Vacuum tubes

Factual Questions
41

Actually, after thinking about this for a while, Zenster, I’m starting to think that one of the things I said was specious as well.

Convection cooling of the filament can’t be a sensible reason. You want the filament at a certain temperature to achieve the result you want. Having convection cooling means that it requires more power to achieve the same result.

And what you said about the effects of ambient atmosphere being offset by pressure in the tube also make sense, not in terms of leakage, but in terms of the strength of the glass required. That is, more pressure inside the tube requires less glass.

42

Yep, that’s exactly what I was thinking, i.e. filling the tube with an inert gas that’s around 101 kPa would certainly reduce the mechanical stress in the glass and seals. And reducing the stress at the seals would help reduce the leak rate…

43

Yes, Vacuum tubes are still produced in Russia and China, I was recently speaking with Eveanna Manley of Manley Labs (high end audio tube gear manufacturer) and she was saying that while they still make new tubes, because of the toxic chemicals required to make them- production has pretty much been limited to “third world countries” and places that have little or no environmental regulation. Both the GE and Military versions of NOS tubes seem to be the most sought after.

44

The reason NOS tubes are preferred is both “real” and imagined. The best were made at the height of the vacuum tube era, while they may or may not have audible differences from recent tubes manufactured in China or Yugoslavia, they last a hell of a lot longer, and don’t fail catastrophically, something the new stuff is noted for, particularly rectifiers.

Good, tested NOS power output and preamp tubes bring some serious buckage from guitar players, in particular. Some common TV tubes can’t be given away. Others are quite common and cheap from various sources. There won’t ever be any “originals” made again, even if WE tools up the old stuff and cranks out 300B’s (the current de rigeur tube for the audiophools) they have a certain cachet - rarity, desirability, and beauty. A glowing tube amplifier has class, even if the snooty, over-engineered salon type stuff is priced with a couple extra superflous zeros. Even the lowly Dynaco 70 (when properly restored) will embarrass very expensive, high-end solid state equipment every time. Interestingly, it is overseas asian buyers who have been snapping up US made tube gear (and largely responsible for the unbelievable prices) for quite some time, rendering once-common PA amplifiers and such to truly stratospheric prices, and buying up whole lots of thousands of tubes and taking them out of the country. In other words, they are more than happy to sell us solid-state crap, but ironically preferring “the good stuff”.

Tubes have a bad reputation for unreliability, this is unwarranted, believe it or not - at least in radios and stereos. The “Neat old radio” that you find in a store or in the attic is an accident waiting to happen - DO NOT plug in any vintage electronic device till it has been checked out by a competent repairman, even if it “works”.

The real problem with vintage gear was/is the capacitors - both electrolytic (which degrade and “leak” electrically with long periods of disuse) HUMMM>> and the waxed paper/foil coupling and bypass capacitors, which also “leak” and destroy power transformers, resistors, coils and the tubes. They weren’t very good to begin with and age takes its toll. They are not repairable. The electrolytics can sometimes be “reformed” by gradual ramping up of voltage utilizing a Variac and diodes to sub for the tube rectifier, but is generally risky. Vintage radio repair is a rewarding hobby and they make excellent gifts once they have been “gone through”; an unrestored/unmaintained set is a firehazard and unremarkable, a few dollars worth of parts will usually restore them to original performance levels, which may surprise you. I have several radios that run generally 24/7 for years and have no problems with reliability.

Here’s a couple websites to get you guys started down the long road to insanity:

http://www.antiqueradios.com

http://www.antiqueradio.org

http://trans-oceanic.fortunecity.net

VTVM’s are useful for troubleshooting, digital VOM’s have digit bobble which is problematic at times, tube gear in general is very forgiving and has wide operating tolerance. Lots of guys build their own amps around specific tubes they may have on hand, winding their own iron and testing various configurations.

45

I’ve always thought that less stress amounted to less leakage. Also, if you’re familiar with ‘crystal ovens’ in the old quartz oscillator heterodyne receivers, then the convection cooling you mention doesn’t seem to make sense. That is, unless, for the sake of component miniaturization, they had tube filaments so close to the shell envelope that convection cooling of the entire mass was desirable, then why bother to decrease filament temperature? Electron emission was of paramount importance to maintain signal (i.e., amplification) strength, no?

PS: I do appreciate you modifying your previous stance.