[Amps] bias for 3cx3000

[Barry Kirkwood]

The original question, as I read it, was about a technique to set the bias
for a power tube.
A cheap and easy way to do this is to put a power transistor in series with
the cathode, and vary the Vcb with a floating power supply of modest
capability.
This is less crude than wiring in diodes one at a time, or running up and
down a long string of them.
One can create a power zener by using zeners to control Vcb and trimming
with low power diodes to set within 0.7 V, it is also easy to vary operating
points by shorting diodes.
In my part of the world power zeners are not easily found, but power
transistors can be had for free from junked gear, or cheap from shops.
If 200A is encountered then the transistor will provide a fast fuse.
Have used this configuration in a modest 2x3-500 amp for around 10 years now
with no problems.
73
Barry

end
Barry Kirkwood PhD ZL1DD
Signal Hill Homestay
66 Cory Road
Palm Beach
Waiheke Island 1240
NEW ZEALAND
ph/fax 64-9-372-5161
www.waiheke.co.nz/signal.htm
----- Original Message ----- 
From: "R.Measures" <r at somis.org>
To: <dezrat1242 at ispwest.com>
Cc: <amps at contesting.com>
Sent: Monday, October 04, 2004 3:21 PM
Subject: Re: [Amps] bias for 3cx3000


>
> On Oct 4, 2004, at 7:33 AM, Bill Turner wrote:
>
> > On Mon, 4 Oct 2004 01:58:50 -0000, Barry Kirkwood wrote:
> >
> >> Beats me why people buy high watts zeners to set the bias of
> >> amplifiers when
> >> there is a better and cheaper way.
> >
> > _________________________________________________________
> >
> > High power zeners are nearly bulletproof when it comes to brief
> > overloads, power surges, spikes, arcs, etc, etc.  Your low power
> > transistor-and-zener approach seems a bit risky by comparison.  And if
> > you use a really high power, high voltage transistor, what have you
> > saved?  Power zeners are pretty cheap anymore.
> >
> -  A string of 3A Si rectifiers is cheap and they are adjustable in
> 0.7v steps by moving the tap.  However, a power zener is not
> adjustable.  A 3A rectifier will withstand 200a peaks.  Can a power
> zener handle 200A?
>
> > KISS is Murphy's worst enemy.  :-)
> >
>    Those who ignore Murphy are their own worst enemies.  Cheerz, Bill.
>
>
> Richard L. Measures, AG6K, 805.386.3734.  www.somis.org
>
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