The AM Forum

I acquired an HT-37 about a year ago and have enjoyed it so far. The radio came with several mods, one a chassis mounted fan to cool the 6146's. Is this an actual benefit for tube life?  Sometimes the added noise becomes really annoying and I've thought about removing it.

Any added cooling in a cabinet full of tubes is an added benefit. Is it positioned for "suck-in" or "suck-out" and how is it mounted? If it's a DC fan, you can generally run them at a lower voltage then their normal operating voltage and probably would never hear it.

(http://i288.photobucket.com/albums/ll196/GSPD44/HT-37/Ht-37%201.jpg) (http://s288.photobucket.com/user/GSPD44/media/HT-37/Ht-37%201.jpg.html)

Here's a picture of the fan. It's mounted on a couple of angle brackets, blows across the tubes and is AC. Any thoughts for a replacement?

Nice looking HT-37!   That was one of my early rigs in '65.

Blowing air in from the side (as it is now) creates a hot area on the opposite side of the tubes. Run the transmitter for a few minutes, turn it off and feel the front and back of the 6146s and you will see.  

I have experimented a lot cooling 4D32s. They are like big 6146s.  I have four in parallel in class C for my tube PDM rig. I run 2KV on them with about 450 watts carrier out, so I had to get the cooling just right.

By far the best way to get a nice symmetrical flow completely around the tube envelopes with effective cooling is to mount a muffin fan directly above them forcing air downwards.  This method takes no prisoners and puts a strong blast of air right down the envelopes. I realize that heat likes to flow up and this sounds counter-intuitive.  

In contrast, pulling air upwards reduces the air flow and cooling effect greatly because the air comes from the whole area.  A direct blast focuses just on the tube glass.  The air hugs the glass and then spreads out over the chassis carrying the heat away.  

This method is not ideal for tubes that need bottom seal cooling like big filament directly heated tubes, (3-500Z) - just the smaller indirectly heated tubes like the 6146, etc.

You can either mount the muffin fan inside the cabinet above the tubes on brackets (if the cabinet has ventilation holes and enough room) - -   Or for best performance, cut a circular hole (5" ?) in the cabinet top above the tubes common mid-point  and mount it there flush, on the inside facing down. (Oh, the humanity!")  Cover the fan/cabinet  with a matching gray screen and it will be barely noticeable.  Use a 120V muffin fan with a Variac to slow it down for minimum noise.  A 12VDC muffin will work too with a power resistor in series for slowing speed.  Mount it on foam or rubber for minimum vibration.


T
 

Hallicrafters thought it got hot enough inside an HT-32 to install a fan on the lid... the unhinged lid of an HT-37 doesn't lend itself as well to a fan but this pix gives you an idea...

Thanks for the replies....I now have a new project! I'll dig out the muffin fans and look at mounting options on the cover.

A friend W5PBN Frank Miller (SK), had a muffin fan with four thin rubber pads 1mm thick on the suction side. He kept this in the HT-32 over the finals area and you could feel the heat coming up, removed by the fan. His HT-32 did not have an internal fan. Later he used it on a Drake TX-4. I think at some point he glued a rubber O-ring on the fan  the right size to make an all-around seal to whatever he set it on. The feet and O-ring were to avoid scratching the equipment more than anything else. The fan was not very noisy or annoying. Some are and some are not.

BTW there are some ceramic trimmer caps in the HT-32 with B+ on them and can be prone to arcing or shorting. I think they are in those boxes in front of the finals.