Valve instability - but not as we know it.

Adrian Tuddenham · 2011-11-17T10:59:17+00:00

I have been commisssioned to design and build a one-off audiopreamplifier using E88CC / 6922 valves in most of the signal-handlingstage. (Don't ask why - it is a strange project but definitely notintended to be a piece of cult audiphool gear)It has to meet an exacting spec for several parameters, including noise.I was experiencing great problems measuring the noise by means of myworkshop bench amplifier because, as I increased the system gain,various forms of instability began to manifest themselves. At first I thought the problem was caused by V.H.F. instability in twoof the stages which were connected as cascodes; in that mode, high gainis maintained to several hundred megacycles and some of my wiring waslong enough to have an appreciable effect at those frequencies. I wasalready using a grid stopper on the bottom triode of the pair, but,unconventionally, I found that another grid stopper on the upper triodewas necessary to cure the problem. H.F. oscillation in the HT PSU(which was transistorised) was found to be another cause of someunpredicatable behaviour - but there was still another effect whichremained:This third effect depended on the gain of the whole system, theamplifier on test plus the bench amplifier. The bench amplifier has amonitoring loudspeaker, with an audio gain control independent of themain calibrated measuring attenuator; the effect was found to vary withthe setting of this volume control. When the volume was advancedslowly, there was a pause, then the level meter slammed hard over. Asthe control was turned back, after a few seconds, the meter droppedback. The effect was just like audio feedback in a very high-Qsituation.Eventually I came to the conclusion that this really was audio feedback,but in the ultrasonic range. I found I could alter the oscillationpoint by covering the loudspeaker with my hand. If I tapped one of thevalves with a small hard object, I could send the meter off-scale -although very little sound was audible...

Nov 17, 2011 23 Replies

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Tauno Voipio 14 years ago

That's right. It was marketed as 'the golden grid tube'.

Anyway, it was intended for VHF use. The dual triode for audio preamplifiers was 12AX7 / ECC83.

Tauno Voipio

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Fred Abse 14 years ago

ISTR reading an article about substituting "modern" frame-grid tubes in the frontend of the AR88. Never tried it, but I always had a soft spot (not a hernia, before you suggest it :-)) for the AR88.

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

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whit3rd 14 years ago

...

Yep, most tubes are at least slightly microphonic; the small ceramic/metal (Nuvistor) types were best in this respect, I'm told. My old Cary electrometer had a preamp section in an eight-pound cast iron case, to keep 440 Hz out of the chopper amp tubes.

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Adrian Tuddenham 14 years ago

Nuvistors had very low overall noise. For many years they gave a better L.F. noise performance than even the best F.E.T.s.

~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply) www.poppyrecords.co.uk

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