[Johnson] Viking I durability?

[Carl]

----- Original Message ----- 
From: "Richard Knoppow" <1oldlens1 at ix.netcom.com>
>
>>
>> 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.

** Not correct, it is rated at 100W Output. At the manuals 600V at 230ma 
that is 138W input and quite a bit more at todays line voltages.

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.

** The design engineers have no problem understanding the issues, it is 
undereducated techs at the companies that publish handbooks and magazines 
that write the 50% nonsense. They overlook losses, tube variations, 
headroom, tube aging, etc, yet the printed word survives forever.
Many of the homebrewers today and well trained ones of the past know enough 
to build for at least 50% overhead as it is a lot easier to back down the 
mike gain compared to trying to find more power without excessive distortion 
or running tubes over ratings.  One of the common better designs of the 50's 
was an 813 modulated by PP 811A's. Another was a pair of 4-400A's at 1KW 
input modulated by PP 810's and with both run off a common 2250-2500V PS 
there was decent overhead.


>     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.

** Totally different subject and a lot more is involved in "thin" sounding 
AM receivers.
In a transmitter modulator the DC in the secondary is what limits its 
capacity. This is why the better BCB transmitters used a modulation reactor 
and an oil filled coupling cap. It is also what I use with the big amp using 
a Gates transformer, reactor, and cap from a scrapped TX.


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

** Feedback is not a free lunch as it creates a problem with phasing the 
modulator tubes. It is better IMO to have a very clean driver and a well 
built modulator and let a small bit of distortion survive; ham AM is not for 
audiophool levels.

Carl