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.
This reminds me a little bit of the ferrite core conclusion. The world
according to Garp.
73, Tom W8JI
w8ji@contesting.com
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