[AMRadio] RE: Modulator design needed


Gary Schafer garyschafer at comcast.net
Tue Mar 28 10:35:01 EST 2006



> -----Original Message-----
> From: amradio-bounces at mailman.qth.net [mailto:amradio-
> bounces at mailman.qth.net] On Behalf Of W7QHO at aol.com
> Sent: Monday, March 27, 2006 10:23 PM
> To: garyschafer at comcast.net; ne1s at neandertech.com; amradio at mailman.qth.net
> Subject: Re: [AMRadio] RE: Modulator design needed
> 
> 
> In a message dated 3/27/06 4:48:02 PM, garyschafer at comcast.net writes:
> 
> 
> > The 807's in the Viking and DX-100 run in AB2.
> >
> 
> I know the VII manual sez AB2 but no way with that triode connected 6AU6
> driver (AB1.25 maybe). A popular mod was to replace with a parallel
> connected
> 12AU7 which doesn't give enough kick either and the stock driver
> transformer won't
> handle AB2 power levels in any case.   The stock VII audio system can
> provide
> just enough output for good sounding 100%   modulation with a hot pair of
> 807s and careful adjustment.   Regulating the screens at 300 volts will
> help a
> lot too.   Don't know what Heath did about the problem, but note that DX-
> 100s
> (pair of 1625s) usually sound quite good on the air.
> 
> I modified my VII for proper AB2 operation with a 6AQ5 driver, 5W driver
> xformer, negative feedback, and regulated screen and bias supplies.
> Works FB but
> a non-trivial undertaking.   Swapped a pair of EL-34s for the 807s in
> another
> VII; regulated the screens but left the rest of the audio string original.
> Worked just as well as the AB2 mod and MUCH easier to do.
> 
> 
> Dennis D. W7QHO
> Glendale, CA

Part of the problem with the Viking II is that the modulation transformer
has the wrong ratio. The modulator tubes are not loaded heavy enough and
they go into clipping too soon. They designed it that way deliberately, I
suppose to get a little high level clipping without the rig being able to
severely over modulate.

I think if you look at the grid voltage with a scope you will find that it
does go into AB2 on modulation peaks.

73
Gary  K4FMX





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