Cooling Power Tubes

[Ed/KB1HYS]

Need a resource or some suggestions on plumbing cooling airflow over power tubes.

The specific tube I am working with is the 4-250 (glass) 

There is no chassis drilled yet, just blank aluminum panels, so things are wide open.

Now is the time to work it out before I start cutting metal...

[K1JJ]

Need a resource or some suggestions on plumbing cooling airflow over power tubes.

The specific tube I am working with is the 4-250 (glass) 

There is no chassis drilled yet, just blank aluminum panels, so things are wide open.

Now is the time to work it out before I start cutting metal...

Hi Ed,

Check out this website for really nice, but cheap lantern glass to fit the 4-250. Get one that leaves maybe 1/2" of gap all around the tube and comes up to near the top while hugging the glass. It's not that critical, but use a real Eimac chimney as a guide.

http://www.rei.com/category/11753602.htm

I bought a set for my blown 813's and they work FB.  The best approach is to build a small sub chassis box to mount the tube on and put a squirrel cage blower on the back. All the grid circuitry goes underneath, of course.  This is the most efficient way of doing airflow. Use a Variac on the blower and it will be very quiet and cool.

73,
T

(Click on the blue url below to get a super view)

[W2XR]

Hi Tom,

That chassis for the 3X 813s looks like a nice welded-construction heavy guage aluminum job, with a vibration-sanded finish. Was this custom fabricated for you, or was this a standard off-the-shelf vendor item?

In my experience, off-the-shelf heavy duty, good quality chassis are all but extinct nowadays, except for that cheap .040 thk crap that you can buy over the counter, unless you want to get one custom fabricated by a sheet metal shop to your specs. Not cheap!!!

I'm building a 1200 VDC PSU for my push-pull class A 845 driver to my p-p 833A modulator and I was lucky; the chassis was a blank leftover from a defense electronics frim I worked for many years ago. It's 0.090" thk with PEM fasteners for the bottom panel, etc. Was saving it for a future RF HPA project, but recently decided to use it for the 845s instead.

73,

Bruce

[W1RKW]

Electronic wicks in them thar lanterns

[K1JJ]

Hi Tom,

That chassis for the 3X 813s looks like a nice welded-construction heavy guage aluminum job, with a vibration-sanded finish. Was this custom fabricated for you, or was this a standard off-the-shelf vendor item?

73,
Bruce

Hi Bruce,

I hear you are haunting the airwaves again lately. Hope to chat soon or at least at HossTraders if ya make it up.

For air chassis fabrication I have a ham buddy, Big Al /K1JCL who has a sheet metal brake and sheer. I bring over sheets of aluminum and he bends them into the forms I need. I use 6-32 nuts and bolts to hold the corners together. He makes little lips on each edge to permit this bonding.

So far I've made up seven air chassis for the new rigs. In the near future, I'll post some shots the the new nearly completed AM transmitters.

Great on your plans for the new tube audio driver. Are you familiar with the new WA3KLR FET, cathode driven audio driver?  I'm now building up two - one to cathode drive a pair of 833A's and one to drive a pair of triode connected 813's. The board is quite simple and Tom modeled it and found it to be very clean. 

The info is in an earlier thread here:

http://amfone.net/Amforum/index.php?topic=6051.0

If you are interested I can post a schematic or you might wait til I get them running to get the bugs out.

73,
T

[W2XR]

Hi Tom,

Thanks for the info and yes, it's good to be back on the air! I too hope to work you very soon. Right now I'm integrating a new Belar modulation monitor into the station (replacing the old tried and true Gates unit), and making some other improvements as well.

Re the 845 audio driver currently under construction here; I really like the sound push-pull class A (not class AB1 or AB2) triodes provide. IMO it's the cleanest, most powerful sound you can get from vacuum tubes and they make superb drivers for p-p class B modulator tubes that take a lot of grid swing and provide a difficult driving impedance such as 833As, etc. Gates used PP class AB1 845s in the driver section of the BC-1F with only about 6 dB of global negative feedback between the plates of the 833As and the cathodes of the 6J7 input tubes and I think that was one of the best sounding plate modulated BC rigs built, although others may take issue with this. This rig also had great quality magnetics in the modulator & modulation reactor as a contributing factor.

I have never been enamored by the sound of pentode power amplifiers in any topology, although I can see the low source driving impedance advantages cathode follower driver tubes would provide to a class B modulator. However, pentodes need a lot of negative feedback to get the linearity/distortion to within acceptable limits (typically on the order of 16 to 20 dB) and this can cause slew induced distortion (SID) which contains a lot of odd-order distortion products and these are very dissonant to the human ear. Not to mention the phase shift and related negative feedback stability  issues going through xfmrs, etc., with this 20 dB level of feedback, although one advantage of the the cathode follower driver is plainly the elimination of the audio driver xfmr and it's related phase shift. My 845 driver has provision for incorporating 6 dB of global feedback, and I have the feedback ladders for this purpose, but I don't think I'll "close the loop" just yet.

Class A power triodes when properly set up usually don't require negative feedback to obtain acceptable levels of distortion, and negative feedback and the resulting SID can be a problem when heavy amounts of audio processing are employed in any audio amplifier circuit, whether solid state or tube.

I guess more to the point, I really like using all-tube topologies here in the shack and in my homebrew stereo and the old-time radio feel they provide, as everyone here is of course is aware of. I have had opportunities to go with all kinds of solid state audio driver toplogies, but I prefer the coolness factor of the tubes!

Not to mention that I work with state of the art solid state electronics 8 hours a day, 5 days a week, and the romance just ain't there guys!

Best 73,

Bruce