Power scaling connection

[Ramsay]

I am bulding a power scaling and I need help of the experts of the forum:
In what point of the circuit (linked below) must to be interrupted to connect the vbias raw and the vbias out?
It would be at point "E" ?
Or it would be at the cathode of D12?

http://i223.photobucket.com/albums/d...100esquema.jpg

[Steve Conner]

Depends on what power scaling circuit you're using. Please post a schematic.

For instance:
Does it regulate the bias voltage? (It probably does in some way, as it has to make it track the screen voltage)
Does it have its own bias trimpot, or do you need to use the existing one in the amp?

If the circuit regulates, and you have to use the existing bias pot, then it has to be inserted before the bias pot, otherwise the regulation will stop the pot from working.

If it has its own bias pot, then you should basically replace the bias pot with the circuit. Having said that, it must go after the filter capacitor to ensure it gets smooth DC.

[Ramsay]

Thanks for your reply.
In fact the power scaling has its own Vbias pot that is a 4.7K trimmer.

The filter capacitor you said is C45 (.047/630V) right?

For the +B voltage reducer I will use a power Mosfet 2SK1466 (heatsinked) and a IRFP 460, are they enough for this amp?

[Ramsay]

Can anyone help me please !

[Ramsay]

Ok, if no one It has condictions to help me (Steve Conner is a exception, because he is the guy) I will go by my self.

I will keep the bias pot of the amp, put it to the clockwise direction to full negative voltage and the raw bias will be conected after the C44 filter capacitor.

But I have a doubt:
Will be sufficient the bias current suplied to feed three small transistors of the bias control of the power scaling?

[Steve Conner]

That seems like a reasonable way to do it. Yes indeed, C44 is the bias filter cap. You break the connection between C44 and point "E", C44 goes to the bias input of your circuit, and the bias output goes to E.

If the bias supply can't deliver enough current, the bias voltage will be too low and your power scaling won't work. The tubes will red plate at high power settings. If you need more current:

Remove R53. (Some current is wasted in P11 and R53)

If that doesn't do it, make C45 bigger. But check that the bias voltage doesn't rise dangerously high at any power scaling setting. If it does, you might have to put some Zener diodes across C44.

[Ramsay]

Thanks a lot, Steve,
I will follow your suggestions.
Greetings.