[ARC5] AM linear amplification

[Richard Knoppow]

     Please note that if your transmitter does not have controlled 
carrier modulation using a linear amp is not similar.  Linear amplifiers 
were standard practice for broadcast transmitters until about the late 
1930s when high level plate modulated transmitters began to displace 
them.  A linear amp has very low overall efficiency for an AM signal. 
The efficiency of a Class-B linear is about 33% for the carrier, a Class 
AB will be slightly lower. This is because the level of the signal must 
be kept low enough to allow linear amplification of the modulation 
peaks. The actual overall efficiency varies with the modulation. A 
linear handling SSB will have an average efficiency of around 70%. There 
were several special amplifier circuits developed to try to get average 
efficiency increased for AM broadcast use to lower operating cost. The 
only successful one was the Doherty amplifier, made by Western Electric 
and a similar amplifier now made by Continental Electronics.  This 
amplifier uses two sections, one operating at Class B when the carrier 
only is present and both tubes contributing when the carrier is 
modulated. The overall efficiency is relatively constant with modulation 
at around 70% These amps require phase shift networks so they are 
difficult to make when the transmitter must cover different frequencies 
although such transmitters were made.
    There is nothing wrong with using a Class AB or B amp for AM but you 
must put up with low overall efficiency and make sure the plate 
dissipation is adequate. Older books on transmitters will have a ton of 
both theoretical and practical design information.

On 12/19/2016 12:36 PM, Kenneth G. Gordon wrote:
> On 20 Dec 2016 at 9:05, AKLDGUY . wrote:
>
>>
>> My BC-AO-230 puts out about 5W and I'm interested in boosting that with a 6146B Class AB1
>> linear amp. The ARRL Handbooks say that AM linear amplification is similar to controlled
>> carrier modulation and caution that efficiency is low, about 33%, but don't give explicit
>> operating conditions.
>>
>> I assume that the (6146B) stage cannot at any time be driven to the same output that it is
>> capable of in SSB amplification. That's because with no audio applied, the level of the AM carrier
>> drive must be carefully set so that the tube's plate is dissipating half of its rated dissipation. The
>> application of audio drives the stage to a level where the plate dissipation is approached but not
>> exceeded. This level is far below the SSB PEP input.
>>
>> Anyone have any comments?
>
> I have nothing to add. You have about covered it.
>
> Using any linear amp in AM service requires keeping the plate dissipation at a "reasonable"
> level.
>
> You obviously understand that.
>
> I would expect no more than about 25 watts output from your amp. Still, that is 5X the
> un-amplified output. I think that would be well worth the effort.
>
> Actually, the 807 or 1625 has a slightly greater plate-dissipation rating than the 6146. Some
> books show it as having a PD rating about 5 watts greater than a 6146.
>
> In addition, controlled-carrier modulation has greater AVERAGE efficiency than straight AM,
> because the carrier falls to a much lower level in the absence of modulation, while it
> remains steady for AM.
>
> CC modulation of a 6146 results in around 35 watts output.
>
> Ken W7EKB
>
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-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL