On Tue, 15 Jun 1999 11:29:10 +0100 Peter Chadwick
<Peter_Chadwick@mitel.com> writes:
>Shouldn't the bias be set for correct zero signal plate current? Then
limit
>the drive. There's also a maximum for grid circuit resistance.
>
>I was thinking of a 'last resort' ALC output for my 4CX1000 provided
when
>the grid voltage gets more positive than -5 or -6 volts. I'm not a great
>lover of ALC for preventing overload - it's a 'shutting the stable door
>after the horse has bolted' approach to my mind - but as a last resort,
>followed by a grid current trip (or maybe a PIN diode attenuator) when
the
>grid current starts (4CX1000s are rated at zero watts grid dissipation),
it
>may be useful. Comments?
>
Hi Peter,
I think I sent you the overall description of my 4CX1500B rediculotron
some time back.
But just to elaborate a bit on that design, I have a floating grid
supply (sourced from a switching supply... one of the old noisy ones!)
which is connected to two grid current sense circuits.
There is a 1K resistor connected across an opto-isolator which is for
fault detection. This is in the negative side of the supply connecting
to the tube.
The positive side of the supply is returned to "ground" at the summing
node of an op-amp (which is protected by a couple of back to back diodes
connected to real ground!). A 1K feedback resistor around this op-amp
creates a -1V per mA grid current signal.
This signal is compared with a fixed reference and creates an ALC
output at a preset level. I have a switch to select between operating
the amp with a 4CX1500B or a 4CX1000A. This selects .1 mA ALC threshold
for the 4CX1000A or 1 mA for the 4CX1500B.
I try to operate so as to NEVER see ALC from the amplifier (or in the
exciter!) as I use an RF clipper type speech processor and any ALC action
greatly reduces the effectiveness of this means of audio processing.
73,
Marv WC6W
*
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