Discussion Topic: Neutralisation

The disadvantage with my 400 watt  P.A. design is that it lacks any form of neutralisation - hence the need for extensive 'padding' around the grid circuit.

Murray Greenman ZL1BPU has written:
"I noticed your appeal for help in neutralizing your 572B amplifier.  Here's my 50p on how you do it. Rather than connect 1000 pf capacitors to ground on each of the grids, connect really tiny (but high voltage) capacitors to the opposite anode.

Now this leads to a real problem because there are HUGE voltages there!

The answer is to (a) leave the 1000 pF capacitors there, perhaps reduce the values, so the neutralizing capacitors become realistic, and part of a voltage divider (check the anode - grid capacitiance of the tubes) and (b) make your own capacitors.  I suggest metal or copper-clad discs about 30 mm diameter, with really good insulation. In the old days pennies and ceramic insulators were used.  Mount the discs on threaded rod or solder them to bolt heads, and mount nuts on good fibreglass insulation (no copper of course!).   I'd also leave the ferrite beads on the grids leads.

Needless to say, the voltage rating of the homebrew capacitors needs to be at least 5000 V.  The commercial ones were usually made of 6mm Al discs carefully rounded on the edges.  I suggest the pillars be ceramic or perhaps glass fibre rod.  Long stainless bolts with the heads countersunk into the discs would be appropriate, but the discs and the heads of the screws need to be very smooth so there are no projections which might start an arc.  If the discs are too small, they'll need to be too close to achieve the correct capacitance.  I'd suggest about 8 mm spacing at least.

You might also want to consider parasitic suppressing damping chokes on the anode caps as well.

These can consist of 5W or more composition resistors (if you can find them!) with a 10 turn inductor wound on them.  Using a wirewound resistor won't help, as the inductance is in SERIES with the resistor, instead of in PARALLEL.  It's worth experimenting with coils wound on wirewound resistors, though.

The tuning procedure consists of firing up the amplifier with no anode voltage, and with full drive applied, adjust the two neutralizing capacitors to null the transmitter output.   You might need to use a sensitive RF meter for this, and there should be a distinct null, which you can deepen by differential adjustment of the capacitors.  This technique works even if the amplifier is aperiodic, since what you are doing is creating a bridge network to balance out the a-g capacitance in the tubes.

The final step is to apply anode power and check that the neutralization gives stable results as you adjust your antenna tuner.  Perfect neutralization won't be necessary, so long as you have significantly reduced the feedback.  The tube capacitance can change slightly with voltage on the tubes, especially when the plates glow cherry red, as the tubes expand internally.  What ever you do, don't try to adjust the neutralizing caps with anode voltage on!

It might also be a good idea to fit a beefy diode or Zener across the grid bias supply to ensure that if there is a flash over in one of the neutralizing capacitors, the grid coil and the exciter won't be damaged."

The picture is from the "Radio Engineers' Handbook", 9th edition, 1943.  The amplifier was the PA from a 400 Watt AM/CW rig with a pair of 8005 tubes in push-pull, grounded cathode.  On the left you see the plate tuning capacitor, and on the right, the grid tuning capacitor.  You'll clearly recognise the two neutralizing capacitors, one behind the other.  The amplifier had plug-in coils, the plate coils off picture to the left and the grid coils just about visible to the right.  My guess is that it would take nearly 10 minutes to change bands and retune the whole transmitter!

I am very grateful to Murray for his contribution, but have not yet had a chance to try out the above procedure.  I look forward to doing so when I next take the lid off the amplifier. 

Incidentally, Murray runs what is probably the definitive Hellschreiber web site, and also the only web sites for MFSK, MT63 and Peter G3PLX's chirp sounding experiments. See Murray's web site for more details.