Low voltage at T2 and OA

[rusty-jade]

I had built my Stereomour some months ago and at that time all the voltage checks were within normal range.  A few weeks ago, I modified the Stereomour to a power amp by removing the connections to the potentiometer and the selector switch, and adding a 100K resistor at each of two RCA input jacks. I did not check the voltages then and the amp sounded good.  Yesterday, having some spare time, I checked the voltages and found T2 to be 170 and OA to be 170 also, quite lower than the 230 in the manual. I resoldered all the connections of the above modification, the wiring and resistors at the RCA Jacks and particularly at 7U, 8U and 9U, but still the same two low readings.  Everything else is normal.  Any advice would be greatly appreciated.   

[Paul Birkeland]

Hello,

How to the voltages at Kreg stack up?  Are they both 2.5V?

Can you take a DC voltage reading at 7 and 9?

Since you removed the 100K pot and replaced it with a fixed resistor (which sould be no big deal), a problematic connection there could result in sufficient DC voltage on 7 or 9 (I think it would be 7 in your case) to throw off the bias of the driver stage, and thus pull down your plate voltage a bit.

If you have 0V at terminals 7 and 9 and 2.5V on both Kreg terminals, I wouldn't worry about the lower voltage.

-PB

[rusty-jade]

Hi Paul,

Thanks for your speedy response.  My voltages at T7 and T9 are 0.  The Kreg measures 2.45 on one side and 2.47 on the other.  I also resoldered one of the two 100K resistors which I installed at the RCA jack.  As I said, sonically it sounds much better since I converted to a power amp.  I only checked the voltages as an afterthought as the Triad CX7 I ordered finally arrived (after 8 weeks) and I was trying to see where I can install it.  Thanks once again!

[Paul Joppa]

If you look on the circuit diagram, you will see that I originally expected 200v at the driver plates and at OA and OB. That would be with an average tube as documented in the specifications. The 230v measurement was made on the actual original production prototype. So your voltage is 15% low from the nominal, and the manual is at 15% high. This much variance is quite normal in this circuit, which has a current source load and a fixed bias voltage. As long as it is between 120v and 300v, the driver can provide more than enough drive voltage. This has been a frequent question on the Forum ever since the first C4S'd Paramour, more than a decade ago!

If you have 170v on OA and 230v on OB, it just means the 12AT7 sections are not closely matched. Usually this has little sonic significance, but you can try a 12AT7 with matched sections if you want to test this for yourself.

[rusty-jade]

Thanks Paul for the explanation.   I had neglected to look at the schematics, and I measured the OB at 201.







Brad