[AMPS] Alpha 77DX Bias Switching

[Tom Rauch (W8JI)]

marv h. gonsior wrote:
> 
> Hello ampers,
> 
> Has anyone had any experience with the problem of excessive Electronic
> Bias Switching speed in the 77 amplifiers, causing distortion and the
> recognizable "crackling" sound that results?

> Are there any mods for the apparent problem?

Hi Marv,

One major problem with that circuit, and circuits that copied the ETO
design (published in QST and other publications)is that the pull the
bias up to a high voltage.

That does absolutely nothing for dissipation, and increases the recovery
time.

What is really necessary is to allow the tube to "seek it's own bias
level" without forcing the bias higher than necessary. The tube should
be set right at the point of class B, but slightly into conduction.

Linearity remains good during the near class B operating period, and the
recovery towards class A is accelerated because the bias voltage has
much less overall voltage swing.

Since the dissipation difference between even ten mA at 3500 volts, and
zero mA at 3500 volts is only 35 watts, I could NEVER understand why
anyone would want to cut the tube completely off.

I'm surte if you eliminated the silly 100 volts plus of bias and let the
tube just float up to NEAR cutoff by self bias the leading edge
distrotion problem would disappear.

73 Tom


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