On Mon, 2006-05-01 at 12:59 +0000, Rob Atkinson, K5UJ wrote:
> >>If I wanted to get into AM operation, no solid state rig could ever
> >>satisfy me. I'd want a big tube transmitter. <<<
>
> There's this mythical idea hams seem to have, that decent AM can only happen
> with a big plate modulated tube rig. I have no idea where this comes from,
> given the fact that broadcasters are ditching this technology for solid state
> transmitters as fast as they can.
Art Collins for one. And he demanded a separate PA plate feed audio
choke to keep the unbalanced DC out of the modulation transformer. A
young engineer once proposed running a 1 KW broadcast transmitter
without the separate choke, having studied the transformer and core
design details to make it work, but when Art saw the "working paper" his
response was that it didn't work in 1938 and it won't work now.
> Turn on your medium wave AM receiver and
> tune around to any radio station. Odds are 3:1 that you are hearing a solid
> state rig, especially if you are tuned into a big 50 kw station. About the
> only ones left running primary vacuum rigs are the dinky daytimers that
> aren't generating enough revenue to go solid state.
The first switching modulation AM rig used tubes for 50 KW. Built at
Gates by Hilmer Swanson. Then he made rigs solid state. Their most
attractive feature is super high efficiency from power line to RF,
something like 85% overall while the classic high level plate modulated
rig rarely did over 50% overall. On a 24/7 load the power savings are
way beyond significant. Saving over 40 KW for a 50 KW signal amounts to
over $4 an hour, depending on the electric rate. And the new 50 KW
transmitter is so small, there's room at WHO's Mitchelville Iowa site to
keep the previous two tube transmitters for spares or museum pieces.
I'm not sure the new solid state transmitters sound better than the high
level plate modulated rigs, but there's no modulation iron to limit the
bandwidth. There very well may be spurs at the sampling frequency each
side of the carrier. Round about 75 KHz in Hilmer's first transmitter.
Low level modulated rigs with linear amplifiers don't do well in the
efficiency department, and don't put out as big a signal if the power
input is the limit as it once was. Actually a 1KW input PA in ham
service puts out more power when plate modulated than a 1.5 KW output
linear and takes less power from the AC line. Old time AM operators
complained to the FCC (to no avail) when ham power levels where changed
from PA plate input to RF output because of that limitation.
>
> the best sounding ham AM rigs out there now are solid state, either class E,
> dsp generated AM from the flex radio, or low level modulated exciters
> driving amps. a few guys are doing well with old tube broadcast rigs and
> globe kings, gold dust twins etc. but for every one of them there are 100
> with dx100s who are unbearable.
Its easy to overdrive a linear from low level modulation and put out a
distorted AM signal from peak clipping. And tempting to do so to see
power on the RF wattmeter.
There always have been some DX100s that sounded bad from trying to get
super modulation effects or simply from schlock tuning not driving the
PA grids hard enough to supply the peaks or from running the PA and
modulator tubes way past their prime.
I was once a temporary chief engineer at a class four AM station where
the management didn't keep spare tubes on hand. One day I noticed the up
tube plate current peaks weren't quite as strong as the down peaks. I
spent a while with the owner convincing him to let me pick up a spare
modulator tube (828). The next afternoon when I arrived the station
sounded terrible. The morning DJ was watching positive modulation and
had cranked up the gains trying to get some while he was running 300%
negative modulation and the up tube didn't have the emission to wiggle
the plate current out of idling current. I shut the transmitter off and
put in that new tube that never sat on the shelf, and after cheating the
interlocks so I could set the bias in seconds instead of minutes, I put
the station back on the air for my afternoon into evening shift at the
board. I have no doubt same failure continues to happen to DX100 and
Viking rigs yet today. And sometimes the "spare" on the shelf isn't as
good as the tube that needs replacement.
>
> rob / k5uj
>
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
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