Hi Andrew,

Yes, the example I quoted was for a 6146, but same construction and
circuit elements apply.

I just wanted to give concrete examples of the elements I was talking
about.

-M

On May 25, 1:17 am, Andrew VK3BFA wrote:
On May 23, 11:23 pm, 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.

ERK! - looks like a 6146 PA, rather than an 807. And, congrats at the
attempt, (LOVE the agricultural look - wheres the PTO for the chaff
cutter?) BUT theres too much loose wiring providing stray coupling
paths - AND you do need to use a metal chassis, even if only to
separate the input/output circuitry. A piece of light gauge aluminium
sandwiched(sp?) on top of your present chassis would do it.....easy to
work with simple tools, no bends.....or do what the old timers did,
go to the supermarket, buy a sheetmetal baking tray from the Homewares
section)

BTW - while your there, buy one of those cheap white plastic kitchen
cutting boards - easy to work, a lovely source of insulating material
for standoffs, insulators etc...

And whats with the 3 big wire wound resistors? - screen supply to the
PA - what a potential source of stray coupling. A 1 watt resistor is
usually sufficient, you wont need the string of 10 watt ones....
(especially sitting under the lead from the anode of your PA stage..)
And the anode tuning is about 100 yards from the tube! - can you get
it any closer?

So saying all that, general rule of RF stability is to bypass
EVERYTHING, including any existing bypass circuitry. (This is why a
metal chassis is so good...no long leads on bypass caps)In this HF
cct, a handful of 0.1 or even 0.01 caps would be a good try....wack
em everywhere. Go buy heaps, they will be most useful for other stuff
as well. 630V polyester/polystyrene are easily and commonly and
cheaply available...

And keep going - its worth the effort -

Andrew VK3BFA.- Hide quoted text -

- Show quoted text -