>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?
>
I am biased toward simple, minimal-cost, glitch-resistant circuits that
work. The [Figure 7] optoisolator/10A, Si-npn switch/3A diode-string
will reliably handle a 3cx3000A7 and it switches in under 7uS.
>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.
>
>
>--
cheerz, Ian
- R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
www.vcnet.com/measures.
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