[AMRadio] Re: WTB: HV Tranny

Geoff/W5OMR ars.w5omr at gmail.com
Thu Feb 7 08:10:43 EST 2008

JT Croteau wrote:
> On Feb 7, 2008 1:14 AM,  <doxemf at aol.com> wrote:
>> Don't forget about the 5881's bias battery or source.
> Yup, got it covered.  Thanks.  A few people have recommended three
> 9V's in series, I will likely do this.

Back in my 'Johnny Novice' days, I remember when I switched modulators 
for my 250TH's, from a pair of 811's to a pair of 810's.  Big mistake 
(*=read below)
I also went from 0 bias, up to a requirement of -45v or some such.  It's 
been a long time ago.  T had found a bunch of old, dead/weak 9v 
batteries and strung 'em together.  Eventually, I wound up with the 
required voltage but there were something like 7 or 8 batteries in 
series ;-)

That darn handbook... said a pair of 811's was only like some 175w in 
Class B.  But, wow, what's this? 810's have 700w?!?  I GOTS TO HAVE ME 
SOME OF THEM (or so I thought).  Contacted John Mohn/W5MEU(sk) and 
struck a deal.

Apparently the 810's I had were either weak, or there was a severe 
mis-match of impedance, using that ol' buzzard RCA 5500ohm 1:1 
open-frame modulation transformer.  But, in order to get the same level 
of audio I was getting with the 811's, I had to run (-4-) 810's in 
push-pull parallel.  Plate voltage was around 1500 on the 810s, common 
Final/modulator B+ supply.  I only ran 1500vdc on the 811's, but a pair 
of 811's has 'beau coup' audio. 

I did some experimenting, and eventually even pulled the parallel tube 
on the negative peak side (two 810s on the positive peak side of the 
modulator, one 810 on the negative peak side.  I -finally- started 
seeing decent peaks on the scope).

I think my next modulator is going to be Four  (4) 811s in push-pull 
parallel.  I might even go with six (6) just to insure there's plenty 
enough dissipation for natural asymmetrical audio peaks.

Having -enough modulator power- to faithfully reproduce your voice is 
what's required to modulate the final to 100%, with as little distortion 
as possible.

100% modulation, to me, is where the negative peaks hit the baseline of 
the carrier.  When you go more negative than that, then that's where 
over-modulation, distortion, splatter and a whole myriad of other bad 
things happen that are -not- in 'good engineering practice'.

The myth is:  for running 500w of carrier, you need 250w of audio power. 
While that works out mathematically for a sine-wave, that is simply not 
a true statement for a carrier modulated with the rich harmonics of the 
human voice.

Driving your AM Rig without a scope, 
is like driving your car at night, without headlights. (K4KYV)

73 = Best Regards,

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