Shield Grid 9 Renewing...........

[Steve Larrabee]

Hi Kent:  I am working on a Shield Grid 9 now that is in very good shape. I have dismounted the SGA (shield grid assembly) because of low filament voltage on the three 22s - max I can get is 2 volts with 5 volts on the entry bus.  Taking that SGA off and opening it was much more of a chore than necessary because the designer apparently didn't think it would ever be required.  The paperwork discusses the short wave ability of the SG-9 and the need to change the coil in the 630 Antenna Coupler copper box (+ the .0008 added capacitor).  Only one of the 4 copper housings has 2 bolts mounting it to the bakelite.  Do the covers of the other three copper housings pull off? or how do they come off?  Is desoldering the lugs from under the chassis to free the assembly required?  The bakelite is so fragile that I don't want to apply any damaging pressure.  Any ideas/help greatly appreciated! 

[Steve Larrabee]

I apologize for the apparent drop off on this topic - I could have sworn I wrote a closing comment after I got the SG-9 going in mid-April.  I had the SG-9 working very well with delicate tuning of both tuning shafts running separate variable condensers + 1 for the RF and one for the oscillator .  By matching the dial scales on both, and then tuning the matched dials together, I was picking up stations quite well - some near and some far away.  The tuning was quite broad and careful manipulation of the variable caps and the RF tube filament voltage controlled the oscillations. I ran the 22s at the 3 volt max which was full CW on the right side rheostat - with 6vdc in for A+. I ended up using aluminum foil around the glass on the 22 tubes in the SGA -- they are not seen with the SGA copper cover installed.

This set was a challenge but also a marvel when you consider this was the 1928 real thing from  the Scott Transformer Company of Chicago, ILL.  A pleasure and thank you all again for the help!

very best, steve

[Joseph W Strickland]

Steve;

Here is one source for large OD aluminum tubing in shorter lengths at reasonable price that I use:

https://www.dxengineering.com/parts/dxe-at1506, Aluminum Tubing, 6063-T832, 3 ft., 2.000 in. OD, 0.058 in.Wall, No Slit, $6.60 each. The ID should be more than the typical 1 13/16 OD of a globe tube like the 01A types. They also have the same length in an OD of 1.875 but the ID I am not sure about.

Joe

[Steve Larrabee]

Thanks, Norman:  I'll make some presentable shields, ground them and then cover them with the very nice copper lid of the SGA -- and only 550 people will know.  best,

[Joseph W Strickland]

Norman;

Were either the Scott or Remier shields plated?

Joe

[Norman S Braithwaite]

The original cast pot metal Scott tube shields are unobtainium but they are close copies of the Remler cast pot metal tube shields. They were even the same color! The Remler tube shields are hard to find but you are more likely to find the Remler version than the Scott. Norman

[Joseph W Strickland]

Steve;

OK, then it does indeed need additional shielding around the tubes even inside an already shielded enclosure. It is relatively easy in an IF strip to have oscillation develop if everything is not just right, particularly when the circuit is designed for best gain. Mr. Scott was going for lots of gain compared to other brands. One thought I had not long ago was to look and see if I could find a source of large ID copper or brass tubing to make a shield for an IF transformer. So far I have not found a reasonable source for the size I was looking for. I wonder if the OD of the globe type tubes was standardized or whether different manufacturers varied from a nominal size. Tolerances can sometimes result in trouble if the range is not known and a different source does not adhere to a specification well. There might be some sizes of aluminum tubing that would be large enough to use.

Achieving a good long lasting connection for a ground would be the issue with aluminum. In assembling aluminum tubing type antennas I have used Penetrox conductive paste. There are two types, one has ground up zinc particles in it and the other has ground up copper particles in it. Some manufactured amateur radio antenna kits include the copper paste to use with aluminum tubing connections where good lasting RF connections are exposed to weather. The copper product would serve well with aluminum using a short length of wire, star washer, nut and screw or a solder lug with an internal star washer end.

If you manage to locate some good OEM type shields that would be preferred from the standpoint of keeping it as original as possible.

Joe

[Steve Larrabee]

OK -- The Shield Grid 9 article (starting on page 96) in the Info Archive parts list contains 4 #680 tube shields and the article further on mentions each tube in the SGA is individually shielded.  As I mentioned, I tried some homemade shields and they helped - I can make those better if no one has the real shields or other shields that will fit + work.  Atwater Kent had some really heavy metal shields they used on their critical tubes - some grounded with a grid cap style connection and others with an open top for the tube grid cap when it had one.  Just hoping someone has some in the "box" they are not needing.  The lid for the SGA does shield the whole assy from the rest of the radio, but the individual tubes evidently need their own shields, too.  Thanks, best,

[Joseph W Strickland]

It would be nice if someone could post a picture of one globe tube shield here so we know what to look for. In the pictures of some Shield Grid 9 receivers shown in the Collector's Guide it appears that the copper shielding you removed was to serve that purpose, so are you wanting round shields that you can use for temporary shielding while you work on the unit? Or alternatively you might fabricate something you can use while working on the set. Some aluminum sheet thin enough to work easily with tin-snips and a few nuts and screws to close the ends that have to meet to form a circular shield, attach a suitable length of wire at one of the screws and nuts with an internal star washer for good contact and then slip it over a tube, and ground the shield to RF/IF circuit ground. If you have some old aluminum pie tins lying about you might be able to use material from that to make shields.

Joe

[Steve Larrabee]

I put the SGA back onto the bakelite plate chassis and wired it up after I reformed all the solder connections on the set.  It's working for the first time: with a lot of oscillations but good control on the front panel.  It is tricky tuning.  Problem with oscillations I think can be helped with shields for the tubes in the Shield Grid Assembly.  I made some with 3x5 cards wrapped with aluminum foil and grounded to each other and that cut the osc down quite a bit.  Does anyone have some shields for the old globe tubes they can spare/sell - there are 5 tubes in the assembly with 3 of them having grid caps on the top (22s).  Great to get the SG9 going........Thank you all for your help!!! best, Steve

[Joseph W Strickland]

If the set used variable air trimmers to peak align the IF's, it may have had trimmers that allowed them to tune anywhere near the 130kHz frequency. Maybe it was just misaligned? Or perhaps had some fixed value padders remove or added?

Yes the set of jumpers looks challenging!

Joe