Self Osc Problem
 

Hi Jack,

Grid stoppers: A low value resistor (100 ohms) attached directly
between G2 and the screen supply to reduce the incidental Q of
circuit. Same applies to G1.

APC - Anti Parasitic Choke. A few turns (10 ?) of 22AWG enameled
copper wire around the out side of and terminated with a 10 ohm 1W
resistor. If the AMP becomes unstable at VHF, the reactance of the
small inductor will increase and the 10 ohm resistor will dampen the
mayhem.

See http://www.somis.org/Oct88qst.html

From your photo, I think you may need to considerably reduce the
length of your wiring before you should tackle stability issues. Anode
circuits should be above chassis, grid below chassis and never the
twain should intertwine. It looks like you make have lots of inductive
coupling between the two at the moment (long wires etc.) which may
well be the cause of your problems.

Also take care of proper de-coupling - directly to chassis with
minimum lead lengths (1 inch max ?).

Get a 1960s copy of the ARRL handbook and read up on HF amplifier
construction techniques. There has been plenty written.

Regards,

Mark.


On May 24, 1:23 am, Jack Schmidling wrote:
MarkAren wrote:
Hi Jack,

Have you tried removing drive to the PA and looked for output RF ?

Roger. Just pegs the meter with no output.

Grid stoppers in place ?

What's a grid stopper?

APC in the PA anode ?

What's APC?



Any chance of a photo to look at construction techniques ?

Sure. http://schmidling.com/807a.jpg

Needs a little cleaning up. Note the careful placement of the grid coil
at right angles to the final and with the Masonite panel between for
good shielding.

js

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Self Osc Problem
 

MarkAren wrote:

Grid stoppers: A low value resistor (100 ohms) attached directly
between G2 and the screen supply to reduce the incidental Q of
circuit.

OK but the supply is a 12K resistor from the plate supply. Guess I
don't see what a 100 in series with this would do.

Same applies to G1.

Where does that go? There is a 47K grid leak going through the meter to
ground and a coupling cap from the driver.

So what is this grid stopper supposed to do?

APC - Anti Parasitic Choke.

Roger. The A caused a brain cramp. I have one in there.


Get a 1960s copy of the ARRL handbook and read up on HF amplifier
construction techniques. There has been plenty written.

I have 49, 57, 61 and 73 one of which I take to bed with me every night.

js

--
PHOTO OF THE WEEK: http://schmidling.com/pow.htm
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Self Osc Problem
 

Hi Jack,

See if you can find a project called "A 75 to 120 Watt CW transmitter"
in any of your ARRL manuals, (1973 ?).

Z1 is the APC, RFC4 is the anode RFC, there is a 10R grid stopper on
G1 (pin 5), Decoupling abounds (including the heaters), band switch
shorts unused turns of the output inductor, top chassis shows anode
wiring only, bottom chassis shows grid and drive electronics.

I haven't read the whole article yet, but it seems like a well
designed beastie and a good starting point.

Keep a change of fresh underwear next to the rubber gloves :)

Regards,

Mark


On May 24, 12:26 pm, Jack Schmidling wrote:
MarkAren wrote:
Grid stoppers: A low value resistor (100 ohms) attached directly
between G2 and the screen supply to reduce the incidental Q of
circuit.

OK but the supply is a 12K resistor from the plate supply. Guess I
don't see what a 100 in series with this would do.

Same applies to G1.

Where does that go? There is a 47K grid leak going through the meter to
ground and a coupling cap from the driver.

So what is this grid stopper supposed to do?

APC - Anti Parasitic Choke.

Roger. The A caused a brain cramp. I have one in there.

Get a 1960s copy of the ARRL handbook and read up on HF amplifier
construction techniques. There has been plenty written.

I have 49, 57, 61 and 73 one of which I take to bed with me every night.

js

--
PHOTO OF THE WEEK:http://schmidling.com/pow.htm
Astronomy, Beer, Cheese, Fiber,Gems, Sausage,Silverhttp://schmidling.com