[AMRadio] RE: Modulator design needed

W7QHO at aol.com W7QHO at aol.com
Mon Mar 27 22:23:07 EST 2006

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

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