Gary Schafer garyschafer at comcast.net
Wed Nov 8 18:33:48 EST 2006

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> -----Original Message-----
> bounces at mailman.qth.net] On Behalf Of Jack Schmidling
> Sent: Friday, December 08, 2006 11:43 AM
> To: Discussion of AM Radio in the Amateur Service
>
> Gary Schafer wrote:
>
> > So unless you have some higher gain tubes in the amp 12 to 15 watts
> drive
> > will not be enough drive.
> >
> > Now you can tune and load the amp to get the 125 watts out with lower
> drive
> > but it won't be linear.
>
> Now I am really getting confused.
>
> I have contended all along that it takes about 20w drive.  Everything
> seems to work well including 100% mod when I drive with 20w.  Only
> problem being that the tubes redden.
>
> As a point of interest, I am using 6L6's in the modulator but assume
> they just won't last as long and not effect the amount of modulation.
> Is this a good assumption?
>
> js

Here's how you figure how much the tubes will handle;
In linear AM operation the efficiency of the amplifier has to be exactly
half of what it is for SSB.

This means that if you tune up the amplifier to 500 watts output and the
efficiency is 66% (typical of a GG amplifier) and will handle 500 watts PEP
output.

For AM operation you would reduce the carrier to 125 watts output (down from
the 500 watts at tune up) and not touch the tuning of the amp. This will
drop the efficiency exactly in half or down to 33% in this case.

So with 125 watts output at 33% efficiency you can find out what the input
power to the amp is (should be) by dividing 125 watts by 33%. That gives
378.8 watts input. Now if you subtract the 125 watts from the 378.8 watts
you will have the amount of power that the tubes need to dissipate or get
rid of in heat.
378.8 watts minus 125 = 253.8 watts of plate dissipation. You subtract the
output power because it is going out to the antenna, the amount left is
dissipated in heat.

253.8 watts plate dissipation for two tubes is 126.9 watts per tube. If I
remember right the plate dissipation ratting on the 572B is 125 watts so you
are right at the maximum for the tubes.

Now when you modulate the power output increases and so does the efficiency
of the amplifier (to 66% on voice peaks) so the amount of power dissipated
in the plates goes down slightly.

If the tubes are getting red in a short time either the tubes are soft or
things are not tuned up right.

** Figure out what the input power to the amp is by multiplying plate
voltage by plate current and take 33% of that. It should come out to the
power output fairly close. Or figure the efficiency by dividing output power
by plate input power and see if it works out to around 33%. If not tuning is
not correct.

73
Gary  K4FMX

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