voltage too high on 0A and 0B

[davidnow]

High,

i have just completed the crackatwoa build. Resistance check is OK but the voltage check has too high voltage on OA/OB on the low current board and OB on both high current boards. here are the voltages :

LOW CURRENT
IA  148V
OA 128V
KregA 5,4V
bregA 148V

IB 149V
OB 129V
KregB 3,9V
bregB 149V

HIGH CURRENT Bside
IA 201V
OA 148V
bA 0V
IB 0V
OB 129V
bB 148V

HIGH CURRENT Dside
IA 201V
OA 149V
bA 0V
IB 0V
OB 131V
bB 140V

Ring and Tip 30mV

May I use the amp as it is or is there any rebuild to do?
Best regards
David

 

[Paul Birkeland]

Failing the voltage checks means the amp isn't working.  You don't want to use an amp that's not working.

I would guess that the LEDs on the 9 pin socket aren't glowing?  Can you see the 12AU7 tube itself glowing? 

With the amp turned off and the volume pot turned all the way down, what DC resistance do you get between ground and A2? How about ground and A7?

 

[davidnow]

Thank you for your answer, always useful to remind basic rules...

The leds of the 9 pins socket are glowing, as well as the 12AU7 tube.
resistance between ground and A2 is 217 ohms and 218 ohms for A7.

 

[Paul Birkeland]

You'll want to check a whole bunch of things on your build.  First of all, you can measure the R1 resistors with them installed on the board.  Are they 237 ohms?

Triple check all of the LM431 and PN2907 transistors to be sure they are in the correct spots. 

Since the LEDs are on, the board is passing current, and the high voltage makes it look like a lot of current is being passed.  This suggests an issue with the center section of that middle C4S board.  Since the LEDs are on and you have appropriate grid to ground resistance on pins 2 and 7 on that driver tube socket, that just leaves the C4S board as the issue. 

It would certainly be worth taking the time to go over all the build steps in the manual involving constructing that board and installing it.

 

[davidnow]

The R1 resistance are 237 ohms. I checked everything twice and did not find anything wrong...

Finally i changed the 12AU7 i received from bottlehead to a spare one i had. And now the voltage check is alright...

now i am going to spend some time to listen to the amp.

 

[Paul Birkeland]

I would keep an eye on things.  When you change a tube and things magically snap into place, my immediate reaction would be to look at the solder joints around the tube socket, as the act of changing the tube can disturb flaky joints and make them work temporarily.

 

[davidnow]

yes, i will keep an eye on it, thanks

don't you think it can also come from a bad tube?

 

[Paul Birkeland]

It's pretty unlikely, as the tubes that we send out get tested on a TV-10, which is a very picky tester. 

 

[davidnow]

I tried several 12AU7 and each time i have the right voltage... except with the tube bottlehead have sent.
could it be that the wrong tube has been sent? I think i can read 12AT7 written on it, though it is really not clear. Would a 12AT7 give these high voltages on OA and OB?

 

[Paul Birkeland]

Yes, a 12AT7 would indeed do that, and one could've snuck in on accident. 

 

[davidnow]

I looked at my stereomour II 12AT7, the tube i have got definitively looks like it. End of story!

 

[Paul Birkeland]

Now that you mentioned that, I think we have had that happen once or twice in the past.  You have discovered on your own why you can't just plug a 12AT7 or 12AX7 in place of a 12AU7.