[AMRadio] Drake L-4B on AM

A.R.S. - WA5AM ars.w5ami at gmail.com
Thu Mar 6 10:15:03 EST 2008

On Thu, Mar 6, 2008 at 8:17 AM, Bob Peters <rwpeters at swbell.net> wrote:
> Bob...I run an L-7 on AM and drive it at about 30 watts and getting a
>  solid 300 watts out and peaking about 500 ...Using a rice box to drive
>  it... Works well... Same amp.

I have a Henry 3K Classic-X with the heavy 4000v supply and 8877.
It's a two holer, but I never have put two tube in there.

Only thing that bothers me about running it on AM is the plate
dissipation due to inefficiency.  Of course I am considering what an
8877 costs these days; $500 to $1200 depending on where you get them,
and if you buy them new.

I conducted a test this morning using my LP-100 digital vector watt
meter to determine true output, and of course the current meter on the
amp for the input calculation.  My results were as follows:

Using pure unmodulated carrier
Excitation power: 25 watts into the amp load
Plate current: 430 mils
Plate voltage: 3900 while keyed (need bigger wire for my run)
Input watts: 1677 calculated (3900 * 0.430)

Output watts: 544
Plate dissipation: 1133 watts calculated (input watts - output watts)
Efficiency: 32.4%

Of course with audio, the efficiency will go near %50 for average,
still not too good.  The tube will actually run cooler at periods of
modulation, however there is still instantaneous drops to the carrier
level of 32% efficiency.  The 8877 is a 1500 watt tube, and the plates
are tough with forced air cooling, but they are also real expensive.
It runs much hotter on AM with very little drive, than it does on SSB
with drive levels up to 65 watts.  Bottom line is that I think tube
life is greatly reduced when running a linear amp on AM, but that's
just my opinion.

Just something to consider when using a linear amp for AM service...

Brian / wa5am

"Money is only temporary, radios are forever" - Jim Little aka "the
old dawg"/K5BAI

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