Tom, if you are operating AM as in steady carrier, the resistor bias is
probably ok. If you plan to operate SSB or cw then the bias voltage will be
changing from zero with no drive to a maximum value with maximum drive. You
have the problem of cutting the tube(s) off when in standby.
In a message dated 12/14/2005 4:28:17 P.M. Central Standard Time,
Thanks for all the replies! cheezz... I got close to 15 private ones and
they're still coming in - really appreciate the ideas, guys. Please excuse
me if I don't answer them all...
K5GZ [good whiskey] that's an good idea about determining how clean the
FT-1000D 200W module is at 90w or so, FIRST, to possibly eliminate a few
linear stages. I've actually idled the finals a little higher in the past,
but hit the pot's limit for now.
Also, the fellas who emailed me about the Harris and Colins amps - Worth
I was looking through the Amps archives and came across talk about using a
cathode resistor for conventional negative feedback. I tried a 10 ohm
resistor in my GG triode amp today and found it idled the amp at about 1/2
it's normal level. I use diodes normally that get switched in and out to
idle it at various levels... so just switched out a few to get it back to
normal idle. Does anyone know how to determine the optimum value for this
cathode resistor and about how many db of IMD/negative feedback improvement
you can expect with an optimum value?
I could actually eliminate the diodes completely and increase the resistor
to 20 ohms - so that all the idling bias is supplied by the resistor. Is
this a good limit?
BTW, After adding the cathode resistor, I didn't see much difference in the
amplifier's drive requirements, so it makes me wonder how little negative
feedback it was adding. ie, if it was a measly 3db, I'd need TWICE the
drive, right? No way I saw anything near that.
Another idea I received - For a 75M mono-band amplifier: How about
building a single hi-mu GG triode final and a smaller hi-mu GG driver ....
How would you connect an addtional feedback loop around them? If a capacitor
coupled RF sample came off the final's plate, where would it be fed back
into the driver? The driver's grid is usually grounded, so that's out
unless another config is used, and I'm not sure of the phase to feed back to
the driver's cathode... What would be a proper hookup?
There's more ideas, but that's enough for now... :-)
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