Then you didn't get my question, I obviously worded it badly.
What to do when I have RF, I have what I think it the right bias volts going
to the cathodes and yet I have NO idle current.
From: Carl [mailto:email@example.com]
Sent: 25 February 2012 09:43
To: Martin Sole; firstname.lastname@example.org
Subject: Re: [Amps] FW: AL572B, a little more help please.
Its explained in the manual, the amp uses bias switching controlled by RF.
No signal....no power wasting idle current.
----- Original Message -----
From: "Martin Sole" <email@example.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
> 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
> to simulate it being turned on. This gives me about 9 volts on the
> of Q103 / bias line to the centre tap of the filament transformer. Amp
> current is about 5mA! Shorting the zener D103 drops the collector of Q103
> 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
> 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
> 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.
> Martin, HS0ZED
> Amps mailing list
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