I have a Canadian 6a7m Rodgers tube that has spray shield on it, now I can replace it with a 6a8g but have to add shield which I can do but is there a product that I can apply on the tube instead like original? These tubes a hard to find. The tube I really badly need is a 6f7m , if I cant find of those I will have to rewire tube socket for a 6b8 tube
(06-08-2015, 07:34 PM)Fred Taylor Wrote: [ -> ] I have a Canadian 6a7m Rodgers tube that has spray shield on it, now I can replace it with a 6a8g but have to add shield which I can do but is there a product that I can apply on the tube instead like original? These tubes a hard to find. The tube I really badly need is a 6f7m , if I cant find of those I will have to rewire tube socket for a 6b8 tube
Fred;
A 6A7M can be replaced with just a 6A8 metal type, no G. Another option is to add a Goat shield to a 6A8G. There are two substances that may work on a G type octal tube to recreate a spray shield, one is slip plate which our own Bob Andersen uses to replace the aquadag on picture tubes, the other is a type of spray paint that some auto parts stores sell to repair rear window defoggers. The problem with the slip plate is that it is not that durable, I would guess that the rear window defogger stuff may be more tough, apparently British and European radio restorers use the rear window defogger paint to repair Phillips/Valvo/Mullard and other tubes with spray shields.
As for the 6f7M from what I can find a 6F7G and a 6P7G can be made to work since they are all triode-pentodes, though you may still have to rewire the socket to use a 6P7G, or make an adapter in the case of the 6F7 with an older seven pin socket and an octal tube base. Perhaps you were thinking of a 6B7M which is a dual diode-pentode, that can be replaced with a 6B8 or 6B8G by rewiring the socket. By the way, the name of the company was "Rogers Tubes LTD." no "D" in Rogers, just as "Rogers-Majestic LTD".
I have a Canadian Marconi Tube manual from 1946 that has specs listed for most of the Rogers type tubes like 2X3s, 2Y3s, 6H7S, 6H7M, 6X6, etc., so I will look up some of these types and see what the cross reference is. I also have a few copies of a Rogers tube substitution pamphlet around here but I don't know where I put them.
Regards
Arran
Thanks Arran but I wont need one now. It turns out that I had tested the tube the wrong way and it is good. I do have the rogers tube chart and the replacement sub chart also.
Fred;
I looked up the 6F7M and the 6P7G, the tubes are identical electrically other then the pinout, same voltage and current ratings, same amplification factors, same transconductance. I think that like many tubes decided to make a spray shield version of a standard RMA tube type such as the 6F7 by coming out with the 6F7S. When octal based tubes hit the market in 1935-36 they decided to manufacture an octal version of the 6F7S and called it a 6F7M whereas when either RCA or Sylvania decided to make an octal version of a 6F7 they called it a 6P7 or a 6P7G but used a different pinout. The reason I looked into this is that I just acquired a Rogers set that happens to use a 6F7M, but unlike your Deforest-Crosley someone has replaced most of the spray shield tubes with metal ones, and I have not investigated if they replaced the 6F7M tubes with something else or how they did so.
Regards
Arran
P.S What model is your Deforest-Crosley? There should be a name stamped or stenciled somewhere in the back of the cabinet. Also what's the chassis model number? It should be something like 7DXXX.
Hi Arran, the name is Lyra and the number 6d932. When I tested the 6f7m tube first I look it up on chart then set all the controls for that tube and test it, well the chart did not ask if it was a Rodgers tube and I didn't know but there's a big difference and the rogers tube has more pins. I'm glad I didn't fry the tube. Its ok thou, thank GOD, that's a hard to get tube like you said. Also Arran , the tuning light on this, does it only come on when coming up on a station , its right In the center of dial with ring around it ?
Fred;
I'm not sure how the tuning light or lights are supposed to behave in this D-F Lyra model, on my Rogers 10-60 the tubing light stays bright when it's off a station, but dims when it's on a station. Some of these Rogers built sets had a tube dedicated to the operation of the tuning light or lights, I can't tell whether this set has one or not, but the light is connected in parallel with a 8600 Ohm resistor, and in series with two 10K ohm resistors, and then those are paralled with a 20K Om resistor, in the B+ circuitry. If the diagram on Nostalgia Air is correct then this set should also have variable IF bandwidth, which is what the arrows through the IF transformers are supposed to represent, it's a form of fidelity control, try playing with the tone control and see if the tuning light changes it's behavior.
Regards
Arran
I thought it was the other way around, Fred. Bright between stations, and then dims when you come up on a station. That's the way it was described to me, but I haven't actually seen one in operation yet.
Hi Fred.. that tuning system is as you said.. The light gets dim when you are on a station and brighter when you are off the station.. Some thing that most don't know about this system is that the bulb is a 24 volt 3.5 MA .
At the moment I can't remember where I got mine .. I did buy it on line from a supplier. Right now my computor is down so I don't have access to my favorites and that is where I have the name of the company I bought it from.
If I remember correctly, the MA is quite critical for it to work properly.
The rogers 6F7M tubes are hard to find , but I have a fellow in town here that says he has a few.. If you like I can see what he wants for one.
Dan in Calgary
(07-15-2015, 12:07 AM)Dan Walker Wrote: [ -> ] Hi Fred.. that tuning system is as you said.. The light gets dim when you are on a station and brighter when you are off the station.. Some thing that most don't know about this system is that the bulb is a 24 volt 3.5 MA .
At the moment I can't remember where I got mine .. I did buy it on line from a supplier. Right now my computor is down so I don't have access to my favorites and that is where I have the name of the company I bought it from.
If I remember correctly, the MA is quite critical for it to work properly.
The rogers 6F7M tubes are hard to find , but I have a fellow in town here that says he has a few.. If you like I can see what he wants for one.
Dan in Calgary
Dan;
From what I can tell whether you really need a 6F7M or a 6P7G depends on how the tube was originally used. In the case of the Rogers 10-12 and the Deforest Crosley Buckingham the two 6F7Ms had their pentode section used as IF amplifiers, the triode section of each 6F7M was hooked up as a diode with one for second detector, the other as an AVC diode. So there are three ways to handle this if you can't find some good 6F7Ms: (1) Rewire each 6F7M socket to work with a 6P7G (2) Make an adapter to use a 6F7 seven pin in place of 6F7M which is octal. (3) Rewire one socket to accept a boring old 6K7 and rewire the other to accept a 6B8 which is a dual diode pentode.
Regards
Arran