Ok, here's a little education: High impedance doesn't matter much; as a fact, bipolar gain stages can be bootstrapped, input currents can be cancelled, etc... to provide a pretty high input impedance as well. But any ingress EMI that keeps the DUT within linear region can safely be ignored anyway, since it is way outside the small signal bandwidth of the amplifier. Because we are talking about small signals, whatever gets in past the input filter will not create intermodulation products.
What matters is the large signal contribution; tubes, like jfet, have a parabolic dependence of the output current on the input voltage. Bipolars have the same dependency exponential, which makes them a much more effective rectifiers (or demodulators, if you prefer). The demodulated signal from e.g. radio stations is in the audio band, hence it may appear at the amplifier output, in the worse case as intermodulation products with the source signal.
The second effect, also large signal, is driving the input stage into nonlinearities; since bipolars have much larger transconductance (read: they amplify the ingress crap much better than tubes/jfets), it is easier to drive them into nonlinearities. Add to this the fact that tubes have usually much larger overload margins compared to bipolars and you'll get the picture why tubes are more immune to ingress EMI.
When designing, you can collapse the large signal effects into one issue, and simply provide a bandwidth limiting input passive filter. This input filter will remove any components above an arbitrary frequency, I would think that anything over 75-100KHz can be considered junk, and even a first order filter will have enough attenuation in the closes AM radio band to avoid driving the input stage into nonlinearities, even if you live on top of the transmitter antenna pole.
You are welcomed.
RNM,
That was mean spirited, it would have been nicer if you wished him success in his future ventures.
Scott,
I actually picked up some military surplus tubes designed with that consideration. They are intended for use in a microphone preamp to be used in high EMI fields, along with transformers of course.
Impedance match and resonances matter more. An input tube without shield can pick up easier 60 Hz (50 in Europe). Theoretically, it can pick up better waves closer in wavelengths to their 2 heights, 4 heights, and so on, the higher the frequency, the worse. But it is not enough to pick up RF waves; they need to demodulate them to make them audible, and volt-ampere characteristics of tubes make levels of demodulated signals significantly lower than the same RF levels would contribute by solid state devices. Also, as I mentioned before, solid state devices on input often have huge open loop gain that is not attenuated by feedbacks like signals on audio frequencies. Plus, tubes often have so called "grid stoppers" to tame up possible oscillations, that with parasitic capacitances form good RF filters. That's why cellphones in a studio with SS gear are chirping, while with tube gear they don't.
FYI this is another Jim Williams, one really knows/knew circuits inside and out in every detail the other, well you can make up your own mind by reading.
EDIT - It's good to know the acrimony can really fly without me around.
What is the name of that tube?
I bought a lot of 6S17K-V tubes that were used in missiles, for the same purpose; they worked on 60g acceleration, so microphonic effect is zero. However, they have tiny 8 micron grids with 18 microns between wires... I don't expect them to survive an EMP. 🙂
It is another Jim; I know him by gearslutz forum. AFAIR, he sells his SS microphone preamps. I would not comment about his knowledge and experience, since we are friendly to each other on the forum.
I heard the rumour, that during Desert Storm they had to bring to Iraq de-comissioned already Collins R390A/URR receiver because new receivers with solid state mixers on inputs and over-the-roof filters after them were overwhelmed hearing so many strong signalls on antennae. That mechanical analog computer with gears and shafts had tubes and very good preselectors.
No problem but it's hard to have a technical discussion when formalism is not in the lexicon. Design by putting things together and just listening ends up with decisions based on multiple simultaneous changes and what really matters is lost.
They were assembled in vacuum from tiny details, so it is pretty hard to match a pair. Also, for so huge gm & mu combination they have grids very close to cathodes; it causes significant grid currents.
Exactly. It resembles clinical studies when they forget to mention that meat was from animals fed by antibiotics, while vegetables did not contain them.
Do you think it's possible to do an SS buffer pretube, that doesn't cause any issues with the sound you're going for?
Sure, why not? The problem is not in presence of solid state devices. The problem is, in power amplification with complex load. Just divide say ((30^2)/4) / ((0.775^2)/20,000) and get the number. 30V is output voltage of an amp, 4 Ohm is a nominal impedance of a loudspeaker, 0.775V is input signal, 20k is a volume control pot's resistance.
From a RF interference perspective a bipolar is probably the worst as it would have a forward biased junction just perfect for an AM radio all its own. Couple in some cable for a tank circuit and you are set.
Probably 20 years ago a customer purchased a pair of Aleph ones when his Sony wireless something was set on top and the RFI didn't interfere with the signal. A fairly good sized MOSFET input pair with 3 Volt turn on. Probably not the only requirement he had but it was a must for him since he developed for whatever it was.
Probably 20 years ago a customer purchased a pair of Aleph ones when his Sony wireless something was set on top and the RFI didn't interfere with the signal. A fairly good sized MOSFET input pair with 3 Volt turn on. Probably not the only requirement he had but it was a must for him since he developed for whatever it was.
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