La2a problems

[bubbi]

Hey,
So I built a pair of La2a's but one of them is having problems. When I turn it on it works perfectly for like 5 seconds and then starts oscillating. The control knobs (and meter switch) all have effect on the oscillation so it's basically working as a weird synthesizer. The other unit is working perfectly and I already build two other ones with the same layout and parts that are working perfectly.

All the parts are the same in both units and I already tried switching out all tubes and T4B but this same one is still having problems. I also triple checked the whole layout and also compared the wiring in them side by side but I can't find any error. I uploaded a youtube video to demonstrate what is happening: https://www.youtube.com/watch?v=lRhPNnxbl1Q

Do you have any ideas what could be wrong? Is it possible that I received a bad cap or something? What should I be doing to troubleshoot? I have run out of ideas...


Thanks!
B

 

[Deepdark]

Be sure you scratch paint. I had oscillation once and get rid of it by scratchng paint and making sure there was good continuity. Check your ground in general

 

[Audio1Man]

I would sure look @ the power supply decoupling.  The PS grounds could be the the source of the problem.
Good hunting
Duke

 

[bubbi]

Alright so I took a better look at my ground and found a cold joint. That did make it better but it is still having problems.  I made a new video: https://www.youtube.com/watch?v=1B7x5elkuH8

So it starts out just fine but then a hum fades in and if I move the gain pot around it starts oscillating.
However I started measuring the voltages on the test points given in the original schematic and found that the cathode voltage of the first 12ax7 is very low (around 1.5V) and then when I measured the plate voltage to my surprise the problems stopped (and the voltage is correct btw.), so just touching the plate with the probe fixes the problem, must have something to do with the internal resistance of my DMM? I already tried switching out the tube but it didn't help. This is something really simple right?

Thanks!

 

[Deepdark]

looks like you have a short/cold joint around