Its explained in the manual, the amp uses bias switching controlled by RF.
No signal....no power wasting idle current.
Carl
KM1H
----- Original Message -----
From: "Martin Sole" <hs0zed@csloxinfo.com>
To: <amps@contesting.com>
Sent: Friday, February 24, 2012 9:23 PM
Subject: [Amps] FW: AL572B, a little more help please.
>I asked this elsewhere but got no feedback. Maybe my questions are unclear,
> or just plain too dumb! This AL572B had 2 bad tubes, a blown grid
> resistor,
> and a failed 3055 bias transistor.
>
>
>
> I figured out from the rather badly drawn schematic how the automatic bias
> switching is supposed to work, but it's still giving me a lot of pain.
>
> Rather than rely on RF to operate the bias I forced the bias circuit into
> operation for testing by grounding the collector of Q101 in the bias
> circuit
> to simulate it being turned on. This gives me about 9 volts on the
> collector
> of Q103 / bias line to the centre tap of the filament transformer. Amp
> idle
> current is about 5mA! Shorting the zener D103 drops the collector of Q103
> to
> about 0.7v as it gets turned on more and the bias current rises to, what I
> think seems a more reasonable 125mA. Pulling the bias line to ground
> entirely just causes the grid current meter to go hard negative!
>
>
>
> With RF applied I see values of plate current that translate to what I am
> seeing for power out, when factoring the plate voltage and 60% efficiency
> so
> I doubt the meter is wrong. So why the very low 5mA with the stock bias
> circuit switched on?
>
>
>
> Something else bothering me, When the T/R/bias relay is open the bias line
> coming from the centre tap of the filament transformer has about 23 volts
> on
> it. This goes to 9v (or0.7v) as above when the relay is energised. Where
> does the 23 volts come from?
>
>
>
> What is the function of D117 and C140? (C140 is a tantalum polarised
> capacitor though the parts list might suggest otherwise.)
>
>
>
> Okay, enlighten me.
>
>
>
> Thanks
>
> Martin, HS0ZED
>
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