[ARC5] AM linear amplification
Richard Knoppow
1oldlens1 at ix.netcom.com
Mon Dec 19 20:22:44 EST 2016
Harris introduced the pulse modulated transmitter for AM some time ago
but I don't remember the date. The idea is that the modulation audio is
sampled at some high frequency, say around 70Khz and converted to a
variable width pulse. The pulses are applied to the final amp plate and
are integrated by the tank circuit. The result is AM but the transmitter
is capable of very high efficiency along with wide, flat, frequency
response and low distortion if done right. The original pulse modulated
transmitters were tube amps but Harris and others have gone to solid
state. The kind of thing you describe would produce the same result
using an array of lower level switched devices.
FWIW, the first generally available 50KW transmitter was the RCA
50-B which came out about 1931. This was a several stage linear with
modulation in a low level stage. The line demand was on the order of
250KW. Some of this was for the water cooling system. RCA built at least
a dozen of these before they were replaced with a newer design. The
first RCA 50KW amp was at WEAF in New York, it was the only 50-A ever
made and was rebuilt into a 50-B shortly after. I don't know what the
rebuild consisted of. Western Electric must have made some 50KW
transmitters too around this time but I don't have specific information.
A modern 50KW transmitter has a line demand of well under 100 KW.
The Doherty amplifier was announced by Western Electric about 1935.
The first one went to WHAS in Louisville. There were a lot of Doherty
amps sold but they had a reputation for being hard to tune up, something
that WE fought until they quit making broadcast equpment. Also, the
arrangement is not really linear since there is a mismatch in the tube
curves. WE used about 30 db of RF feedback around the amp to get the
distortion down but, due to difficulty in keeping the phase constant the
feedback was less effective at high modulation frequencies so at 10Khz
it could reach nearly 10% at 90% modulation. James Weldon, of
Continental, modified and improved the Doherty circuit improving its
stability and lowering distortion. The Continental transmitters proved
very popular partly because they were built to a very high standard of
quality. Actually, William Doherty, of Bell Labs, the inventor of the
circuit came out with a number of improvements but only after WE was out
of the business.
Too bad that we now have AM transmitters capable of very high
fidelity being fed over-processed mostly junk programming.
On 12/19/2016 3:20 PM, Jim Wiley wrote:
> I am told by a broadcast engineer friend (Terry, AL7CE) that modern AM
> transmitters use a collection of dozens to hundreds of "building blocks"
> that are switched on and off at an audio rate, with the resulting RF
> being combined by a (hybrid?) network of some kind. He converted
> (replaced) the KICY 50-KW DX50 Harris (now Gates) transmitter at Nome,
> Alaska this way and achieved something on the order of a 90% efficiency
> (AC power in to RF out) power after the change. Failed RF modules can
> be "hot switched" with no loss of air time. Transmitter has NO
> microprocessors controlling the process.
>
> How effective is it? The transmitter experiences a 3-degree rise in
> cooling air from outside air intake to exhaust, when running at full
> rated power.
>
> A different scheme used by Nautel (sp?) uses a pulse modulation
> technique with similar efficiency to the Harris system. The combining
> system is touchy - and as someone else here said, don't try this at home.
>
>
> - Jim, KL7CC
>
>
>
> On 12/19/2016 1:24 PM, Ian Wilson wrote:
>> Good excuse to use a large tube (4-125, 813, etc). No need for air
>> cooling until you
>> get to one size of plate dissipation higher :)
>>
>> 73, ian K3IMW
>>
>> On Mon, Dec 19, 2016 at 12:36 PM, Kenneth G. Gordon
>> <kgordon2006 at frontier.com <mailto:kgordon2006 at frontier.com>> 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
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