Mesa Boogie Mk4 design shennanigans

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One of the design issues with valves is the vacuum tight seal between the p ins and the glass base - cos steel and glass do not have the same tempco of expansion. Typically, steel has a higher tempco so the fit get tighter as the glass envelope heats - conversely it gets looser at very low temps.

Prolonged exposure at very low temps could let air ( or N2) into the vacuum and spoil the operation.

.... Phil

One of the design issues with valves is the vacuum tight seal between the pins and the glass base - cos steel and glass do not have the same tempco of expansion. Typically, steel has a higher tempco so the fit get tighter as the glass envelope heats - conversely it gets looser at very low temps.

Prolonged exposure at very low temps could let air ( or N2) into the vacuum and spoil the operation.

.... Phil

My bad, their website makes no mention of N2. Maybe they just stick them in the freezer and call it Cryo. Interesting info though.

Anyway, any thoughts on 5.1v heater voltage as a viable propostion?

Cheers,

Gareth.

One of the design issues with valves is the vacuum tight seal between the pins and the glass base - cos steel and glass do not have the same tempco of expansion. Typically, steel has a higher tempco so the fit get tighter as the glass envelope heats - conversely it gets looser at very low temps.

Prolonged exposure at very low temps could let air ( or N2) into the vacuum and spoil the operation.

.... Phil

Oh, OK, rather late to the party on this one - so there is no bonding at all between the pin and the glass, just an interference fit?

Gareth.

oy them instantly by cracking the glass. Even if done some other and safer way, the whole idea is completely nuts.

pins and the glass base - cos steel and glass do not have the same tempco of expansion. Typically, steel has a higher tempco so the fit get tighter a s the glass envelope heats - conversely it gets looser at very low temps.

um and spoil the operation.

I do not believe they do, either. Here is one "Cryo" vendor describing thei r process: _______________________________________________________________

The BLACK SABLE process is far more than putting tubes in a chamber and low ering the temperature. The BLACK SABLE process starts with tube selection. Only the best preamp, power and rectifier tubes are chosen to undergo this arduous process. All tubes undergo a 24 hour burn-in under load. The tubes are then placed in the Cryo-Processor where a computer-controlled proprieta ry system lowers the temperature to -300 Fahrenheit / -184 Celsius for 24 h ours and then slowly ascends back to room temperature. Preamp tubes are tes ted and rated for Gain, Microphonics and Noise. Next, all 7/9 pin tubes hav e their pins cleaned for a better electrical connection. Power tubes are ma tched to within 3% for both Ip (Plate Current) and Gm (Transconductance). N o other cryogenic process comes close. When only the best will do, choose B LACK SABLE. __________________________________________________________________________

Which is 21F above the boiling point of liquid nitrogen, a cheap and readil y obtainable material in liquid form. That being written, most metals, incl uding coated steel, copper, cupro-nickel and other solderable materials hav e a vastly greater coefficient of expansion/contraction than even the softe st glass (Pyrex). So either the pins are going to shrink out of their surro undings, or all this is done in a *hard* vacuum. There is no real danger of shattering the glass if done slowly, but as the pins will be *smaller* tha n the opening during the process - that is where all the 'magic' must take place.

Color me dubious.

Peter Wieck Melrose Park, PA

Especially dubious because if the pins really did shrink away from the glass the glass/metal seal would be broken and the tube would leak air in once exposed to the atmosphere again. When tubes are made the glass really does seal to the pins, just like glue. But glass is elastic (more elastic than the metal pins) and the movement would be small so maybe the glass does move with the pins. Steel pins on a tube would shrink about .0001" going from room temp to 300 below. Glass would move about .000067" so about 33 millionths of an inch difference. So the glass/steel seal would probably hold. Eric

But this happens at a very high temp ( about 1500C ) so during cooling thermal contraction will put considerable stress on this seal even at room temp.

.... Phil

I did a quick test on a few 'X7 and 'U7 types and found good operation was maintained to under half voltage on the heaters. At about 2 volts instead of 6.3V, voltage gain is low or sub unity.

.... Phil

How would they do this in practice? If it had to be done in a hard vacuum, how would you transfer the heat away from the valve to cool it down to -184? Maybe they hire a rocket to fire the valve into space and let it cool down by radiation, obviously keeping it out of sunlight while it's up there. ;-)

And why would they do it to a rectifier tube? What would they expect to gain?

... Phil

Your money. if you are gullible enough