12AP4 Rebuilding
Some time Dave Johnson contacted Clinton Electronics in Rockford, Illinois about the possibility of rebuilding prewar picture tubes. We sent an email to Randy Schroedl, describing the project:
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Jeff Aulik suggested that I write to you concerning my
attempts to get several prewar CRTs rebuilt. As you probably
know, tubes made before the war used Pyrex or some other high
temperature glass, and, as a result, can't be rebuilt using
standard methods.
There have been a couple of successful rebuilds in the past.
They were done by welding several pieces of glass tubing, each
with a lower melting temperature, to the bulb. The last piece of
tubing could be welded to a modern stem. In this approach a
1960's electrostatic focus gun was used.
I would like to attempt a different approach. I believe I can
get the original guns rebuilt by John Bishop (formerly of
Global). Then, I'd have stems made of high temperature glass.
All that would be needed is to get a piece of tubing of the same
glass and the rebuild could be done like a normal tube. Of
course higher temperatures would be involved in the process.
There are quite a few 9AP4 and 12AP4s out there that need
rebuilding. I have a database of the surviving sets using these
tubes, and I could contact the owners to see how many need
tubes, or want spares. The one good 12AP4 that came on the
market recently (not new, but with reasonable emission) sold for
over $1500, so it would be worth quite a bit to a collector to
get a new tube.
In addition, I have a number of other prewar tubes that need
to be rebuilt, and I'm sure there are other collectors with
other tubes.
Successful rebuilding would also guarantee that the 400 or so
surviving prewar sets will be operational far into the future.
Without a suppy of tubes, they will all be dark within a few
years.
Please let me know if you have any interest in persuing this.
Thanks for your help.
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Randy responded:
| Thank you for your inquiry, we will review your request and advise within the next week. There is always a concern with rebuilding CRTs with potential implosions as a result of bruises or scratches on glass that has been in the field. I am not sure how this issue is currently handled but we would not be in a position to take responsibility for any shrinkage as a result of our attempt to rework the CRT. We are also interested in any input you may have to determine to what extent the CRTs must be reworked once we receive them (i.e. from a simple re-sealing to a complete rework). Your input will help us to determine the project feasibility |
Dave Johnson agreed to provide a dud 12AP4 for them to work with. After a few weeks, and several emails about sources for rebuilt guns and Pyrex stems, Randy reported on their progress:
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I just wanted to give you an update on our progress. We
were able to remove the gun and getter assembly from the dud
without problems. We believe we have located the proper
neck tubing to reneck the CRT and we are currently awaiting
tubing delivery. I faxed a proposed stem drawing to ETI for
review and as long as they can come up with something close
with the right glass we should be able to use it. We have
run some initial screening test to be in position re-screen
and aluminize the CRT for 6kv operation. A review of the
e-gun shows we may have the parts in-house to build an
equivalent design once we have the proper stem. We will
know more once the tubing arrives for sealing tests.
Today we received the first samples of a Pyrex-compatible stem for the 12AP4. We will start evaluating them and hopefully, the stem is the last high hurdle in this project. |
Then, almost a year passed without any progress. In April of 2005 Randy again reported on their progress:
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The re-build progress is going slower than we anticipated. We
have all the major hurdles covered but just need to get more time
on our production sealing equipment. We can now seal the Pyrex
glass but must work on reducing the residual strain. We are
hoping to get a little time on the production equipment later this
week and should have more time available on the equipment next
week. We have the process and materials defined for reworking the
glass, re-screening for the low anode operation, new modern gun
design with a Pyrex stem, exhaust cycle defined with auto stem tip
off, and just need to complete the sealing procedure. We still
remain optimistic that we can run the older tubes on our equipment
and any one that gets one of our re-built tubes will have a tube
that looks brand new capable of many hours of use.
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So, it appears that we are close to having a place to get prewar tubes rebuilt. There are limitations to the type of tubes they can do. Only magnetic deflection tubes with necks about 35mm in diameter are possible, but this includes the 9AP4, 12AP4, and a number of British 7, 9 and 12 inch tubes. Later that month, Randy wrote:
| As you know from Randy's last e-mail, one significant problem we are working on is sealing the electron gun into the tube. We have made a number of good seals but they all develop a crack within an hour or so. There is too great a difference in the coefficient of expansion of the Pyrex stem and the Pyrex neck tubing; there are different types of Pyrex which have different coefficient of expansion rates. |
In July, 2005 Randy informed us that the project had been put on hold, because the company didn't want continue to fund the experiment with no guarantee of success. Glass was located in Germany that Randy felt might solve the problem. The Early Television Foundation offered to pay the $800 cost for the glass to keep the project going. Randy wrote in July:
| You are correct that the project is currently on hold. We have put a lot of engineering time and material cost into the project with limited success. Management current has no additional material expenditures budgeted for the project. Your proposal to fund the special neck tubing to proceed with the development was well received by our management. We have been approved to allocate additional engineering time to the project if you fund the tubing. We feel the tubing is the key to solving our next hurdle but like all projects "Murphy" is always around the corner. We are finding very little information on the material properties for the prewar Pyrex which has made the progress a lot more difficult to develop the process. We know the tubing we have located will be a closer match to the available Pyrex glass stems but we are still guessing on the actual Pyrex glass that was used to make the prewar bulbs. Once we solve the technical issues we feel the actual price to re-build the old prewar tubes and convert them to an aluminized screen will run in the $700 to $750 price range. If you feel the market is willing to pay this price for the prewar Pyrex version then the tubing is worth pursuing. |
I replied that I was confident that collectors would pay $700-750 for a rebuilt prewar tube. As a result, the glass has been ordered and the project is underway again.
In February, 2006 Randy reported that the German glass had successfully been welded to the 12AP4 glass. An attempt will be made within the next few weeks to rebuild a 12AP4.
After several attempts, Clinton was unable to keep the stem from cracking as it cooled, and determined that a custom stem would be required. So far, Clinton has been unable to find a stem at a reasonable cost.
We have some glass tubing that was used in a previous successful rebuild of a 9AP4. This tubing is of an intermediate melting temperature, and can be welded to both the Pyrex shell and the soft glass stem. Clinton is considering trying this approach.
Clinton has sold its rebuilding operation to Video Display Corp., and is no longer interested in pursuing this project. However, a French company, RACS, has recently been successful in rebuilding a Pyrex tube.