Vincent Fedele wrote:
>
> Re: [AMPS] Alpha 77DX Bias Switching
> The crackling or popping sound between syllables of speech is heard when
> the tube is switched from partial cutoff at 20V abruptly to 8.2V full-on
> as RF is detected from the exciter.
At 20 volts of bias, you would never hear the distortion. That bias
level actually places a typical 8877 a few milliamperes into conduction.
The tube is class AB, but sitting right on the edge of class B.
Something else is going on.
A tube has to be far into class C, or into severe flat-topping to
produce audible distortion. Long before you get it to the level of audio
distortion, any people working weak signals up and down from you would
form an angry lynch mob.
> the EBS circuit. Allowing the tube to switch into nonlinear bias at any
> time when the RF relays are in the transmit position is not good
> engineering practice.
Amen to that, going into cut-off provides no advantage. At the edge of
class B, dissipation is so low any further reduction in quiescent
current is a wasted effort.
73 Tom
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