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 <[email protected]> wrote: > 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
