 
> of the final, without distortion. 
> 
> Good Engineering Practices.  Isn't that what it's about?
> 
> Besides, I might wanna change out the 4-125 for a 4-250 on 
> Heavy Metal 
> night, and run 350w of carrier (with 4 811's in the modulator)
> :-)
> 
> The more I think about it, the more I like that idea.  a 
> single 4-125, 
> modulated by (4) 811's in push-pull parallel with the option 
> of going up 
> to a 4-250 or even a 4-400.
> 
> Check http://www.g8wrb.org/data/Eimac/4-125A.pdf for what Eimac says 
> about screen modulating the 4-125 final.
> 
> --
> 73 = Best Regards,
> -Geoff/W5OMR
> 
> 
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