prewar Marconi
 

This has been up a couple of days. Anyone know the story?
http://cgi.ebay.com/1938-Marconi-707...item2c5cc4c404

Saw that 'un...Don't know the histoire'...Bet it'll go for Megabux, though.

Being totally ignorant about these won't stop me from venturing a guess anyhow. I'm betting it goes for around $12K.

What am I looking at that says "Pyrex Brand"? Is it the CRT or something covering it?

John

Pyrex makes/made laboratory flasks, beakers, stuff like that, & as such, had lots of experience in commercial/industrial glassblowing. They MIGHT have made the envelope for the CRT...

Yes, that is most likely the CRT, and the Pyrex glass also used in the prewar USA CRTs has been a big obstacle to rebuilding them.

The EMI pre-war CRTs were made from Pyrex (borosilicate) glass. Virtually all later CRTs were made from ordinary glass. Being Pyrex makes it more complex to rebuild these CRTs. The only company in the world that can do this is RACS in France and even they have to use some outside services.

When rebuilding a CRT it is usually essential to fit a new "pinch". This is the part where the lead out wires pass through the glass. Sealing metal to glass is a tricky business as the thermal expansion coeffts have to be matched. Pyrex pinches are no longer available so soft glass ones must be used. Soft glass has a different coefft of expansion so a direct join to pyrex would crack. The answer is a graded seal, using 2 or 3 intermediate grades of glass. Making a graded seal takes glassblowing skill of a very high order.

Wow...The amount of technical "Smarts" that is displayed here & over at AK NEVER ceases to amaze me...

This set was originally in the collection of Danny Gustafson, who sold it before he died to another collector.

Or rather, a graded section of neck glass to 'adapt' the Pyrex progressively to regular glass. It looks like this:


http://www.earlytelevision.org/image...7---Detail.jpg

I like that photo, was it done with polarised light? Whoever did that glassblowing was very good at his job, the profile is very smooth.

Some useful links on the subject:
http://www.transitionglass.com/
http://www.moores-glass.co.uk/graded-seals.html
http://www.robsonscientific.co.uk/qu...aded_seals.php
http://www.glassdynamicsllc.com/Pref...to%20Metal.htm

Hi to All,

Hi jeffrey, Thanks for the super advert for RACS!

Yes, the colored ring photos were taken under polarized light.

Here are other pictures showing the Pyrex to electronic glass transition, this time under ordinary light.

On the left you see the new neck section with "P" for the Pyrex section and 8250 probably a type designation number for the electronic glass section.

The center item show the new electronic glass base assembly with the slender glass tube which will be connected to the vacuum pumps when the CRT is evacuated.

The item on the right shows the CRT's original gun now rebuilt with new cathodes and filaments which has been transfered from its original base (discarded) and mounted on the new electronic glass base. This completed gun assembly is now ready for mounting in the rebuilt tube.

Much skill is needed to rebuild guns and sometimes an entirely new gun must be assembled from scratch re-using the original parts: Wehnelt cylinder, G1/G2/G3 anode assemblies, etc. X/Y/Z positionning of the parts in space must match the original specs to within fractions of a millimeter so that the electron optics performance are satisfactory. This is no mean feat, and for color CRTs (Think 15G..) the amount of work is tripled.

Through experience, RACS discovered an unexpected source of failures when pumping the rebuilt tube: in prewar CRTs, the EHT final anode is a large metal cylinder which goes through the glass and the quality of the seal can be questionable, especially as you are asking a 70 year old assembly to withstand once again the high temperature and pressure ordeal of vacuum pumping.
Also, the anode cylinder is ordinary steel and depending on the conditions of storage, invisible (except under a microscope) rust microcracks and metal fatigue can weaken the through-glass section and make it a failure waiting to happen.

The workaround is to either remove the EHT contact altogether and fill the hole with Pyrex or leave it there but totally resealed with new Pyrex. The EHT contact is then recreated on the CRT's base which is entirely potted in a EHT resistant compound. Tubes with EHTs of up to 12KV have been modified in this manner with no risk of arcing at the base. A male/female adapter is supplied by RACS to mate the enlarged potted base to the CRT's original connection to the TV. The EHT connection is brought out separately on a high-insulation wire to mate with the original EHT rubber-cup contact.

The last two photos show the renecking process where the new neck cylinder is fused to the CRT's bell assembly and the gun sealing at the other end.
About this last photo, it is interesting to note that when the temperature is right, the straight sides of the glass cylinder cave in under the pull of gravity to come into contact with the stem/base assembly. Once fused, the excess glass just falls off under its own weight.

For those interested by the entire CRT rebuild process, my PhotoBucket slide show documents the entire process from start to finish:

http://s281.photobucket.com/albums/k...RACS%20France/

Best Regards

jhalphen
Paris/France

A couple of things bother me . First , the supplied spare tubes , , , , I see a rather newer looking octal there . And is that a mounting for some kind of Tuning Eye setup , , , I dont see one on the tv . Well anyway , beyond the spare tubes , Wont being missing the speaker be a major hit to its collector value ? Yea , Im sure a suitable substitute most certainly can be found , but points off for not being 100% original ? Also , how hard would it be to find/manufacture the broken knob/switch ? .

Course , none of that really matters , , , its at $3150 with 4 days left , , I'll bet it hits 5K easy .

