[Boatanchors] AM Amplifier

[Bry Carling]

It’s almost impossible to get to the AGC line on my FT950. Without a fixed DC source there, the AM it produces is awful. Mostly downward modulation and it can’t make good AM. I’ve seen the same thing with other transceivers also. Not just Yaesu. I think certain older Icom models will produce properAM.

Best regards - Bry Carling, AF4K





On Jul 9, 2018, at 2:21 PM, Rodger Singley <wq9nsc at live.com<mailto:wq9nsc at live.com>> wrote:

Bry,

Set up properly most Yaesu rigs can do decent AM but the particular setting depends upon the rig and the included operator’s manual typically varies between useless and wrong.  With the FT-817 you have to adjust the carrier level (via the menu) down so that it is no more than ¼ of the rigs capable output or otherwise the protective ALC in the RF amp section will result in very poor modulation with the default settings.  If you understand AM this makes perfect sense but because Yaesu markets this as a “plug and play” rig they should have the default settings ready for the AM appliance operator who doesn’t understand much beyond pressing a mic button.

With the Yaesu FT-1000/1000D the manual is simply wrong.  This rig has front panels controls for drive and RF power with the RF power control acting as a limiter for power output.  For AM operation, Yaesu has you set both controls to 12 o’clock resulting in horrible AM operation while the correct setting is to leave the RF power (i.e. limit) control in its full power position and use the drive control to drive to the final for the safe AM power level.  The same thought applies for CW, don’t use the RF power control to set the level but instead use the drive control or otherwise you will end up with horrible key clicks as the output amplifier will turn the proper keying waverform into a near square wave.

On AM, set up as Yaesu instructs the rig will limit modulation to just over the carrier level which not surprisingly sounds truly horrible, something like a Johnson Valiant whose owner has not read the manual and thinks setting the clipper control to full CCW provides minimum clipping when in actuality that is the maximum clipping position which of course sounds terrible.

One difference between Yaesu and vintage AM rigs was with vintage rigs you had to carefully follow the manual (ditto Heathkit which copied much from Johnson with their own twist on the clipper control in the Apache TX-1 by hiding the most needed gain control behind the CW key jack helping to create the “scratchy Apache” legend).  With Yaesu (and some other modern rigs), extracting good AM requires a good understanding of the mode and the radio but it can be done.

Rodger WQ9E

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________________________________
From: Bry Carling <af4k at hotmail.com<mailto:af4k at hotmail.com>>
Sent: Monday, July 9, 2018 12:20:04 PM
To: Rodger Singley
Cc: Rob Atkinson; Boat Anchors List
Subject: Re: [Boatanchors] AM Amplifier

FT-817 ?

I’m guessing that provided you with the typical negative modulation that is produced by most Yaesu rigs.

Best regards - Bry Carling, AF4K


On Jul 8, 2018, at 8:13 AM, Rodger Singley <wq9nsc at live.com<mailto:wq9nsc at live.com>> wrote:

The overall efficiency for the apparent amplifier output will be less than the actual tube efficiency since output tank losses come into play and these can be very significant with the typical tapped or roller inductor pi net output circuit.  The older single band plug in coil setups can do better but there will be losses in all types of output networks.



The only amplifier I own that will safely do legal limit AM is a contest duty amp I built back in the early 1990s using three 4CX800 tubes in AB1 with a large blower and a Dahl transformer rated for 3KW CCS.  I am impressed with the overall design of my Heathkit KL-1 “Chippewa” outside of the exposed plate meter that is directly in the HV feed.  I had some fun using it to greatly increase the AM power of a Yaesu FT-817.



For high power AM I prefer my Desk KW and Viking 500.  A Johnson Valiant can produce about the same level of AM output as the typical twin 3-500Z amp designed primarily for SSB.  For those willing to run controlled carrier AM (as found in many popular 60s and early 70s rigs including the Drake 4 line transmitters and transceivers, Heathkit DX-60, Knight T-60, etc.) then any of the  SSB rated amps will work well.  Otherwise I would find an amp rated for RTTY use, load it to rated RTTY input, and then run it at no more than ¼ of this input at carrier level.



The cooling of typical amps like the SB-220 is marginal and it was common for contesters in the 1970s to run them with the outer case removed and just the protective shielding in place which helped them live under contest conditions.  Drake did a better job with the blower plus chimneys L4 series (cheapened when the nice looking L7 was released).



Rodger WQ9E



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________________________________
From: boatanchors-bounces at mailman.qth.net<mailto:boatanchors-bounces at mailman.qth.net> <boatanchors-bounces at mailman.qth.net<mailto:boatanchors-bounces at mailman.qth.net>> on behalf of Rob Atkinson <ranchorobbo at gmail.com<mailto:ranchorobbo at gmail.com>>
Sent: Sunday, July 8, 2018 5:24:17 AM
To: WA5VGO
Cc: Boat Anchors List
Subject: Re: [Boatanchors] AM Amplifier

He said his amp was class B.  Actually Class B is usually a little
better than 50%.

73
Rob
K5UJ

On Sat, Jul 7, 2018 at 9:23 PM, WA5VGO <hbrnut at suddenlink.net<mailto:hbrnut at suddenlink.net>> wrote:
The problem isn't voice peaks. At 100% modulation the amplifier should be
about 60% efficient. However, as the modulation level drops, so does the
amplifier efficiency. With no audio, it's down to 25-30%. I suppose if
someone talks fast and loud, they can push things.

Darrell
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