Thank you Andy :-)

now by the first I have to find a way to fix the coil (which is quite 
bulky..) I used 2 corks :-)) and hotmelt glue, but after 1 hour of 
testing the coil falled :-(((

looking on the notes, the first tank coil had 92T and the link had 25T 
with this combination the voltage across the link coupling was about 
Vpp = Vdc but in this conditions, this afternoon when testing at 50Vdc 
without the guard circuit I had Idd 0,87A and Wout 29,5 connecting the 
guard Idd dropped to 0,41A and Wout to 13,2W while Ig=0,16A. On the 
other hand with 25 ohm load, Idd was 0,31A and Wout 8,4W with Ig 0,26A.

So let me borrow another cork :-)))

Thank you again for support

CU Marco IK1HSS
----Messaggio originale----
Da: [email protected]
Data: 4-giu-2017 18.42
A: <[email protected]>
Ogg: Re: Re: R: Re: LF: Re: I: Fw: For today the FETs survived...

Yes.
You need to adjust the link so current is driven back into the supply 
when
RF amps exceed the maximum that you have designed for.   Up to that 
point
there should be no guard current as the induced voltage in the link, 
when
rectified , is less than the supply.

If the link coupling were to be perfect, ie it behaved as a 
transformer,
then at full power into your designed load the voltage across the tank 
coil
would be Q * Vdc * 4/pi  * SQRT(2)    (since you calculated Rload this
way).    The SQRT terms comes from the DC being the peak

So the turns ratio (for perfect coupling) if made equal equal to 1 : Q 
* 4
/ pi * SQRT(2) ,will start to generate load current just as maximum 
output
is reached.

But things aren't perfect; the mutual coupling is never 1 in an air 
wound
coil (mine is about 0.6), the rectification doesn't   give exactly peak
volts, so you have to experiment, changing the link turns until you get 
the
limiting you want.   Fortunately it scales with DC volts, so guard 
circuit
testing can be done at low supply / power out.

'jnt

On 4 June 2017 at 16:47, [email protected] <[email protected]> 
wrote:

> heyheyhey..
> now I'm testing with the new coil still at 50Vdc ;-)
> ops I was forgetting: without dissipator :-)) just with the mounting
> alu block it runs at room temp, maybe it is worth later at full 
power!
> The
> picture of Andy is not in thema: is the consequence of qro ampli on
> undersize external loading coil :-)
> By the way when this ampli will finally run on the antenna I need to 
re-
> size the variometer because it warms already with 250W..
>
> Also the new tank coil with 2,5mm² wire after 1 hour has a bit of
> fever.. but is not warm as the former.
>
> Just a quick question Andy: speaking of the guard circuit, if I read
> Iguard whithout mismatch is correct the assumption that the link has
> too many turns?
>
> Alan: I'm vintage.. and I'm sure I have a variac in the basement :-)
> thanks for the hint ;-)
>
> Thanks all for the psycological and praxi help !!
> Marco
>
> ----Messaggio originale----
> Da: [email protected]
> Data: 4-giu-2017 17.49
> A: <[email protected]>
> Ogg: Re: R: Re: LF: Re: I: Fw: For today the FETs survived...
>
> Something like this ...
> http://www.g4jnt.com/QRO_LF_DoesThis.JPG> 'jnt :-)
>
>
> On 4 June 2017 at 16:25, Chris Wilson <dead.fets@gmail.
com> wrote:
>
> > Hello Alan,
> >
> >
> > No, what he needs is a video camera, and the courage 
to post the
> > result here for scrutiny, we need a bit of fun now and again ;)
> >
> > Good luck Marco, what's the worst that can happen.... :)
> >
> >
>
>
>
>
>