Self Oscillation Problem

[W9GT]

Here is a picture  http://schmidling.com/807a.jpg

It needs a little cleaning up but the good news is that all the trash I was hearing was created by the S38 I use on the bench to listen. 

Note the Masonite panel is between the osc coil and the final so it is pretty well isolated.


js

p.s. I abhore smileys so work it out.

Hi Jack,

Looks like a 6146 in the picture.  I thought you were using an 807.  The 6146 is a little different and doesn't exhibit the same characteristics with regard the the need for shielding around the base as the 807.  It has a metal base and shorter leads connecting to the tube elements.  The 6146 can also be squirrely though, as it is a high gain tube.  It is a good idea to use short leads where possible and keep the input and output circuits away from each other.  Keeping the input below the chassis and the output above the chassis provides good shielding and isolation.

I understand that you might be just "prototyping" and testing out your circuit on a piece of wood with a masonite panel, but a metal chassis is definitely the way to go for your permanent configuration!  Masonite offers no electrical isolation whatsoever, and a metal chassis-common ground/shield will provide a much more stable platform.

73,  Jack, W9GT

[K9ACT]

Masonite offers no electrical isolation whatsoever, and a metal chassis-common ground/shield will provide a much more stable platform.

Guess I need to reconsider my war against smileys.  That picture was intended as a joke and the Masonite "shield" was to make sure everyone knew it was a joke.

It was my tinkering prototype to develop a layout.  The 807 turned into an 866 and I substituted the 6146 to keep going until I could get a new 807.  It was originally xtal controlled and then I got the urge to add the VFO which is the junk on the left.

A current pic is as http://schmidling.com/807b.jpg

Unfortunately, it looks a lot better than it works.

In case anyone is wondering why the 6CL6, it's an emotional thing.  When I got home from a hamfest with my 6AG7, I was appalled to find a metal tube in the box and to learn that is the only way they were made.  I am thinking of going to a 6L6.

Please also note the 1/2 shield for 807 per your suggestion.

I think I am close enough to solving the original problem or at least know where to concentrate to move on to another subject.... the VFO.  This is afield enough to start a new thread.

Again, thanks to all for the ideas.

js

[K1MVP]

Jack,
Your metal version looks mucho better than you plywood version, --But there are a couple of things
I question,--Is that that a grid current meter near the plate current meter?
If so I think, I would move that grid meter under the chassis to prevent any coupling between the
grid and plate current meter.
Also, is that variable cap in front of the oscillator, for grid tuning?--I would also move that under
tht chassis to minimize osc-final coupling.
Just a couple of thoughts, that might help.
                                                  73, K1MVP
 

[K9ACT]

I question,--Is that that a grid current meter near the plate current meter?
If so I think, I would move that grid meter under the chassis to prevent any coupling between the grid and plate current meter.

Never occurred to me to put it under.  In fact, it never occurred to me that there would be a coupling problem with meters with RF chokes and bypassing .

>Also, is that variable cap in front of the oscillator, for grid tuning?--I would also move that under tht chassis to minimize osc-final coupling.

You're gonna love this... it's the VFO tuning. The grid tuning is underneath.  Both the VFO and grid coils are toroids so the coupling would be minimum between them.  The only thing on top is the cap and it's grounded and the tube is metal.

I suppose the whole VFO should be in a box but this is sort of my Mr Wizzard Science project and it's informative to see the results of not following the rules and fixing things as needed.

The final self-oscillation seems to have been fixed but now I have a self oscillation in the VFO.  That seems like an oxymoron but it's a very unusual one.  It is a ticking at about a 500 ms rate when I unkey. I have the oscillator on all the time and key the grid bias for a nice clean, chirpless key but the ticking between is nutty.  I fixed it with a 9V battery as a fixed bias supply.  Don't ask me how this solved anything but it did.  Now I need to find out what the real cause it and fix it right.

I also added a fixed bias supply for the 807 to cut it off when the key is up.  Works real nice.  Used a backward fil trans on an unused fil winding on the power trans.  Same trick I used on the 8000.

Lot's more problems to resolve yet but it wouldn't much fun if it worked the first time I plugged it in.  Things that do that are called rice boxes and are boring as hell.

>Just a couple of thoughts, that might help.
                                                 
Thanks.  Never too many thoughts,

js