"Sander deWaal" wrote in message

"Arny Krueger" said:

OTOH, tubed amps are more prone to EMI problems due to unique
tube-only problems like grid rectification, Miller capacitance and
their high input impedances. I hard a lot more RFI problems during
the days of tubes than I do now.

Grid rectification: the voltage span of an average power amp input
tube is about 2 volts.

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.

You're not thinking of auto-grid bias, are you?

No.

A method that isn't used anymore in "modern" tube amps, at least not
that I know of.

Agreed.

This might also explain why you think tube inputs have such extremely
high input resistors, 10 megohm wasn't unusual is such an arrangement.

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.

The voltage span of an average base-emitter junction is what...20
mV?

Irrelevant because transistor inputs stages are always far lower
than with tubes and almost always run with emitter degeneration.

Partly agreed, see above for the same rule that applies to cathode
resistors.

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.

Miller capacitance: dependent on circuitry.

Also dependent on the tubes themselves.

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.

*Every* amplifying device with inverting output suffers from mr.
Miller.

It's all in the quantification, and the quantities are vastly
differnt for tubes and SS.

Also agreed, but certainly not just "due to unique tube problems like
Miller capacitance [.....] ".

Yawn.

Examples of said circuits would be CCC, CEC, CSC.

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.

A tube amp with a 10 k resistor at the grid to ground won't be more
susceptible (sp?) to RFI than an average bipolar input.

Perhaps, but irrelevant due to the common practice of using far
higher grid resistors than 10K.

They're 25 kohms in my tube amps.

That's a choice you get to make. But, it says nothing about tubed amps in
general.

Not that I'm alone in this, you will note.

Not that you set the rule or the average, either.

Let alone FETs or MOSFETs.

Oh my goodness, it's a red herring. Actually two of them.

Actually, no.

Yawn.

One of the main *advantages* of (MOS)FETs is their voltage-driven
nature.

What was the first transistor again in your FM-frontend, Arny? ;-)

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.

If the same technique is used with a triode- or even penthode input
circuits (NOT hexodes or pentagrid tubes!), cross-modulation is almost
not an item to worry about.

See previous comment.

Noise may be, though.

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.

BTW, SS amplifiers are also inherently low pass filters.

In general, SS circuitry is capable of handling higher frequencies
than tube equipment, consider an average output transformer with a
corner freq. at 50 kHz or so.

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.


Of course, a good practice is to add an anti-slewing filter at the
input of the amp (even with tubes this might be beneficial!), but
some designers select a corner frequency of 150 or 200 kHz.

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.

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?

hence even if Shakti stones work
as described on their website, they won't do very much for tubed
systems IMO (from a technical POV that is!).

From a Sander technical view, that is. ;-)

Gratuitous personal insult ignored.

Inability to gracefully perceive a gentle stab due to a
technological disagreement noted.

Please stop being so hostile! ;-)

Yawn.