Low voltages at OB and T14 [solved]

[rusty-jade]

My Stereomour is a 45 tube power amp, with Triad CX-7 chokes,  47 uf Mundorf cathode bypass caps and 27 uf (630v) Solen film caps substituting for the 100 uf 450 v electrolytic caps.
Recently, I increased the capacitance of the 27 uf, 630v Solen caps by adding another 33 uf Solen cap in parallel. 
Subsequently, on testing the voltages, everything was normal, including the b+, Kreg on both sides and the four Leds all lit up.  However, the OB and T14 were 126 volts. ( OA was normal, - 186 v)  Since the OB is connected to the driver tube pin 6 and T14, I did the following:

-resoldered the OB, pin 6 and T14 joints

-resoldered T8 ( ground) and T9

-resoldered  the Rca input jack connections ( wiring and 100K resistors)

-resoldered T14 and checked the capacitance of the coupling cap (.1uf)

However, the OB and T14 voltages still remained low at 126v.

Any advice or suggestions on the resolving the problem will be greatly appreciated. 
   

[Paul Birkeland]

If the Kreg sits at 2.5V and the OB at 126V, I'd try a different 12AT7.

I'd also triple check that the voltage at the output of the 0.1uF coupling cap is zero (T4/T12).

-PB

[rusty-jade]

Thanks Paul.  I forgot to mention I changed the 12AT7 as part of the investigation, but no change in the low voltages.  I just rechecked the voltages at T4 and T12. Both are zero.  In looking at the schematic diagram, I wonder if the MJE5731A has anything to do with it. I do not have a spare on hand, so I am reluctant to tinker with it.

[Paul Birkeland]

That voltage is set by the actual 12AT7 used.  If you swap the wires leaving OA and OB, that low voltage should switch sides (this isn't a bad test to do!)

If you aren't giving the 12AT7 it's 2.5V of bias, then that voltage can shift.

If the 12AT7 has issues, that voltage may wander (though generally it goes up).

If the 0.1uF coupling cap is failing, this voltage will also wander.

-PB

[rusty-jade]

The 12AT7 is definitely not the cause of the problem.  I used three different ones while investigating, including the stock tube.  The second and third tubes gave even a lower voltage reading.
I doubt too that the coupling cap is the cause. It is a Clarity Cap. I measured the capacitance of both  coupling caps with my DMM which can measure capacitance.  Both came in at .094 uf.  The bias is also OK as the Kreg on both sides measures 2.47v. I even measured the stopper resistor 220ohms, and it is OK.  The grid voltage ( between pins 1 & 4 of the power tube is a bit high, 2.66v, but the outlet voltage in my area can sometimes as high as 126 V.   So by the process of elimination,  I feel ( and dread because I do not have a spare) that it can be the MJE5731A since it is connected to OB.
Your suggestion of swapping the OA and OB wires is a good one as it can confirm whether that is the case.

[rusty-jade]

Per PB's suggestion, I swapped the wires of OA and OB.  The low voltages now showed up on OA.  So at least the thought of a failing MJE5731A is eliminated.  Thinking that the 0.1 uf coupling Clarity Cap is indeed the culprit, even though it measured OK, I swapped it with the stock coupling cap.  Unfortunately, the low voltage still persists.  However both the coupling cap and the transistor have now been eliminated as the cause of the problem.  I will swap the Mundorf 47uf bypass caps to see if that is the problem.

[Paul Birkeland]

I should also ask, does the amp still sound about normal?

It's possible that a flaky 220 Ohm resistor going to the grid of the 12AT7 is giving you some biasing headaches.  It's possible to jumper this resistor temproarily, then recheck the voltage.

-PB

[rusty-jade]

It sounded OK before I added the additional 33uf Solens caps in parallel but I don't know how it sounds now as I've unscrewed the chokes from the chassis and all the parts are lying on a table.  However, I know it is not the power supply as the HV voltages on both sides are fine.  I did check the resistance of the 220 ohm grid stopper resistor and it was right on the money.  Same too for the 249K resistor.  It is really strange.
I'll take it a step at a time - I'll swap the Mundorf bypass caps and see it that is the problem. If not, I'll replace the 220 ohm resistor.  Thanks for your advice! 

[rusty-jade]

Problem Fixed

I swapped the cathode bypass caps.  It was not the problem.  Then I rewired OB to B6, to T14. Also replaced the 220 ohm stopper resistor at B7, even though it measured OK.  I noticed then that the red wire going to T9 from the RCA input jack ( mine is a power amp) was nicked about one and a half cm from the terminal.  Doing a continuity test revealed no signal.  That, I believe, was the problem.  Subsequent to fixing this wire, all voltages are back to normal.

[Paul Birkeland]

Yeah, that's roughly the same issue as having an open 220 Ohm resistor.  The grid doesn't get a path to ground, and will collect electrons from the cathode and develop its own bias voltage.

[Grainger49]

The grid doesn't get a path to ground, and will collect electrons from the cathode and develop its own bias voltage.

I like this explanation, I hope you don't mind if I use it.