Vacuum tubes require, as should be obvious in their name, vacuum. Over the years, vacuum tube manufacturers and suppliers have turned to lower quality vacuum to cut costs. The high-quality vacuum supply is harder and more expensive to acquire.
As vacuum was mined, the easily reached sources of good quality vacuum (near the surface) were quickly exhausted leaving only more expensive and dangerous mining operations as the only source. As a result of several large and devastating mine implosions the larger vacuum mines that produced good quality vacuum were closed leaving only small boutique vacuum mines to supply the vacuum. Often, this vacuum contains impurities that must be removed (by what else than a vacuum cleaner!).
In the mid-80s, the Russian Kamakiskorvitchski brothers (Kamis for short) had a corner on the vacuum market. They had several tanker cars on a railroad siding in Siberia that contained most of the high-quality liquified vacuum available at the time. They carefully supplied only the Soviet and Eastern Block tube manufacturers with the high quality vacuum. Some smaller tube manufacturers were forced to use counterfeit vacuum containing large amounts of impurities. Again this vacuum needed cleaning.
Unfortunately, the supply of good quality vacuum has dried up in recent years. Only the older tubes still used high quality vacuum in their manufacture, but some tubes of the same era used counterfeit vacuum.
So one must be careful. Check your tubes and don't forget to use your vacuum cleaner.
As vacuum was mined, the easily reached sources of good quality vacuum (near the surface) were quickly exhausted leaving only more expensive and dangerous mining operations as the only source. As a result of several large and devastating mine implosions the larger vacuum mines that produced good quality vacuum were closed leaving only small boutique vacuum mines to supply the vacuum. Often, this vacuum contains impurities that must be removed (by what else than a vacuum cleaner!).
In the mid-80s, the Russian Kamakiskorvitchski brothers (Kamis for short) had a corner on the vacuum market. They had several tanker cars on a railroad siding in Siberia that contained most of the high-quality liquified vacuum available at the time. They carefully supplied only the Soviet and Eastern Block tube manufacturers with the high quality vacuum. Some smaller tube manufacturers were forced to use counterfeit vacuum containing large amounts of impurities. Again this vacuum needed cleaning.
Unfortunately, the supply of good quality vacuum has dried up in recent years. Only the older tubes still used high quality vacuum in their manufacture, but some tubes of the same era used counterfeit vacuum.
So one must be careful. Check your tubes and don't forget to use your vacuum cleaner.