Topband: Neutralizing your 833As

[Mike Waters]

No problem, Tree. I fully agree, and your point is very well taken. Thanks.
(And thanks to Dave and everyone else who offered advice!)

I really don't have the time to start a thread on the amps reflector right
now (but I'd like to soon).

73, Mike
www.w0btu.com

On Sun, Jan 17, 2016 at 8:20 PM, Tree <tree at kkn.net> wrote:

> 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
>> >>
>> >
>> _________________
>> Topband Reflector Archives - http://www.contesting.com/_topband
>>
>
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