Valve instability - but not as we know it.

Tube (valve) microphonics are/were common with audio types too. Other components can also exhibit the effect. I've come across it with capacitors (ceramics are notorious), even resistors and jacks.

[...]

I had expected some microphony, but not at ultrasonic frequencies. I was quite prepared for a bit of audible 'ponging' when I tapped the input valves, a small amount of which did occur.

The effect of turning up the monitoring amplifier volume control was a sudden *drop* in background noise (as it became overloaded with the ultrasonic signal), simultaneously the meter hit its end stop. It was initially very puzzling.

--
~ Adrian Tuddenham ~
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I can't recall a personal experience of instability caused by microphonic feedback at ultrasonic frequencies, but I do remember some occuring at high audio freqs. One case that comes to mind happened in a movie theater run by a friend back in the 70s. I isolated the offending valve by tapping them in turn and cured it by inserting a wad of cottonwool between the tube and its retaining clamp. The owner was amazed by what seemed like magic to him.

Yes, of course! I would have expected this immediately. Vacuum valves are notoriously microphonic, and also the exposed plates can radiate a lot of electrostatic field, especially with their large voltage swings. Those the the two most likely sorts of unwanted coupling you run into with them.

I find it hard to believe you can't deliver better performance with a transistor circuit. The only place for tubes these days is VHF and UHF transmitting applications and such.

Jon

Don't forget about the millions of magnetrons in microwave ovens. Most radars still use magnetrons and the telephone microwave system uses vacuum tubes. Traveling wave tubes are used in satellite transponders.

I started my engineering career at Bell Labs in the tube department. I finished it working with traveling wave tubes in communication satellites.

I did teach transistor electronics for UCLA extension courses, so I'm not really unaware of their existence!

--
Virg Wall, P.E.

Why on earth cascodes for audio?

The cascode is a way to get a good VHF/UHF amplifier with triodes, simulating the properties of a pentode with two triodes. The tubes do not know that you're aiming at a frequency well below the capability of the amplifier. If you're building an UHF-capable amplifier, you have to have the layout, shielding and bypassing to match. Have you paid attention to proper bypassing of the heaters?

--

Tauno Voipio

The voltage gain of a single triode is about 30, which would not have been enough for the required open-loop gain before feedback. A cascaded pair of triodes (both in common-cathode mode) could be expected to give a gain of 900, but the output signal would be in the same polarity the input, so feedback would have to be returned to the cathode of the first stage. Two of the stages in this design needed the 'virtual earth' configuration to obtain a response which fell to zero at certain frequencies or certain gain control settings, so I needed a configuration which gave inverted output polarity.

A pair of triodes in cascode gave sufficient open-loop gain (about 100) for the feedback to work properly and still allowed enough voltage swing (about 20v p/p) to give a good overload margin. I could have obtained even more gain from the pair by increasing the anode load resistor and by-passing the upper valve with a resistor so that the lower valve could work at higher anode current. This gain would have come at the expense of bandwidth, which I could easily have afforded to lose, but unfortunately the D.C. conditions of that configuration are not particularly stable with valve ageing.

The input stage of the chain had to be designed for the lowest possible noise and had to present a load of 47k to the source. If a physical terminating resistor had been used between signal and earth, the S/N ratio would have been worsened by 6dB; so a virtual source input impedance was generated instead, using separate current and voltage feedback paths. The virtual earth current feedback from anode to grid determines the signal input current; a resistor in series with the cathode bypass capacitor allows the correct voltage swing at the cathode to make the grid circuit look like 47k. (This means that D.C. heaters are an absolute necessity.)

Again, this could have been done with a cascaded pair of triodes, taking the inverted polarity current feedback from the anode of the first stage, but the cascode gives a reduced component count and a tidier layout.

By using a grid stopper on the upper triode, the vhf response is killed and the design becomes a lot less layout-critical. It seemed a pity to lose all that hard-earned bandwidth - but I still had enough left to allay any fears about whether the H.F. response would be high enough for audio.

Very helpful advice. During my attempts to track down the cause of the instabiity, I tried by-passing the heaters to earth with a short-leaded capacitor, but that didn't make any difference. If the cathode-heater capacitance is low, the inductance of long cathode leads can improve stability by reducing VHF gain - although I wouldn't recommend this as a design technique.

--
~ Adrian Tuddenham ~
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The thing which surprised me was that the microphony was so marked and of such high Q (it took several seconds to die down when the gain was cut).

The other surprise was that a cheap and nasty loudspeaker, squirting sound through a few holes drilled in the bottom of a measurement amplifier casing, which was screwed to the underside of a shelf, should give enough output at ultrasonic frequencies to cause the effect. I suppose I had not taken account of the loop gain, which was being used to measure the noise level of the input stage.

The voltage stage gains of the pre-amp on test were approximately x10, x1, x1, x2, x5, x10; giving a total gain of 1000. The bench amplifier was set to "50dB" , which is actually a voltage gain of x316. A further

10dB could be added by the volume control on the bench amplifier to give a total of 10^6 voltage gain betweeen a 47k input load resistor and the 3-ohm moving coil loudspeaker.

When measured this way, with a bandwidth of 25 Kc/s, I am happy to say that the noise level of the pre-amp was equivalent to the Johnson noise of the 47k resistor within the limits of my measuring equipment.

--
~ Adrian Tuddenham ~
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You seem very angry that the electrons don't seem to have learned your laws of physics - perhaps you should take the trouble to learn theirs.

--
~ Adrian Tuddenham ~
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"Adrian Tuddenham"

** So you snip my post to just one line and write a totally stupid reply.

Electrons are embarrassed to know fools like you.

Piss off.

... Phil

Why not?

A friend of mine is building

The devil is in the details.

Tubes are different beasts due to the impedance levels and physical dimensions. 100 MHz and up are challenging with tubes, but with semiconductors even far higher frequencies are pretty easy.

The tube used, E88CC is a twin triode designed especially for TV receiver VHF front-ends, and using it needs VHF

-compatible layout, shielding and by-passing techniques.

For parasitic taming, I'd attempt using ferrite beads.

--

Tauno Voipio

"Adrian Tuddenham"

** A noise figure of close to 0dB in the audio band is not possible with valves.

And you still will not say what the signal source really is.

** A crazy, unsupported assertion refutes nothing.

... Phil

[...]

Checkmate.

--
~ Adrian Tuddenham ~
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"Adrian Tuddenham"

** What sort of asinine f****it are you ???

Yet again, you snip everything you do not feel like answering and post another arrogant, s*****ad remark.

Guess what - that makes you an arrogant s*****ad.

... Phil

Sometimes when I read your replies I think of these WAV files:

"Bitrex = TROLL "

** FOAD

Asshole

nice files, I saved them :)

Jamie

ECC88 is/was the consumer, TV frontend thing.

E88CC is/was a special quality industrial variant, which probably cost twice as much.

--
"For a successful technology, reality must take precedence 
over public relations, for nature cannot be fooled."
                                       (Richard Feynman)