This sounds like a topic for the Amps reflector - thanks!! Tree N6TR
On Sun, Jan 17, 2016 at 5:02 PM, Mike Waters <[email protected]> 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 <[email protected]> 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 > _________________ Topband Reflector Archives - http://www.contesting.com/_topband
