Earlier this summer I found this radio at an auction and bought it because I needed a Philco fixerupper project for the Winter. So, I am just beginning on the project and as I get more deeply into it I will post more about it. Here is a photo of it and as it shows, it is in good condition with a few common issues like pushbuttons, grill cloth and dial cover.
Also I found out that someone had changed the 6A8 tube and socket for a 6A7 tube and socket.
G.
(11-03-2014, 11:54 AM)philconerd Wrote: [ -> ]Also I found out that someone had changed the 6A8 tube and socket for a 6A7 tube and socket.
Could be a wartime repair, as parts were short and repairmen had to do the best they could to "Keep 'em Flying" with whatever they had on hand. A 6A7 is electrically identical to a 6A8 but of course the pinouts are different with different basing, too.
That should be a nice project for you. Don't forget the rubber-covered wiring
including the wire leads from the power transformer. Good luck!
The 6A7 was the predecessor to the 6A8, 6A8G, or 6A8GT, electrically the tubes itself has the same specs except for one detail, the inter-electrode capacity is definitely higher then that of a 6A8 or 6A8GT, perhaps even a 6A8G as well. In this case it won't matter much, the 39-30 only has two bands, and the shortwave band runs from 4.9 mc to 18 mc, so the inter-electrode capacity is NOT an issue. I don't know is these sets were all equipped with 6A8Gs from the factory or whether some were built with 6A7s, Philco was notorious for changing tube layouts in their sets if they got a deal on one type of tube verses another, if the 6A7 socket is held in wite screws and nuts then a serviceman changed it, if it is held in by hollow rivets it may be original.
Even so the shortwave band is more or less just a token feature on this set, with a pentagrid converter, no RF amp (tuned or untuned), or an extra IF amplifier stage, the set is not going to be very selective or sensitive on shortwave anyhow, to say nothing of the frequency spread. The addition of a 76 tube for AVC seems rather pointless, I think the sales and marketing department had the engineering department add it just so make the 39-30 chassis add up to six tubes, rather then five, to claim it was a better set then a 39-25.
Regards
Arran
(11-04-2014, 12:26 AM)Arran Wrote: [ -> ] The 6A7 was the predecessor to the 6A8, 6A8G, or 6A8GT, electrically the tubes itself has the same specs except for one detail, the inter-electrode capacity is definitely higher then that of a 6A8 or 6A8GT, perhaps even a 6A8G as well.
Yes, but as you said, in this case it does not really matter.
Quote:I don't know is these sets were all equipped with 6A8Gs from the factory or whether some were built with 6A7s, Philco was notorious for changing tube layouts in their sets if they got a deal on one type of tube verses another...
As far as I know, they all had 6A8G tubes from the factory. Philco didn't do that det-osc tube switching until the following (1940) season where some radio models use a 7J7 and others of the same model use a 6J8G.
Quote:The addition of a 76 tube for AVC seems rather pointless, I think the sales and marketing department had the engineering department add it just so make the 39-30 chassis add up to six tubes, rather then five, to claim it was a better set then a 39-25.
Agreed. There really is no difference, electrically, between the 39-25, 39-30 and 39-31, save for the fact the latter two have an extra tube handling the 2nd detector function and AVC. In the 39-25, the 75 tube (being a dual diode, triode) handles 2nd detector, AVC and 1st audio functions. In the 39-30, the 2nd detector and AVC are handled by a 37 tube connected as two diodes while the 75 is used only as an audio amp.
Philco did this again in the 1940 and 1941 seasons on a few of their models. Nothing like padding the tube count.
Well, anyway...
Philconerd, the 39-30 is still a very nice radio and a surprisingly good performer. I own the Canadian version, the 39-330AT, which is very similar to the USA 39-30T. The main difference is six pushbuttons instead of eight. Being Canadian, it also has cloth-covered wiring instead of rubber, although the IF transformer leads did have that awful rubber stuff which I replaced. I think you are going to like this one.
Hi Guys.....
Thanks for the information.
This radio must have come from my area of the country when new because two of the push buttons are marked for WCCO and KSTP stations.
