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Tubby i mean tube ? - Printable Version

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Tubby i mean tube ? - Fred Taylor - 06-08-2015

 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 Icon_think

RE: Tubby i mean tube ? - Arran - 06-09-2015

(06-08-2015, 08:34 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 Icon_think

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

RE: Tubby i mean tube ? - Fred Taylor - 06-09-2015

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.

RE: Tubby i mean tube ? - Arran - 06-10-2015

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.

RE: Tubby i mean tube ? - Fred Taylor - 06-10-2015

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 ?

RE: Tubby i mean tube ? - Arran - 06-11-2015

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

RE: Tubby i mean tube ? - palegreenthumb - 06-11-2015

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.

RE: Tubby i mean tube ? - Dan Walker - 07-15-2015

    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

RE: Tubby i mean tube ? - Arran - 07-15-2015

(07-15-2015, 01:07 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