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RE: Microphonics? Take them away!

Posted by Bambi B (A) on August 19, 2010 at 21:31:51

In Reply to: RE: Microphonics? Take them away! posted by Anacrusis on August 19, 2010 at 09:39:35:

PHOTO: Tungsram E88CC featuring red pin stuff

Anacrusis,

I'm enjoying your threads on microphonics- a demonstration of the collision of mechanics with electronics- a technological artifact of the 19th Century that we'll soon not see around- except with speakers- probably until the end of time,..

"Why aren't we using carbon fiber circuit boards?"

Carbon fibre does have some inherent damping characteristics- it has a low vibration capacitance because of the low mass/volume, but it is very hard and therefore more resonant than it may appear. Tap a board of it and it will make a quite high pitch dry, plink sound- well up in the audible frequency range.

Structurally, it should make good circuit boards by nature, except the way it's made- top face down in a mould that has to be placed in a vacuum bag and then cured in an oven for a day or so.

These traumas mean there's no way to place the traces first and not have them damaged later in the other processes, which often includes a planing/polishing of the face. And if the board were thusly finished- that odd appearance of heavy, woven cloth encased in transparent black plastic, it will have a hard, smooth- slick surface that mean the traces wouldn't have a good long-term adherence. To add the traces after finishing, the board would probably have to be machined where the traces went- extra steps plus hand labour. And, if the traces were added before finishing- the board couldn't be finished,.. Interesting how complex a black plastic plank can be.

While carbon fibre seems so high tech, it actually involves hand cutting and placing strips of the cloth, brushing resin, hand sanding, and other considerable and messy hand labour and craftsmanship judgments. -Low and high tech together.

-These are conjectures, but it happens recently I've been looking into carbon fibre and syntactic foam- (like "fluffy" carbon fibre with glass beads instead of carbon fibres floating in it)- recently for a project I'm doing on auto and aircraft structures. Carbon fibre is magic stuff for engineers,.. The 19th C. was a mechanical/manufacturing age, the 20th C., of electronics/information, and the 21st C. I think will be an age of materials/robotics.

"6922 Teslas,.. seem to have some kind of red damping material on the pins where they go through the glass."

Red pin stuff seems to be mainly on Eastern European and Russian pins- Sovteks and EH6922's use it also. You can see that same red coating on the pins, as used on the Tesla 6922 pins at the glass, also used on the pins/glass on one of my favourite 6922's, the Tungsram E88CC- (photo above) and this is also present in the old Sovteks and EH6922's. I'm not sure about the old Ei's, but I think they have it as well.

I don't know the composition of this material, but I believe it's purpose is for long-term, effective vacuum sealing in the region where the metal pins pass through the glass bottle. Dissimilar materials placed adjacent and having to seal always mean trouble. In tubes, there is a differential cooling of the pins and glass bottle both during construction and the heating/cooling cycles of on/off use. -I'm quite sure I've heard a slight crinkling-tinkling sound from cooling output tubes. The pins when cooling would contract and at a different rate than the glass- the expansion coefficient of steel is many times that of glass.

The red stuff on the pins- probably paprika in the Tungsrams as the Hungarians put paprika in everything (more likely a resin) simply fills the gap as a gasket and as a substrate/coating that can maintain a glass-to-metal bond and therefore vacuum seal under all the operating conditions.

My intuition is that tube pins and their leads represent a significant role in the heat dissipation of vacuum tubes. This is due to the vacuum in the bottle- a vacuum can't conduct heat to the glass. The heat must be conducted along the heater, grid, plate leads to the pins and the pins, having a good surface area contact with the glass where they pass through, send the heat along into the massive glass of the base- the mass acts as a heat sink.

I believe that to some degree, heat dissipation imposes a limitation on the output of tubes- in effect, if the heat could find a proper home, the wick can be turned up. There are those Frankenstein's laboratory looking, multi-kW, giant broadcast tubes that are water-cooled (and can be rebuilt) and I believe this is in the form of a circulating water jacket- only around the tube base. And effective cooling apparently increases tube longevity subtantially- there's a water-cooled broadcast tube somewhere that operated continuously for something over 100,000 hours-

Again, I've used mostly conjecture on this topic- perhaps Victor will comment and correct any engineering lunacies.

Cheers,

Bambi B

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