[ARC5] AM modulating an "ARC-5" transmitter...

[Kenneth G. Gordon]

On 10 Oct 2015 at 14:56, Mark K3MSB wrote:

> 
> >>Does that 60 W PEP mean 30 W fixed carrier and 15 W per sideband max when
> >>modulating?
> 
> Wouldn't that be for a pure sinusoidal modulating frequency?

Most probably...

> I don't think PEP is relevant to AM.

Well, yes it is. At least the FCC sure thinks it does.... !!! This is the reason 
the power limit for AM now is 275 watts. This figures to around 1100 watts 
(275 X 4, roughly) PEP according to the FCC's reckoning. Many AM 
affciandos vehemently disagree with that though.

>   When I do AM I simply state the carrier
> power I'm running.

But with controlled carrier AM, the carrier can be as small was zero watts 
output during pauses in speech. So what would you state in this case? The 
most accurate measure of output power in the case of CC modulation IS 
PEP. There are a number of PEP wattmeters available these days, (the 
Heathkit IP-2140 for instance) which fairly accurately measure PEP.

In any case, the maximum power output one can reach with CC modulation 
is the maximum plate dissipation rating of the tubes being modulated. Oh, 
sure, you could force it over that, but that would limit the life of your tubes, 
and besides, there really is no good reason to do that.

>   If anything else, I'd state the modulation percentage
> too, but my modulation meter is not working.  I'm currently using just an
> SB-610 monitor scope.

Modulation percentage with a good CC design is 90%, on average, but can 
be either higher or lower depending on how it is set up. FYI, to properly set 
up a CC modulation system, one really should use a 'scope and a single or 
two-tone audio input.

> I just got a T-17, so I hope to try my BC-456 out soon on AM.   Assuming the
> T-17 still works, which I gravely doubt.

I have not seen one of those within the last 30 years that still works. Back in 
the late 1950s, when I was first licensed, there were many that worked...and 
many that didn't too.

>     I think there's a way to use a
> dynamic mic in a circuit meant for carbon mics -- I thought I saw a thread on
> that some years ago.

Yes. In fact, one can remove the old carbon element and replace it with 
some modern element which will allow it to work properly in the old 
carbon-mic circuits. However, I don't have that info at my fingertips.

You might also remember that the T-17 (and others like it) were purposely 
designed so that the operator must shout into it in order to get enough audio. 
This was due to its primarily being used where external noise was very 
high...like on a battlefield.

We used to remove the front cover from the T-17 and drill many more holes 
in it in place of the three holes which were usually there. That made a big 
difference.

We would also routinely replace the original element with one from a 
telephone....but you can't find those any more either.

Sometimes we could get a recalcitrant T-17 element to work, at least for a 
while, by beating the crap out of it to loosen up the granules. That didn't 
often work though.

vy 73,

Ken W7EKB