Second, will there be detrimental effects if the amp is switched on with the tubes below freezing?
Hi Mark,
There does not seem to be any official Eimac data out there on the subject that I could find when I looked in the past. However, here is some info you may find interesting:
There was a time when I used a blower mounted on the outside wall (sucking in outside the house air) to cool a 4-1000A final modulated by 4-1000As. The object was to have low blower noise. During the winter, the air was sometimes so cold that even the 4-1000A rig, when operating, did not warm the shack. The air coming out of the operating tubes was barely luke warm. The blower hose kept the tubes at the same temperature as the outside air when the rig was turned off. So the tubes started operation cold, just like yours.
It worked FB for a few years this way. However, I was worried about the seals for both the socket area and plate, especially when first turned on and the tubes were sitting at sub-freezing air temps and the fils came on.
Anyway, I axed a friend who was once an engineer at Varian (Eimac) about it. Basically he said that it's a matter of degree. The constant thermal expansion and contraction cycling from say room temp to 200F was normal, of course, and should not affect the tube seal longevity. However, when we consider a cyclic run from 0F to full operating temp, this puts
additional stress on the tube seals and is probably not a great idea. He basically thought it would be OK, but it was pushing the envelope. The fact that mine ran OK for a few years this way shows at least this sample worked. I still use the same tubes and have since gone to all inside blower air at room temp.
I also ran an 8877 external anode tube with outside air at the same time and had no adverse seal effects. The 8877 tube also continues to function FB with inside room-temp air.
So, bottom line, it appears to be OK, at least for a few years sample size test. However, my guess is that over a 50 year period of use, the cold air tube would have more of a tendency to become gassy compared to a room-temp tube, due to seal deterioration / vacuum leakage, though I cannot prove this.
T
