[HomeBrew] Neutralization circuit question

[N2EY@aol.com]

In a message dated 8/1/02 5:37:08 PM Eastern Daylight Time, 
catman351@yahoo.com writes:

> I have a question with regards to the 1962 ARRL Handbook "90 watt 
amplifier" 
> neutralization circuit. I modified the input circuit so that I'm not using 
a 
> link input per the original circuit. Instead, I'm running a conventional 
> plate blocking cap to LC circuit (which comprises of a 140 pf variable and 
a 
> bandswitched coil taken from the osc circuit of a Viking 1for my 6AG7 osc 
> circuit). 

For clarity, let's refer to the original circuit before modification. The 
principles are the same.

>The output is fed to a 15 ohm/1 watt carbon resistor going into the 
> 6146 grid (which I assume has something to do with lessening the amount of 
> grid drive? 

No. 

>I dunno. Please let me know why the 15 ohm grid resistor).

It's a parasitic suppressor. Keeps the 6146 from oscillating at VHF/UHF 
frequencies, where the tube is self-resonant.
>  
>  Anyway, the ARRL circuit calls for the bottom of the 

[6146 grid]

> tank circuit to be 
> connected in series to a 680 pf cap which is in turn, hooked to ground. In 
> between the tank circuit and the 680 pf cap is a connection to a 10 pf 
> variable cap to the plate of the 6146. This is the neutralization 
adjustment 
> cap. From what I see, the variable cap and the 680 pf cap forms a signal 
> voltage divider for neutralization purposes.

No.

That's a bridge neutralization circuit. Here's how it works:

The 6146 grid tuned circuit, when tuned to resonance, has its ends 
out-of-phase with each other. That is, when one end is positive the other end 
is negative.

The way a tuned amplifier becomes unstable at the operating frequency is for 
enough voltage of the correct phase getting from the plate back to the grid, 
setting up a tuned-plate-tuned-grid (TPTG) oscillator. The feedback path in 
the 6146 amp is through the grid-plate capacitance. Also involved is the 
input capacitance of the 6146.

To counter this feedback, the neutralizing circuit sets up an 
equal-but-opposite feedback path. The neut cap and the 680 pf do that job. 

When the ratio of the grid-plate C to the input C equals the ratio of the 
neut C to the 680 pf, you have a balanced bridge circuit and the feedback is 
cancelled.

See the Handbook chapter.

> Now why must the 680 pf cap must 
> be connected between the LC tank circuit and ground? How will this affect 
the 
> drive coming from the 6AG7 osc? From what I see (using the formula for cap 
> reactance 1/(2piFC), I expect to see an impedence of several meg ohms as a 
> result of the 680 pf cap. 

Do the math again.

Xc = 1/2piFC is the right formula.

It's a lot less than megohms of reactance.

> Normally, the 6AG7 tank circuit's bottom is hooked 
> to ground. Am I missing anything here or is this o.k? 
> 
When you tie on the driver by capacitive coupling, the 6AG7 output 
capacitance is in parallel with the 6146 input capacitance and the circuit 
values need some adjusting.
  
>  Finally, I was advised to expect at least a 2KV "operating/breakdown 
voltage"
>  when plate modulating the 6146. I'm going to ensure that the 10 pf neut 
cap 
> will have that rating. How about the 680 pf cap? Must this also be of a 
> significant voltage rating or will a convention mica cap do ok? I suspect 
> that the neut cap should be effective enough to block any plate developed 
> voltages from entering the voltage divider circuit.

The neut cap must stand the full peak plate voltage, which is about twice the 
applied plate supply voltage. The 680 pF cap only has to stand the voltage in 
the grid circuit.

The "applied plate supply voltage" is the B+ on CW. But on plate-modulated 
AM, it's the B+ added to the peak output from the modulation transformer 
secondary. You can get really high peak voltages that way.

Do yourself a favor and put a high voltage disc ceramic (.001 or so) between 
the neut cap and the 6146 plate. That way if the screwdriver slips while 
adjusting the neut cap, or the neut cap arcs over, you won't pop the 6146 so 
easily.

Read the Handbook chapter on HF transmitters, particularly the section on 
neutralization.

73 de Jim, N2EY