[Johnson] Viking I durability?

[Richard Knoppow]

----- Original Message ----- 
From: "Carl" <km1h at jeremy.mv.com>
To: <johnson at mailman.qth.net>; <tbs50a at aol.com>
Sent: Sunday, May 26, 2013 9:30 AM
Subject: Re: [Johnson] Viking I durability?


> it seems like that would be easy to fix with a phase 
> splitter circuit. throw away the transformer. I don't see 
> why a Viking one would be any less durable than any other 
> AM transmitter.

     I was curious about the modulator design. The RCA 
handbook shows the output of a pair of AB2 807S to be 80 
watts with the voltages in the Viking. The Viking is speced 
for something over 100 watts input for AM. So, if my 
understanding of plate modulators is correct, its marginal 
even if the driver is sufficient. A lot of the older books 
state that the modulator needs to put out 50% of the carrier 
but this makes assumptions about the nature of the 
modulating wave which are not always valid. Of course, total 
power in an AM signal at peak of modulation at 100% is four 
times the carrier power.  Average power with a sine wave is 
1.5X carrier and I think that can mislead designers into not 
providing enough modulator capacity.
     DC in a transformer winding partly saturates the core. 
In an audio transformer it limits the low frequency 
response.  This is one reason the typical single-ended 
amplifiers found in tube communications receivers sound 
thin.  Feedback can improve matters but there is still the 
problem of the transformer core having too much flux in it 
due to the DC. A bigger transformer helps but getting the DC 
out is a better answer.  The problem is that for a given 
available plate voltage the gain and power output of a 
transformer coupled amplifier is substantially greater than 
an RC coupled one.  Another answer is to use a push-pull 
driver but then the cost goes up.  Johnson was aiming at a 
low sales price but some much more expensive transmitters 
were not any better.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com