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Thursday, June 2, 2016

Manhattan-style Vacuum Tubes: "An Evolution of Thermatron Homebrew Techniques" by Grayson Evans



Grayson Evans was at Dayton.  Scheduling problems prevented him from being interviewed by ace correspondent Bob Crane, but Grayson was kind enough to e-mail us the essence of his presentation.  And it is really wonderful.  He brings the advantages of the Manhattan construction technique (fast prototyping, all components on the same side of the board, easy modification) to the world of tubes (aka valves or, as Grayson prefers, thermatrons).  We also see in Grayson's work an admirable willingness to bridge the digital-analog design, to bring into his rigs the best of the old and the new.  Thanks Grayson!

Grayson writes:

An Evolution of Thermatron Homebrew Techniques

For a long time I have been trying to develop some techniques to prototype Thermatron projects as easily as the typical “Manhattan style” solid-state construction.  Thermatron projects you see now-a-days still use the traditional technique of mangling aluminum-drilling and mounting everything to a “bud” style chassis. This just takes too much time and my projects always look disappointing.

Fortunately, around two years ago, Rex Harper, W1REX, came to the rescue after hearing an earlier talk of mine and developed a set of thermatron socket pads. The MeTubes panel from QRPme consists of 10 prototype pads for thermatron sockets. The panel has v-scores for breaking the panel into single tube pads. The panel has pads for mini 7 & 9s, octal, compactrons and acorns. Awesome.



The best sockets to use with the pads are PCB style.  These provide a large pin area to bend out and solder to the pad (see photos). 


I pre-mount a dozen or so of the 7-pin and 9-pin sockets on MeTube pads so I have them ready to go when prototyping. The “crude” example below shows and 7 and 9-pin socket on one of my prototypes.  I think this was a microphone amp for my AM transmitter.  Pardon the mess.


The nice thing about the pads is that they provide plenty of room to tac solder lots of parts to a single pin–easy to add or remove parts.  This is a lot easier than using the traditional tube socket pin. 

Prototyping thermatrons in this way is FAST.  No more punching out holes to hold thermatron sockets in aluminum chassis!

But it is still nice to be able to have the thermatron on the “top” of the board and the components on the “bottom” of the board.  To do this and still use the MeTube pads, the thermatron has to be mounted on the other side.  I did this by mounting the socket through the MeTube pad.  This requires making a hole in the center of the pad to pass the socket through and then soldering the pins in the usual way.  This is way easier with PC mount thermatron sockets and make a very nice installation. 




This technique has some great advantages over using the traditional socket with pins.  The pad has a lot more room to mount components to each pin.  Normal thermatron socket pins are difficult to attach more than two wires and it’s a bit more difficult to get a good solder connection.  The pads are easy to solder to and allow components to be easily attached in any direction since the “socket” is now flat.



It is also easy to attach the socket/pad to a copper clad board.  The same hole must be made in the copper clad board to pass the top of the socket through, then the pad is superglued to the board in the regular way.  



This is another example of “right side up” thermatron mounting on a prototype test board for crystal filters.  I mounted a small “plug board” (not sure what you call these things) in the center to allow me to easily swap filter components.  Notice the acorn thermatron soldered direct to the pad.  The board works great, the filter design sucked.  I gave up.  Maybe too much distributed C.


I used this technique, combined with Roger Fell’s idea of using inverted aluminum chassis, to build my latest project, a QRP AM/CW transmitter.  I’ve been wanting to try out a few new ideas and this seemed like a good project to try them on.  I also wanted to build the transmitter in modular “blocks”, interconnected in a similar way to Rogers. It worked pretty good although I am still trying to get the thing to work right.  Even new construction techniques can’t cure my screwups.  BTW, the ANALOG VFO is ROCK SOLID.  +/- 10 Hz over 30 min.   Even I was impressed.  The Hartley oscillator is the best circuit for thermatron circuits by far.



I’ll send an update when I get it fully working on AM and CW.
73,

Grayson
TA2ZGE

























2 comments:

  1. I met Grayson and his lovely wife at Dayton. Wonderful couple. I even managed to get Grayson to autograph a copy of his book!

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  2. About 1958, there was an article in QST pointing out the value of using circuit board as a "plate" to build on. People had already been using a plate on top of a metal chassis, this guy just replaced the metal plate with circuit board. Easier to punch holes in for tube sockets, it of course, offered a really good ground surface, one which could be soldered to. No need to string to a ground lug in an existing hole, or drill a hole for a ground lug where you needed it, just solder.

    Frank Jones used to build converters on those plates above the chassis, but I'm not sure if he built any tube converters with circuit board. Once he moved to transistors, he definitely did.

    And of course, Bill Hoisington, K1CLL, built endlessly on copper circuit board, and he started out early enough that he was using tubes in the early days of "73", then moving to transistors. About 1966, he even described a multi-band linear using a 4CX250 type tube, and that was with copper circuit board. Easy to cut, easy to drill, and easy to solder to.

    Michael

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