I remember the 707 from some years ago. I recall the following details on this set:

The octal valve (tube) base was introduced to the UK about 1937. The Marconi 707 uses a KT63 for the frame (vertical) output and line (horizontal) output. The KT63's US equivalent is a 6F6!

If I recall correctly, there are four octal tubes in the 707: the frame oscillator, frame output, the line oscillator and the line output.

The 707 is an early superheterodyne for vision and sound. When this set was built, Alexandra Palace broadcast the BBC vision signal carrier (45MHz) with full double sideband modulation. The AM sound carrier was at 41.5MHz. In the Model 707, the sound IF (for AM sound) was 4.5 MHz and the vision IF was 8MHz. The earlier Marconi EMI TV models were TRF vision and superhet for sound only.

Ah , so there is an octal or few in there , , , well that explains the newer looking tube . The bidding seems to have chilled a bit at the $3150 price level , , its been at that number for almost 24 hours .

If I spent $3500 for a new big screen , I'd be in the doghouse for a while , , , , , ,

But If I spent $3500 on an awesome antique that I'll never watch , , , they'd have me commited to the nuthouse !!!!!!

Sure would look good in my old radio room .......................... :D

I suppose all prewar sets fetch good money these days. The 707 eventually sold for US $7,601.54.

Is the buyer on this forum?

I am still amused how Ebay bidding usually goes. The set went from about $3400 to $7600 in the last thirty seconds of the seven-day auction.

That is because many people (including myself) use snipe bidding services.

Out of my league.... And I'm astounded that a British prewar set went that high at all! They're nowhere near as rare as USA prewar sets.

Charles

I still bid on Ebay the old-fashioned way, "sniping" manually. :)

Actually, I wonder how that term started, because it implies cheating or something else negative. I figure, bidding early just exposes you as a target for anyone else to beat. There is nothing wrong or sneaky about wanting to keep your maximum price hidden.

That would be a very nice set to own, but I would much rather have a prewar set from here in the USA. Not likely to ever happen, though, with my budget...

There were certainly many more British prewar sets made, but Steve's data base shows fewer remain than American ones.

Darryl

I'm sorry, but these things just DON'T really "Excite" me...It likely won't work, & quite possibly CAN'T be made to work, either. Part of the fun of having these things is to turn 'em on once in a while, & at least show a raster to amazed guests...Or showing off the "Glorious Lollipop Color" of a well-tweaked Roundie...There IS a "Kick" from having a pre-war TV, but not THAT much of one if you can't at least show a raster...

I just figure how much I'm willing to spend, and I place the bid with that value. If someone wants it more (bids higher) so be it.

These sets can be restored to work. Standards converters are available to produce the 405 line signal. Even the CRTs can be rebuilt now.

Also, VHS tape holds 405 lines pretty well (apart from the lack of drop out compensation). Considering that the bandwidth of 405 is only 2.5 to 3MHz, VHS quality looks a lot better on 405 than it does on 525 or 625.

Hmmmm...Well, I thought since Scotty has retired, that was it as far as rebuilt CRTs went. I KNOW the electronics can generally be rebuilt, & even the standards issue isn't THAT hard to overcome, if you're really determined, but I thought that the door had closed on CRT rebuilding... Its all academic for me, though, 'cause a prewar set is so totally out of my means to be not even worth considering.

If it's a Pyrex tube, it would have to be sent to RACS in France for now. But I hold out hope that something will materialize with the ETF's efforts. Too bad I couldn't volunteer myself to help out with the project, I just have too much other stuff going on ATM.

The guns on the 9" and 12" pre-war EMI CRTs are a particularly complex hexode design. Not sure how much needs to be dismantled to replace the cathode/heater assembly.

I hadn't heard about the EHT connector problem. Possibly because the 6/6 that I had rebuilt in 1986 was pumped cold. There were 2 reasons for this. The CRT was too long to fit in the available oven and the guys doing the glassblowing were worried that the stresses caused by heating could crack the glass. I'm not sure about the stress problem, the heating might well anneal the glass and make it stronger. I admit that for that project we had a choice of 5 dead EMI 6/6 CRTs. The choice was made on the basis of which glass had the fewest bubbles and other defects.

Jeffery,

Just out of curiosity, how long did you "cold pump" that EMI crt to get a good vacuum? WE had been considering a low temperature pump down, in the range of 450 to 500F for our 15GP22 rebuilding project, but we did not get that far. We have a special epoxy that we wanted to use to seal the leaks, but it degrades around 500F so we were considering using a low temperatur pump cycle, but we had not data as to how long it may take at a lower temperature. It would be useful to know how long it takes to pump down a crt that is at low temperature versus at 650F in a pump down oven.

Thanks!

Sorry, can't help you there. I wasn't present during most of the rebuild, I only co-ordinated the different companies that were involved. The guy who did the work is now dead (at least I assume so, he'd be about 115 now) and had vast experience. I don't know what other knowhow he used for doing a cold pump. One thing I remember is that while the gun was being rebuilt he rough pumped the tube and sealed it off. This was mainly to minimise damage to the phosphor as it was not being rescreened. I think he also used double the normal amount of getter.

For you, I suspect it's a matter of doing some tests with worthless CRTs and seeing how they respond when pumped cold. You have the additional unknown of how the epoxy will outgas during pumping so you might put a bit of that in the worthless CRT for test purposes.