Swr 750x pre-amp problem!

[robo]

Hi,have a wierd problem in the tube pre-amp input area of this SWR 750X.
The 1st half of the 12AX7 has full plate voltage both sides of the 100k plate resistor,and a 1Khz sine wave is clipped off at the top like a distortion circuit would be?The 2nd half(same value 100k plate resistor) is working normally.Tried also with a few new 12AX7's with same problem. Can anyone advise why this could be happening?All othe reistors are measuring ok and no burning etc.
Also if a 12AT7 is substituted it works fine!??
I am baffled!Can someone shed some light.I replaced the plate resistor with no difference.
Would apprecite greatly a SCHEMATIC for this amp too?Thanks.

[g1]

Can you see that both sides heaters are lit?
What are the plate and cathode voltages for the bad side with 12AX7? With 12AT7?
Did you check the cathode resistor?

[Jazz P Bass]

Agreed, check the cathode resistor.

You should see a voltage drop across the 100K plate resistor.

If it is not dropping, the tube is not conducting.

[Enzo]

Your 12AX7 was not working, but you stuck a different tube in there and it works? That is pretty much a certainty that the first tube was bad.

[robo]

Hi,thanks for the help:
G-one:
.. the Heaters are lighting up and the ac 6.4v is present.
The cathode resistor measures ok at 1.5k and has a few mA voltage for the 12ax7 and virtually no voltage dropped on the plate resistor = 230v HT side;229v plate side.
With the 12AT7 in it conducts correctly with=230v HT side and 179v plate side.
Enzo:
I substituted 3x new 12ax7 and all acted like this with no conduction on one side,this is the 1st stage input,the 2nd stage input works fine but the signal has been chopped off like a square wave from the 1st stage.Now the wierd bit is when a 12at7 is used BOTH SIDES conduct normally and produce a clean signal?!This is what has baffled me.
Does anyone have the circuit diagram of the amp i can have a look at?
I am assuming the problem must lie in the valve circuit and not further in with the TLO72's?
What about the Aural Enhancer part?but this is on the 2nd stage i think and that seems to work ok.
I am looking at a 350X circuit but it is not the same as the 750X,could anyone post this if they have it?
Thanks.

[Jazz P Bass]

SWR 750X Schematic

Here is the 750X schematic.

As it is true that 12AX7 & 12AT7 are different tubes, I would not expect one to work & one not too.

Two things come to mind:
- You have a bad connection on V1A.
- it could be a broken socket tab, bad solder joint
- the cathode resistor is losing it's connection to ground.
-All of the 12AX7 tubes are bad,

[robo]

Ok i have finally got this amp working!

Took out the whole pre-amp board and re-soldered around the 12ax7 socket and took the opportunity to trace the circuit into the TLO72 chips directly connected to the 12ax7 (The 550x pre-amp schematic is different to this 750x pre-amp),i could not find any bad solder joints or loose valve base connections but re-soldered them anyway.I also replaced both plate resistors and the 1st stage cathode resistor for good measure!
Re-assembled and put new TLO72 chips in.Sprayed sockets and tightened all nuts etc.
Put in new JJ12ax7 that I tried before........and it is now working!!
I am thinking it was a TLO72 maybe dragging the output of the 1st stage 12ax7 down?????

Anyhoo,thanks to all whom responded and thanks Jazz P Bass for the schematic.

[g1]

The fault will have been strictly related to the tubes and their connections, not the IC's. The 12AX7's that were not working are now working. Some brands of tubes have thinner pins, perhaps one of the pins of the AX7's was not making good contact and when you cleaned the sockets it solved the problem.
Or it could have been an intermittent solder joint at the socket and the AT pressured the pin in a way that made it connect. In this case resoldering the socket will have solved the problem.