Tube Cooling Elevated Bases?

[Ed/KB1HYS]

In a handbook article I found online for building an amp/class C final using 2 tubes, it shows the sockets elevated above the chassis on 1 inch standoffs. There are holes drilled under the pins for air flow to hit the pins presumably. There were no chimenys, just holes directly above the tube for exits.

My first thought was that how can you keep the grid circuit adequately shielded if you elevate the tube sockets above the chassis? Is the inch of wire just not enough coupling to worry about?

Secondly, wouldn't this design require a larger (and therefore Louder) blower, since there is loss through the holes in the chassis?

Attached is a pic from the article.  Very nice construction through out of course.

[The Slab Bacon]

Ed,
     It would depend strictly how hard you run them. Air blown up / across them will always help. If you mount them like that, at least put a good sized muffin fan close to them.

Also anywhere 4-250s are used 4-400s are a drop-in replacement! And 4-400 broadcast pulls are still cheap and plentiful!! ($10-$20 range) and a lot easier to get a hold of than 4-250s.

                                                           The Slab Bacon

[N2DTS]

That is wrong accoring to eimac, they strongly said to mount them directly on the chassis even if no forced air is used, to allow air flow through the holes around the tube base.
They also say sub mounting is bad (for tubes like the 4-125/250/400/1000).
The tubes were designed to allow air flow around the pins with or without air pressure when mounted on the chassis.

Other tubes like the 811, 4D32, 6146 do better sub mounted to allow air flow up around the tube.

Above chassis mount of any tube just looks like a bad idea in every respect.

Brett

[WQ9E]

It looks like someone didn't want to deal with mounting a socket in the chassis.  That construction doesn't make sense from either an RF or thermal point of view.

The 3-500Z's that came with my SB-220 (original kit built by me) had a ceramic spacer around the pins so that the tube base sits around 1/2 inch above the socket.  I don't think this was standard for all 3-500Z's but maybe it is.  In any case it does make sense for amps like the SB-220 because it allows the fan to effectively blow air across the base seals and probably also provides additional protection against damage to the pin/glass seal from improper tube insertion.  I remember the tubes also had a little sheet included warning about the dangers of beryllium oxide although it seems odd that Eimac would use that for this purpose.  In any case, the amp still has its original tubes and it is in semi-retirement now but it was my main (in fact only) amp for about 18 years.

Rodger WQ9E

[W3RSW]

Interestingly, in the same issue (1957) the single 813 in "A 500 watt Multiband VFO Transmitter", page 192, is mounted in the same way.

The 4-65A in "75 to 300 Watts with VFO Control," same issue again, page 184 is also top mounted on standoffs.

-Must have been a trend at the time.  

Since the 813 was designed as a naturally cooled tube this method has to be less critical and even better with a small fan evacuating the cabinet and having air enter from under the chassis.

In the case of the 4-65 design it also was convection cooled with no fan, but the builders realized that the pins had to be cooled and not isolated or sealed off from sub chassis, hence the large cutout under the tube socket.

Much discussion in previous posts about shielding between grid and plate with above chassis spacing, shield rings on 813's, grounding of such shield, placing top of shield even with chassis, etc. All placements have worked well for me on 160 through 20.  Just about all the designs are shown in the various handbooks through the years.

The 4-250/400 as mentioned was designed for forced air cooling up throught the tube socket. I think I'd follow Eimac's recommendation on that one.

[Ed/KB1HYS]

Ok I thought it was oddball tube mounting here.  I figure I'd mount the thing on the chassis, an blow air up through the socket, just like Eimac says to.

Were do you find 20 dollar 4-400's???  At that price a pair modulated by a pair is possible, providing the iron is there.

[The Slab Bacon]

Were do you find 20 dollar 4-400's???  At that price a pair modulated by a pair is possible, providing the iron is there.

You gots to git up off you ass and dig around at some hamfests. I saw 2 of them go at a recent local fester for $25. You just gotta dig around. I got 3 of them a few years back for $10 each. (all 3 were usable)

Used tubes for those kind of prices is definately a crapshoot, but once you average it out they are still cheap. I have seen (and bought) used "run hard and put away wet" 4-1000s for $20! Believe it or not, most were still good!

[WQ9E]

Also watch for the slightly oddball variations.  I have 5 4-500A tubes I picked up used for a total of $5 a few years ago.  Very similar to the 4-400 except for a 10 volt 100 watt cathode and an extra 100 watts of rated plate dissipation.  With a simple filament transformer change I have a few low cost spares for one of my 4-400 powered rigs.

They were all pulls in their original boxes and all tested fine in a little test stand I built.

[W3RSW]

at Timmonium a few years ago someone brought in a whole boat load of brand new in box Eimac 4-400's.  Sold all of them for $100 each.  I know a few AM'ers who bought them.

[KM1H]

I think it was National that sold those goofy sockets with the above chassis standoffs as a package. That idea was gone fast.

The 4-250/400 and other similar family tubes with metal bases are fine at chassis level as long as the metal ring is using the grounding clips that were packaged with Johnson sockets.  Otherwise they can all be squirrely when grid driven.

Carl
KM1H

[N3DRB The Derb]

the H&K datasheet that comes with the 4e27's shows even at vhf flush mounted sockets underneath the chassis are fine with them, but they are very hard on that the tube bases must be grounded for RF using wide grounding strap. They specifically call for strap and not wire.

one o the rf decks I got from skippy and the 4-125 socket mounted above he chassis by a half inch. It was apparent that the builder just didn't want to widen out the hole enough to properly mount the Johnson 275 socket. all the by passes were in the rf field above the chassis.

[KM1H]

Could you email or fax me that data sheet? Its one I dont have on file.

With a common sense layout they are correct about the mounting. But with typical ham construction strange things can happen.  But again I like to run things at 10 and even 6M just to see what they are capable of.

Like PP 100TH's and a single 6C21 on 6M decades ago.

Carl
KM1H

[K1JJ]

I like to mount 4-1000A's and its class of brothers and sisters, flush with the chassis. Then drill a circle of 1/2" holes around the outer edge of the socket so that the chimney JUST covers them.  This allows plenty of air through the outer chimney to cool the anode as well as lets some air past the pins via the socket holes.   The Tron taught me that trick and uses it himself.

It's really important to use a chimney with these kinds of tubes. (3-500Z, 4-400, 4X1, etc)  A muffin fan will work, but the life of the tube under full strap cornditions is reduced.  Seals have no sense of humor...

T

[N3DRB The Derb]

here's the gammatron datasheet. note the vhf mounting layout using vhf tuned lines for both grid and plate. that 'all band' transmitter is the one hole deck I'm making.