Speedball: High Voltage on OA and OB [resolved]

[Wejats96]

Got the Crack working but ran into issues while assembling the Speedball specifically high voltages on the Big Board and Small Board's OA and OB.  Both tubes and LEDs all lit up, using all stock parts, photos attached.  Any suggestions?

Small board
OA: 100v <-- somewhat high, outside the 60-90v range
IA: 185v
B-A/B: 0.6mV
IB: 185v
OB: 107v <-- somewhat high, outside the 60-90v range

Big board
OA: 129v <-- this is way outside the 70-100v range
B+: 185v
G: 0.7mV
OB: 124v <-- this is way outside the 70-100v range

Wall Outlet 122.5v

 

[Paul Birkeland]

Was the small board working OK at the first check in the manual?  I see a lot of flux on your 9 pin socket and kind of all over everything, which exact product did you use?

Are the LEDs on the 9 pin socket still glowing?  Do you have lots and lots of use on the 12AU7? 

 

[Wejats96]

I recall the small board OA/OB voltages were about 94-99v which was within the 10% margin before assembling the big board.  I used a 60/40 rosin core solder and RA type/non-corrosive flux.  Will check for the LEDs on the 9 pin socket but attached a photo of the top side leds.  The 12AU7 has been used for less than an hour and the flux still need a bit of cleaning.

 

[Paul Birkeland]

If your solder spool says "rosin" on it, it would be best not to use more flux.

There are a few possibilities for what's going on, and ultimately your 12AU7 may need some run time for those front OA/OB voltages to come down, and once they do the OA/OB voltages on the large PC board will follow.

Slightly less likely would be a flaky solder joint at A9 since both halves appear to not have full emission.

 

[Wejats96]

Resoldered A9 and a few other pins on that socket, both LEDs on the 9 pin socket are lit but there is no change to the OA/OB voltages.  Going to use it for the week and check the voltages again.

 

[Wejats96]

Ran the amp for about a week and reheated some joints but the voltage hasn't changed.  Eventually ran out of ideas and reheat all the connections, even removed the small and big board to get to them and OA/OB reads about ~4 volts higher (small board OB @ 111v, big board OB @ 129v).  I noticed when I'm listening to the amp without source there is a slight hum right after turning it on that goes away a few seconds later.  Any suggestions?

 

[Paul Birkeland]

You could try a different 12AU7.

 

[Wejats96]

I used another 12AU7 and OA/OB are now within spec and glows slightly brighter so it appears that the supplied tube was the issue.  Thanks for all the help and the amp sounds great.