I took a closer look at the socket for the 6A7 tube and it's made by American Phenolic Corp. Chicago. This type socket was added by a repairman and is held in place with a retaining clip under the chassis. I can see where they removed the hollow rivets holding the original 8 pin socket to the chassis.
Yes, the poor rubber wiring from the power transformer must be repaired. Other than that the power transformer ohms ok.
G.
It's up to you of course but if it were my set I would probably change it back to using a 6A8G, as the 6A7 normally uses a larger and taller envelope. As both Ron and I mentioned, electrically it won't make any difference electrically but I don't really like keeping mods if putting something back to original, if feasible, which is one of the reasons I also remove aftermarket phono jacks and switches. The exception being something like Ron's recent 42-720 project that has a bad power transformer but originally had a 6X5 rectifier, it's difficult to find a tube type power transformer without a 5 volt rectifier winding so why not change it to a more rugged type that also has an octal base.
I know the type of socket you mean, Amphenol made decent sockets, some molded Bakelite, other Micanol, others porcelain, you could get their sockets both with and without a mounting plate, both use that clip or retainer. If you look at the bottom of the socket you will notice a series of key-ways to keep the socket in place on it's plate, which has a locating key. Amphenol sockets and connectors were very popular with test equipment manufacturers, many tube testers have the sockets mounted the very same way you describe, but it means that the socket sticks up above the chassis/panel up to 1/8'', and if the socket is in line with a row of others along the back edge it's pretty noticeable.
Regards
Arran
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attachment=5833]
This photo of the 7 pin socket will be helpful to show the location of the 6A7 tube on the chassis.
Good old Amphenol type S socket. Well, it isn't original, but as I said before, it may have been done during WWII because of wartime shortages. So it becomes a question of "Do I put it back to its original state, or do I leave it as a reminder of a possible wartime repair?" It is up to you.
One thing is for certain - that Amphenol socket is far better quality than the original light tan Phenolic, which Philco liked to use for the det-osc tube in many 1939-1941 models, and which have a tendency to fall apart under stress of heat just when you are soldering on one of the pin contacts. Don't ask me how I know.
I thought that the 6A7 or 6A8G may have been in a location like that, at least the socket is not too noticeable from the back. Depending on the quality of the installation it may or may not be worth changing back, it's something you could do whilst the pushbutton bank and IF cans are rewired. Some servicemen did decent work, others liked to cut things out, tack things together, or piggy back old components with new ones.
Regards
Arran
I just thought of another possibility - that light tan socket could have been damaged by a previous serviceman, who then replaced it with the Amphenol socket and 6A7 tube. Who knows.
Ron;
What I meant by putting back to original would be putting a 6A8G or a 6A8 or a 6A8GT, not the original wafer type socket, those were so so at the best of times, the octal ones used by Electrohome in the late 1940s were horrible. The large wafer sockets Philco used in some of their Transitone sets were almost as bad, they sometimes crack from the stress. If I were switching it back to using a 6A8 it would get a molded Bakelite, Micanol, Micalex, or a ceramic socket put in there, depending on what sizes and types would fit the existing holes.
Regards
Arran
I know, Arran. I just mentioned the original socket because those light tan sockets are very fragile and easily damaged by heat from a soldering iron, and could have been a reason why the socket was changed - maybe whoever made the change did not have an Amphenol octal, but did have a 7-pin socket and a 6A7 tube.
The brown Phenolic wafer sockets Philco used seem to hold up much better than the tan sockets. I misspoke when I called the tan socket Phenolic, it must be made from some other type of material that, like I said, is very fragile but perhaps exhibited lower loss properties than the brown Phenolic?
This sure is a great looking radio. One of my favorite designs.
Thanks All. This will probably be my final post about the 6A7 tube and socket in this section.
So I have been watching the YouTube videos by
bandersentv for restoring this model Philco and they were very helpful.
I have decided to leave the 6A7 tube in this radio and do the repair work necessary to get it playing. I will be doing the repair of the power transformer first and so I will continue posting in the Elecrical Repair section of the forum.
G.