Hi, I am new to tube electronics, I purchased an old elmco pa/phono amp, one 6v6 tube glows, the other doesn't. on the tube that doesn't, the #7 pin has voltage, the #2 pin doesn't. The #2 has a cap/resistor that has one green stripe going from the #6 pin of the tube that does glow. Then the #2 pin has a postage stamp capacitor going to the microphone pot.

Does any one have any insight on what needs to be replaced and how i can figure what to replace it with? Thank you.

Check resistance of the tube on pins 7 and 2; should be low. If not, resolder the pins. Check connections to those pins on the socket. Resolder them even if they look OK.

For the tube that is not lighting, pull it out of the socket, then check the resistance between pin 2 and 7. It should be about 12 ohms. If it is infinite the tube filament is open and it will not work.

You cannot do a valid resistance test of the tube filament while it is plugged in. All you will do is measure the resistance of the filament transformer (fraction of an ohm).

Curtis Eickerman

Thank you for you help. when i tested the tube socket that works, it showed no resistance at all, the meter went past 0 between #2 and #7, The tube socket that doesn't work didn't show anything. The meter didn't budge.

also, i tested it from the back side, from the actual solders, with the same results.

If you have a soldering iron try resoldering the heater pins. Hotter running tubes like power tubes sometimes develop poor connections.

Perhpas I was not clear.

Pull the tube out and check the resistance between pins 2 and 7 of the tube itself. You are trying to find out if the tube is burned out. The tube filament should read about 12 ohms. Not pins 2 and 7 of the socket.

Curtis Eickerman

Hi Curtis,

I am sorry, the amp didn't come with tubes so i bought new ones, if i reverse the tubes the one socket consistantly doesn't glow.

You may need to resolder the connections on the socket, or on the wire that goes to the socket; the poor connection could be at another socket.

There is the possibility that the tube socket does not have a good connection where the tube pin touches. To tighten them, use a large sewing needle and force it between the metal connection and the bakelite. This will make the hole smaller.

No problem. I just wanted to make sure you had a good tube in the socket before chasing circuit problems.

It appears that the filament connection from the transformer to pin 2 is missing or extrememly poor for some reason.

On the good 6V6 socket with the tube removed check between pin 2 and the chassis/ground. See if it is 0 ohms. Then do the same thing at the bad socket.

I believe the two pin pin 2 connections are supposed to be connected together or both connected to the chassis/ground and am wondering why one is not.

Curtis Eickerman

thank you for all your help. I am still stuck. see if this makes sense to you.

the socket that works has 0 ohms from the #2 and #7 "terminal" to ground and has wires coming from the transformer.

there is a jumper wire from the #7 to the #7 socket that doesn't work. this also has 0 ohms when grounded.

Now, the #2 terminal from the socket that works is jumpered to the #8 on the socket that doesn't work and this grounds out to 0 ohms.

The #6 terminal of the socket that does work is connected to the #2 terminal of the socket that doesn't work by a cap/resistor that has a single green stripe. The #6 terminal grounds out with 25 ohms.

Is it possible that this cap with the single green stripe is the problem? and would you have any idea what this cap could be?

The number 2 pin of the socket that is not working needs a connection to the number 2 pin of the socket that is working. This can also be accomplished by a jumper wire from pin 2 to pin 8 on the bad socket.

There might be additional problems, but that for starters should get the tube "fired up".

Curtis Eickerman

thank you very much. that makes sense to me also. I will let you know how it works out.

It sounds like either the socket is wired wrong or perhaps it was designed for a different tube. Pictures?

Hi Guys,

Mescalero got me thinking and I checked out this link below. My confusion about why the heater jumper goes to pin #8 instead of #2 is consistant with the wiring for a 6SL7 preamp tube. I was using two 6v6's. I think I should purchase a 6SL7 before I change the wiring. Does this make sense?



http://www.docstoc.com/docs/43944071/VA ... OUT-SHEETS

I think that we made the assumption that the two 6V6 tubes were in a push-pull amplifier. You said that it was an Elmco; do you have a model number?

My assumption was that you plugged the correct tubes into the amplifier. If you don't know what the correct tubes are, there is nowhere to go from here until that can be determined. Amplifer model number, schematic or some other useful information becomes needed. Sorry about that.

Curtis Eickerman

Ok guys unfortuately I couldnt up load pictures, it says the files are too big. I had to put them on photobucket, the link is below. The black metal tube is a 6SC7. When I got the amp it was missing the other tubes and I read that 6v6's were used on amps that used the 6SC7 tube. So I put two and two together and got two 6v6's because it looked like they belonged.

There is no model number anywhere on the amp and I couldn't find a schematic either.

The link that I put in the previous post has pinouts for different tubes and the only ones that showed heater pins at #7 and #8 were 6SL7, 6SN7, 12SL7 AND 12SN7. I know that it is not too scientific but I figure that one of these pre amp tubes must be the one that fits.

I think I might try this. If anyone can suggest how I should proceed next, I appreciate your help.

Thank you again for all your help.

http://s1170.photobucket.com/albums/r536/jawittdc/

You can forget the 12--7 tubes since I think you have established that you have 6 volt filament wiring in the amplifier.

The 6SN7 and 6SL7 are interchangeable pin-wise. The 6SN7 is the one that is most commonly seen. It has higher gain than the SL, but also higher filament current. They are both dual triodes. Are there any other tubes in this thing?

Maybe the best approach is to draw your own schematic by documenting what is in your amplifier, assuming either of these tubes, and see if the result makes sense. It's just a thought.

I'd be leary of just plugging one of these in and just flipping the switch to see what happens, but it's your amplifier.

Curtis Eickerman

Ridiculous! If you don't know what ou have, no schematic, or even a hint of what tubes are supposed to go where, please recycle the set and find a new hobby.