[AMRadio] Globe Champion

[Brett Gazdzinski]

Wayne,
I am not familiar with the globe chump rigs, but
noticed on a globe king 500 they had part of the
output network under the chassis mixed in with the
grid and driver sections...

Bill Orr handbooks are very useful in reducing all sorts
of problems. The addition of parasitic suppressors
(50 ohm 2 watt resistor with 5 turns of enameled wire around it)
in all the grid and plate sections of the output and driver
stages may help.

Neutralization can also be tricky, as you can neutralize a stage
but have it out of balance.
I discovered this with the home brew 812 rig.
You can have one neutralizing cap screwed in much more than 
the other and have it indicate a good neutralization.
This is sure to cause trouble, and I found I could match the caps
and fine tune each slightly to get the rf deck neutralized.

If things are shielded, I don't think you need to pay
much attention to the early stages, but its very important
to isolate the inputs and outputs from each other, and pay
attention to the bypassing of the screens and filaments.
If these stages are not well shielded, maybe some shielding 
can be added, and/or you can run some stuff in small coax.

It would be useful to find out what stage is oscillating.
An O scope might help, or even meter readings if the driver
stage is metered.
I would suspect its the driver stage causing the problem,
I don't think your typical push pull power stage is prone 
to problems, as they normally take quite some drive to get things
going with triodes....

I have a friend with a GK400 and he seems to have no problems
with oscillations.
 
On my home brew rig, there is no shielding between the neutralizing
caps, the grid coil, the plates of the tubes, etc.
In all the push pull rigs I have looked at, the layout was similar,
no shielding between the grid coil and the output stages,
but there was shielding between the power stage and the driver.
Your typical push pull deck in the Bill Orr books has
the tubes, coils, caps, grid coil, neutralizing caps, all
on top of the deck with no shielding at all, and no problems.


I think a 6146 would substitute a 2e26 very well.
My rig takes about 20 watts of drive for 811 or 812 tubes.
I could use less and reduce the biasing, but figure on
something around 20 watts.
A 2e26 might do this fine, not sure, but the plate and screen
voltage would have to be up there.

Perhaps the lack of drive is a symptom of a problem that also
shows up as the instability.

I sure would look at that driver stage carefully....

Brett
N2DTS 
 
> 
> Re the GK275
> 
> I also have a GK275 which I found about 15 years ago and just 
> renovated it
> about two years ago, Actually it is kind of a hybrid or mixed 
> up GK. It has
> a 275 rf deck and an early 500 mod deck.  When I found it, it 
> was in the
> middle of a modification with a 5 prong socket where the 2E26 
> goes and an
> 807 in it's place and a bunch of wires hanging loose.  After 
> determining it
> was a 275, I put it all back orig. as a 275.
> 
> Here are my problems. I don't think the 2E26 has enough power 
>  although it
> works ok on 75.  I see why the upgrade to an 807,really no 
> problem here.
> The real problem is instability.  After neutralizing , all it 
> takes is a
> bump on the table , lift the lid, close the lid, BREATH on 
> it, and it takes
> off like a shot. I have neutralized other P-P amps and have 
> never had this
> much trouble.  I notice that this early GK275 has a skimpy 
> grid coil shield
> (does not wrap around the coil).  There is a line of sight 
> between the neut.
> caps and the coil.  I don't see this skimpy shield on other 
> V70-D GKs. I
> think I'll experiment with a bigger shield when  get some 
> other projects out
> of the way.
> 
> Anyone have any ideas on this.  Brian's post kind of hit home on this.
> 
> 73
> Wayne, N0TE
> 
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