"Arny Krueger" said:
A serious problem because of the high impedances involved and the
fact that tubes are frequently run without local feedback.
A cathode resistor isn't local feedback?
Not if its bypasssed.
Ah. Another design tirck from me.
I rarely bypass cathode resistors.
My, I must be an exceptional tube amp designer ;-)
470K still is common. that's about 50 times the last number you mentioned,
Sander. Hunt around, sooner or later you'll find a believable number.
In older tube amps, such high input resistors were indeed common.
Modern tube amps, as far as i've seen them, use lower values from
10k....100 k.
Look up "Ebers-Moll" for a better understanding about driving bipolar
transistors.
Here we go, the big tube eggshpert is now talking down his nose at me.
Please find a quote by me where I pretend to be an expert (or
eggsphert) in *anything* , except tinkering.
Furthermore, the Ebers-Moll approach starts with the premise that
bipolars are voltage-driven.
But you knew this, right? ;-)
Also agreed, but certainly not just "due to unique tube problems like
Miller capacitance [.....] ".
Fess up Sander, Miller capacitance is a far more significant issue with
tubes.
I replied to your statement that those problems are "unique with
tubes, like Miller capacity etc".
Fess up Arnold, Miller capacitance is apparent in *every*
signal-inverting amplifying device.
High input impedance: dependent on circuitry.
Of course, but as a matter of course tubed circuts have far higher
input impedances.
Which is compensated for by the much karger grid voltage span.
Nope.
Yep.
Yet another red herring. BTW, the cross-modulation characteristics
of SS FM front ends are generally better than what we had when tubes
were king.
Only due to clamping diodes.
Nope. Cross modulation is about nonlinear distortion, and clamping diodes
increase that, not decrease it.
Remind me again when MOSFETs without internal clamping diodes are
available to designers.
Non-linear distortion, again due to the smaller BE voltage span as
compared to the larger GC span of a triode or penthode.
Granted that most tube amps have higher value input resistors.
Thanks for agreeing that your argument based on unrealistically low
grid resistors was irrelevant, Sander.
Not unrealistic at all considering my design views, but then again,
*I* am able to put off my blinders ;-)
What blinders? The vacuum tube horse has been dead in the mainstream of hi
fi for about 30 years. Putting blinders on a dead horse makes very little
sense.
Thanks anyway for discussing obsolete technology with me Arnold. ;-)
Actually 50 KHz or so is not an uncommon corner frequency for SS
amps.
Due to what property, exactly?
Desire to have an amp that is nice to work with.
I meant what causes the LPF character of an average SS amp?
In what stages is that most apparent?
Slewing was a problem in someone's mind by the time it was
publicized.
Since Otala, Garde et al, we know better ;-)
Yes, Otala is known to have been an alarmist, to say the least.
Otala was an optimist ;-)
Besides, that is mostly a simple RC 6 dB.oct filter, not steep
enough to supress real strong RF energy.
A 6 dB/oct filter cornering at 50 KHz is down over 20 dB at 500 KHz,
40 dB at 5 MHz and 60 dB down at the 50 MHz that Mike origionally
mentioned.
One wrong grounding point may wreak havoc with that 60 dB number.
How many red herrings does this make?
What was the minimum requirement for measuring S/N again?
When I once mentioned I couldn't measure better than -80 dB at one
time, you had a good laugh.
Think about a bipolar differential input, CMRR, and the actual signal
voltage needed to excurse it.
You may use Google, if you like ;-)
--
Sander de Waal
" SOA of a KT88? Sufficient. "
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