Topband: Neutralizing your 833As

[Tree]

This sounds like a topic for the Amps reflector - thanks!!

Tree N6TR

On Sun, Jan 17, 2016 at 5:02 PM, Mike Waters <mikewate at gmail.com> wrote:

> Hi Dave,
>
> Thanks for the advice! What you describe below is an improved version of
> what I had planned to do.
>
> I'm likely over-thinking this. Much ado about nothing, as the old saying
> goes. I've been accused of doing that before on this amp project (and he
> was right. :-)  Let me elaborate a little on what my concern is.
>
> I don't think I ever drew a schematic of the PA grid and plate circuits,
> but I'll try and describe the important details, leaving out things like
> the grid biasing arrangement, RF chokes, DC blocking capacitors, the TO-220
> swamping resistor string, circuit protection, etc.).
>
> 1. There is no tuned grid circuit. (If there was, it would be simple to
> neutralize!)
> 2. There's a 200 ohm 100w non-inductive swamping resistor between the grid
> and ground.
> 3. From the grid to ground is the 200 ohm secondary of the input
> transformer.
> 4. The input is the 50 ohm primary of that transformer.
>
> Here's the idea I had in mind, good or bad: Add a tertiary (third) winding
> to that input matching transformer --grounded at one end-- to obtain the
> voltage for neutralizing. The other end would be in series with an air
> variable (already have that)* and a fixed capacitor connected directly to
> the anodes.
>
> Maybe I'm over-thinking this, but what I expect is too much or too little
> feedback voltage
>
> *Photos of the neut cap is at
> www.w0btu.com/files/misc/833C_linear_amplifier/?C=M;O=D
> neut-cap_5445.JPG and neut-cap_5444.JPG
>
> The reason for the fixed capacitor between the anodes and the variable
> capacitor I mentioned was to reduce the chances of the air variable arcing
> over, which would be catastrophic. That concept is
>
> 73, Mike
> www.w0btu.com
>
> On Sun, Jan 17, 2016 at 12:20 PM, Dave Olean <k1whs at metrocast.net> wrote:
>
> > Hi Mike
> >    You need a signal generator and a crystal detector to sniff in that
> > amp. Feed a low level signal in with the filaments on but HV off and see
> > how much RF leaks thru to the output connector, then mess around with
> > neutralization schemes until the RF drops way down. I do that all the
> time
> > and it works great. I have neutralized a six meter KW with a 4CX1000A, a
> > bunch of 4CX 250B amps and driver amps, etc. It never failed. Shoot for
> 10
> > dB or more attenuation than the tube can produce and it should work FB.
> > The nice part is no high voltage to worry about. You can get the
> > neutralizing wire or the cap set for a perfect null at your frequency
> > pretty fast. I used to use a signal generator wiyth an HP 415E VSWR
> > indicator. Put 1000 Hz audio modulation on the signal generator and you
> can
> > read attenuation right off the meter on the 415.. Slick!  Now I have two
> > port a vector network analyzer, so the HP 415 sits on the shelf, but
> either
> > method is quick.
> >
> > [snip]
> >
> > Maybe you would let me pick your brain sometime about my swamped-grid
> >> dual-833C legal limit amp. It works fine on 160 and 80, but it needs
> >> neutralizing if it's going to be stable on 40 (and work at all on 20). I
> >> have a plan to neutralize it using a tertiary winding on the input
> >> transformer, but I foresee a lot of trial and error. But I don't have
> time
> >> to work on that right now.
> >>
> >> I wasn't even sure if two of those triodes in parallel would even work,
> >> so I didn't spend a lot of time making it look pretty.
> >> www.w0btu.com/833C_linear_amplifier.html
> >>
> >
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