|[AMRadio] Subj change - HB designs|
brett.gazdzinski at verizonbusiness.com
Thu Jul 26 10:17:08 EDT 2007
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
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
> -----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,
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