Yes, Gerald, I thought about the DC bias variation too... the fact it would
be swinging under ssb modulation... and thought that was not a good thing
unless the tube was run in class A. This is class B amplifier, of course.
I saw that Tom/W8JI recommended it back in 2000 or so, so figured it was a
good trick. He said "unbypassed".
Maybe I misunderstood something.
I also saw guys talking about using an RF choke across this resistor. THAT
would cure the DC bias variation problem, and maybe be transparent to RF as
required?
T
>
> 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 still
> have the problem of cutting the tube(s) off when in standby.
>
> 73,
> Gerald K5GW
>
>
>
> In a message dated 12/14/2005 4:28:17 P.M. Central Standard Time,
> K1JJ@comcast.net writes:
>
> 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
> looking at.
>
> 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... :-)
>
> 73,
> Tom, K1JJ
>
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