[Patrick]

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Apparently in run 4, the 7J7 was replaced with a 6J8. I can find the pinouts on the 7J7, but I can't find anything on the 6J8. Does anyone have not only the pinouts of the 6J8, but how it should be wired? I'm replacing resistors around that tube, and R29 keeps heating up. Don't know if the radio ever worked or not. Caps have all been replaced. There is also a 150ohm resistor added to the plates of that tube, but I'm not sure where as I don't know the pinouts. Can someone help me out?

[Radioroslyn]

http://www.radiomuseum.org/tubes/tube_6j8.html
Check voltage on triode plate (29) nom 150vdc  if low check for shorts and #29.
Terry

[mikethedruid]

Here is data on the 6J8 http://www.radiomuseum.org/tubes/tube_6j8g.html . And here is the 7J7 http://www.radiomuseum.org/tubes/tube_7j7.html . They are electronically the same tube, but based differently, and the 6J8 uses a cap.. Hope it is of help

[Patrick]

http://www.radiomuseum.org/tubes/tube_6j8.html
Check voltage on triode plate (29) nom 150vdc  if low check for shorts and #29.
Terry

That was one of the first things I did, and I noticed the voltage was about 90+. But how would low voltage make the resistor heat up?

Pat

[Patrick]

Here is data on the 6J8  http://www.radiomuseum.org/tubes/tube_6j8g.html  .  And here is the 7J7  http://www.radiomuseum.org/tubes/tube_7j7.html  . They are electronically the same tube, but based differently, and the 6J8 uses a cap..  Hope it is of help

Thanks Mike,

That's what I needed. It's a start.

Pat

[Radioroslyn]

Hmmmm... Seems like you got 60v missing. Pull the tube and see if it going up to 150v if so may have a bad tube or something is causing it to haul down a lot of current (incorrect voltages on the grid or cathode).

Terry

[Patrick]

http://www.radiomuseum.org/tubes/tube_6j8.html
Check voltage on triode plate (29) nom 150vdc  if low check for shorts and #29.
Terry

That was one of the first things I did, and I noticed the voltage was about 90+. But how would low voltage make the resistor heat up?

Pat

Whoops. Engaged mouth  (or fingers) before brain. Forgot my basics.

[Patrick]

Hmmmm... Seems like you got 60v missing. Pull the tube and see if it going up to 150v if so may have a bad tube or something is causing it to haul down a lot of current (incorrect voltages on the grid or cathode).

Terry

I think that my biggest problem is that it's been almost 50 years since I've done basic electronics work. I had forgotton who Kirchhoff was, amongst others. Time to get the old textbook out.

[Radioroslyn]

Don't forget MURPHY too. I don't think he's in the book.
Terry

[Raleigh]

I recently finished a 40-190 with the same 7J7 to 6J8 change. The schematic shows only the 7J7, but in mine, the wiring and components to the 6J8 were the same as the shown on the schematic, taking into account the connection to the 6J8 grid cap wire instead of a grid pin on the base.

[Patrick]

I recently finished a 40-190 with the same 7J7 to 6J8 change.  The schematic shows only the 7J7, but in mine, the wiring and components to the 6J8 were the same as the shown on the schematic, taking into account the connection to the 6J8 grid cap wire instead of a grid pin on the base.

Mine has a 150 ohm resistor from pin 6 of the 6J8 to ground. Maybe that's why the 4.7K (R29) is getting hot. Even with the 6J8 pulled, I get 140V before R29, and less than 5V after it (right on the tube socket pin).

[Radioroslyn]

Disconnect the wire from the socket (where the 4.7k is attached)and see if the voltage goes up. If so your socket has a internal short. If not check for shorted rubber wire going to the pushbuttons.

Terry

[Patrick]

Disconnect the wire from the socket (where the 4.7k is attached)and see if the voltage goes up. If so your socket has a internal short. If not check for shorted rubber wire going to the pushbuttons.

Terry

Thanks Terry,

I hadn't even considered a shorted socket. I did find a pic online of the 150 ohm resistor on the chassis. It is attached to an inuslated standoff screwed to the pushbuttons. I've circles it in red. 

   

But I cannot find it on the schematic.  The schematic show only two 150 ohm resistors, #34 and #60, which I can account for. 

Pat

[Radioroslyn]

After taking a second look I think I would unground that resistor. Circuit wise it doesn't make a lot of sense cause it will pull the plate voltage on the osc section of the mixer way down. Tube manual sez the triode section can take up to 250vdc which less than the chassis is providing. Is your 4.7K resistor in spec?

Terry

[Patrick]

After taking a second look I think I would unground that resistor. Circuit wise it doesn't make a lot of sense cause it will pull the plate voltage on the osc section of the mixer way down. Tube manual sez the triode section can take up to 250vdc which less than the chassis is providing. Is your 4.7K resistor in spec?

Terry

Yes. I'm going to keep this radio, So I've replaced all of the resistors, and checked the new ones as I install them. I'll go eliminate the possibility of a shorted socket first, then go from there with your second suggestion. But only after another cup of coffee. It's almost 8 A.M. here.

Tanks,

Pat