[VHFcontesting] looking for info - am-494/gr motorola amp

[Ron Hooper]

Hi John

I was the proud owner of a 494 back in the early 70's and ran it for several
years. It is also called the green machine. The units run a pair of 4x150's
in class C operation. I used it on 2 meter FM for years and converted it to
run linear for SSB after the Kenwood TS 700 came out around 1976. To get the
power output up on the unit, the power supply choke has to be removed from
the HV circuit to increase the plate voltage. I ran about 500 watts out on
FM. I also replaced the 150 tubes with 7580W or 4cx250R tubes and readjusted
the grid bias to the new tube specifications. When you convert the unit to
class AB operation for SSB, you will need to regulate the screen voltage and
grid bias voltage. This can be done in the location where the plate choke
came out.

As you know the 494 is big and heavy. It did not give very many problems
that I can remember other than it was hard to move around. I sold my 494 amp
in the late 70's and replaced it with a home brew pair of 4cx250R,s that I
built from the VHF handbook. Jack K4GL said he was getting a KW out of his
HB 250s for eme so I went for a visit and asked how he could do it.
Basically you have to increase the plate voltage to 2.4 KV on the tubes. I
believe I ran about 300 volts on the screens. From what I know now, I don't
think the 494 output circuit would handle a KW out reliably due to the
cooling required. If your going to push the tube ratings, it also becomes
critical that the tubes draw equal power so some tube matching/selecting may
need to be done.

Since the elements inside the 250 tubes are so close, the filaments need to
be on for a minute before applying plate voltage to reduce the possibility
of an internal arc, especially with a higher plate voltage applied. If you
have a meg ohm meter, you can pretest the tubes before placing them in the
amp by measuring the internal resistance before and after the filaments are
on. Good tubes usually dissipate the resistance quickly after the warming
starts.

I hope my memory is correct and has helped you.

Ron W4WA
Lula, Georgia


On 7/29/07, John Robinson <johnr at cavalry.com> wrote:
>
> Just wondering if anyone knows about an old military Motorola Amp I have
> sitting on my shelf. I purchased it in an estate sale a couple years ago
> and finally have decided to try to learn something about it. According
> to the placard on the front its a Amplifier Radio Frequency AM-494/GR
> Motorola Inc.
>
> I have been unable to find out much at all about it on the internet
> using google, so I'm hoping someone is familiar with this unit and would
> be will to share that info. The one website that listed a little info
> says its a 45w in, 250 watt out VHF amp. Can this amp be made to work on
> 2m?
>
> Thanks for any info or direction on this amp.
> John
> w5hb
>
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