[Amps] Tube Upgrade for Motorola 2 Meter Amp

[Carl]

There is nothing wrong with using an old style PS as it was designed for any class of service. The problems start when the tube rectifiers are removed and SS rectifiers with a single fat cap are used.
A well designed choke input filter will give .9 DC output referenced to the listed AC output of the xfmr. The regulation will be better than the typical C only filter and the peak output under full load will be better. This is without a tuned choke.

There are many examples of Ham and commercial amps using a regular choke input PS.

I have that 4-125A amp or a close clone and the circuit is right out of the Eimac spec sheet. I believe mine was used in Class C AM for the airlines or FAA on the 130 mHz range. With the lines shortened it does a comfortable 300W out on 2M FM for contests and that is with the tubes that were in it.

A VHF FM High Band amp will probably need the lines lengthened a small amount to get down to 144.

With a little work a pair of 4CX-250's, 8930's, or Russian 4CX400A's could be fitted; all using the same socket. Depending on the PS used the SSB output would be in the 600-1200W area. Maybe 400-500W PEP with the existing PS and just SS rectifiers.

Line lengths can be calculated by comparing the output C of the 4-125's against the ceramics.

Carl
KM1H


----- Original Message ----- 
From: "Al Waller" <k3tkj at qsl.net>
To: "'Steve Berg'" <wa9jml at tbc.net>; <amps at contesting.com>
Sent: Saturday, August 23, 2008 6:48 PM
Subject: Re: [Amps] Tube Upgrade for Motorola 2 Meter Amp


> Steve,
> 
> The juice may not be worth the squeezing.  Ideal amp would be a 3cx800a7 at
> that plate voltage but the old Class "C" power supply would be rather poor
> in SSB service so at a minimum new plate transformer and more capacitors in
> the power supply.  3cx800 is a triode so you can scrap the screen supply.
> Good Luck with the project.
> 
> Al
> 
> -----Original Message-----
> From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
> Behalf Of Steve Berg
> Sent: Saturday, August 23, 2008 6:38 PM
> To: amps at contesting.com
> Subject: Re: [Amps] Tube Upgrade for Motorola 2 Meter Amp
> 
> Years ago when I was a service tech, my boss gave me a really nice 1963 
> vintage Motorola 1 range tube type quarter kilowatt station.  The final 
> deck uses a pair of 4-125 tetrodes in push pull configuration.  As I 
> recall, the plate supply is about 2200 volts.  If I were to convert this 
> to class AB1, and regulate the screen voltage, I would get barely enough 
> power to be noticeably louder than my 160 watt Mirage brick.  I would 
> like to figure out some way to make this into a usable 2 meter weak 
> signal amp.  The tuned input and output lines are there, but I need 
> better tubes.  What sort of changes might I consider to get this clunker 
> back on the air where it belongs?
> 
> Steve WA9JML
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