[R-390] thoughts on tube shields

[wli]

Thinking about thermal issues, it occurs to me that we must recognize that half the tubes are up-side-down once all the modules are mounted inside the R-390A.

So for the tubes that are on the top deck, heated air from the tube shields will rise through convection, and conducted heat could go to the chassis. Heat conduction to the chassis frame is why IERC shields were designed to be mated to an IERC mounting collar held to the chassis by the tube socket bolts. At hamfests, these collars are rarely found, as uncommon as any IERC tube shield. Its importance is often overlooked. Without the collars, heat dissipation through conduction is small. 

For tubes on the bottom deck (audio deck and power supply) convectional currents will also rise... to the chassis frame itself and the large power transformer case, both which act as kind of a heat sink. Here, for conduction to work optimally, the IERC mounting
 collar is required. It is designed to conduct heat directly to a chassis. Without the collar, the only way for heat from the shield to dissipate is through convection. Note that the 26Z5's are real heat producers and are underneath. Conversion to silicon rectifiers should reduce heat production.

We agree that for best heat conduction from the tube to any shield, some mechanical contact to the glass envelope is required.  Shiny shields are bad as they reflect heat back into the tube, as Chuck and others have noted. 

Compared to high-end audio amplifiers or transmitters, these receivers do not produce all that much heat. The R-390 with its 6082's are another matter. Heated trapped air inside the cabinet can also be detrimental, making the addition of a small quiet muffin fan logical. 

W. Li
Mercer Island, WA