2 wrote:
>Cuting off the tubes intermittently during speech with RF-actuated bias
>results in switching rapidly in and out of linear-bias during
>soft-syllables. For this reason, Alpha 77s are well-known for their
>harsh-sounding audio and spit-spit sound on adjacent channels +/- c.
>3kHz. [note -- this sort of thing does not appear with a 2-tone test].
>The fix is to sync the bias with the coil-current in the RF-relays.
>
>>
>>
>>
>>> The problem with a 50w zener is that its Vz is not adjustable. A
>>> fwd-biased Si diode string can be adjusted by changing taps.
>>>
>>> //'Tis true. However, I don't know why I'd need it to be adjustable. I
>>> suppose for an amp having two HV taps (CW and SSB) this might be a nice
>>> feature, however my amps don't. Using signal-activated electronic bias to
>>> cut off the tube(s) between words, etc, is a cool feature that would
>>> require more than tap changing.
>>>
>>> -WB2WIK/6
Paul and I favor the TL431-TIP147 bias circuit because we supply our
different control boards for a large range of tubes, so we need a 'one
circuit fits all' solution. Maybe that makes us biased?
However, if you're also going to use EBS, the TL431-TIP147 circuit has
everything you need - the power transistor and the switching facility
too. To bias off the tube, simply switch the control gate of the TL431
to its anode (collector of TIP147). Switching time is a few
microseconds.
--
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
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