[Boatanchors] general PA question

[Sandy]

The Mackay MRU-35A transmitter ( shipboard) used a single 8122 class A 
driving one 4CX1500B.  They had around 2700-2900 on the plate as I remember 
and delivered a kw. out at around 750 MA of plate current.  I don't remember 
now what the "idling" current was.

RF Communications had an amplifier that used two 4CX1500's in parallel and 
operated 2 mhz. band segments from 2-26 or so Mhz.  It too delivered a 
little better than a kw out, but was a mess mechanically and troublesome 
piece of gear.  It only required 100 Mw. of RF to get a kilowatt out!  The 
Mru-35 was driven by the same amount, or actually a bout 1-1.5 volts of RF 
to get a full KW out.  As I remember, the Mackay version's final stage  was 
running Class AB1 (linear) and the 8122 driver was running in Class A.

73,

Sandy W5TVW

----- Original Message ----- 
From: "Brian Clarke" <brianclarke01 at optusnet.com.au>
To: <Boatanchors at mailman.qth.net>
Sent: Saturday, April 11, 2009 7:27 PM
Subject: Re: [Boatanchors] general PA question


> Be careful Eugene,
>
> The cathode idle current depends on the Class of operation. For Class C, 
> key
> up, it should be zero; Class B will be higher and Class A will be even
> higher.
>
> Unless the Class of operation is specified, all 'statements' of idle 
> current
> are spurious.
>
> Bias determines, in large measure, the Class. However, there are other
> possible problems - parasitic oscillations and incorrect neutralisation.
>
> 73 de Brian, VK2GCE.
>
> Eugene asked:
>
>> Hi there. I am working on a power amp.
>>
>> The unit runs 2 4cx1500B's in parallel driven by 2 8122's in parallel.
>>
>> When idle (no rf input, no key) the plate current is supposed to be 0 ma.
>> However, I seem to get about 100ma.
>>
>> When keyed at 1KW output, I get a little 750-800ma plate current. The
> > unit is not supposed to exceed 750ma (800 absolute max).
>>
>> Does this sounds like a bias problem? Could an incorrect bias voltage
>> cause about 100ma too much current overall?
>
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