Speedball voltage issues and suspect 12AU7.

[Wickerduck]

Hi, I completed the Crack build last night and had four wonderful hours of sonic bliss apart from some dropout on the right channel which was rectified by tapping the 12AU7. During the next couple of hours the right channel dropped out 4 more times once again rectified by a light tap of the valve. The socket connections are good and both LED’s were lit during the fault and the value seems quite microphonic. Could this be a faulty valve?

I started the Speedball upgrade today and built the small PCB. I removed both 22.1k resistors and fitted the PCB. Voltage checks were a problem though as connections 0A,1A,1B and 0B read around 240v DC. I went through the flow chart which led me here. The valves are both lit but the 12AU7 LED’s are not lit. I removed the PCB the refitted the 22.1k resistors and the voltages returned to normal.

Cold checks on the PCB resistors were fine. All, orientations are correct on the transistors and LED’s. My only concern is that I was supplied with JC2907 transistors which I cannot find a pinout for.

Any help would be most gratefully received. Cheers. Mike

[Thermioniclife]

I thinkthe first thing to do is reflow all the connections on the 12u7 socket except the led's which are sensitive to heat, they are probably fine. Having said that the center terminal must be well soldered and grounded soundly, but do not apply heat unless the connections there look crusty. After doing that run the amp without any speedball boards installed until you get your amp running properly, listen to it for a few weeks before proceeding to the speedball upgrade. just my 2 cent's worth. Btw I prefered the crack with just the small board installed and not so much with the large board installed.

[Paul Birkeland]

I started the Speedball upgrade today and built the small PCB. I removed both 22.1k resistors and fitted the PCB. V

You need to put the 22.1K resistors back and debug what's wrong with the amp before putting the Speedball in.  A channel dropout in the stock build will come along with voltages that change and those new voltages would be of interest to figure out what's going on.

Tapping the tube to restore operation is nearly always a problematic solder joint.