>
>Hi Peter,
>
>> Rich says;
>>
>> >However, with RF-derived bias switching, the tubes rapidly switch in and
>> out >of linear bias during normal speech when the relays are in the
>> transmit >position
>>
>> Do I understand from that comment that the majority of these bias schemes
>> actually try to switch to follow the RF envelope, rather than 'hang' for a
>> few tens to hundreds of milliseconds after the RF disappears? If that's
>> the case, I can understand why they are so bad for splatter. Given even a
>> hundred milliseconds hang time, it should be OK for splatter (provided the
>> attack is fast enough), but without.......................Rich is too kind
>> in his comments.
>
>Rich apparently does not exactly understand how auto-bias works
>in many cases.
>
>In the AL-80B and AL-800 series, auto-bias turns on hard with
>drive power as low as a few dozen milliwatts.
>
>But even when the auto bias does not turn on, the tube still has a
>few mA of quiescent current, and so remains in class AB.
>
>The circuit also has hang time.
>
>Rich apparently bases his prejudice on old data, from the early
>ETO amps and articles by Pittinger and others where the tube was
>forced to high levels of cutoff bias by a transformer operated bias
>supply. In that design, the auto-bias had to pull the tube out of
>cutoff, requiring extra time to discharge capacitors and restore
>conduction. Those system did not use a fast attack slow decay
>switch in the bias system, because it was necessary to move the
>bias voltage so far and the tube was forced hard into cutoff, instead
>of resting at a low dissipation but still linear state..
>
>The AL-800 is typically over -40 dB IMD in a three tone test, with
>no detectable change in IMD performance as the auto-bias is
>disabled and replaced with conventional bias.
? But of course!. . . A three-tone test does not simulate the
problem of using RF-actuated bias-switching with humanoid speech. .
Curiously, G2DAF-enthusiasts (which Mr. Rauch quite rightly ridicules)
use multi-tone tests to "prove" that the G2DAF circuit really and truly
does not have any IMD problem whatsoever.
>
>This reminds me a little bit of the ferrite core conclusion. The world
>according to Garp.
>
? The ferrite manufacturer rates u=125 core material for 10MHz. In my
application, there was a problem at 14MHz. The fix was to switch to a
ferrite core material rated at 50MHz.
- cheers, Tom.
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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