Oscillate me Not!

[sr1200]

Im building a second La2a point to point, since my last one went so swimmingly... 

I finished the build, being the second time around, i made the wires even neater than before, powered the unit on and voltages are perfect everywhere!  8)

Only problem, when i bring up the GR knob, the meter pins to the right (all the way up) and i get a nasty distorted signal out of it.  I've been trolling the all things la2a thread and found someone with the same issue.  Apparently he put a .1u cap somewhere on his PS section to filter out some noise (which seems to be my case since even with nothing plugged into it, im getting voltage on my t4 pins and quite a bit if i CRANK the knob up)  I reversed the power leads (read that somewhere as well) and it made it voltage that was there drop significantly, but the issue still remains.  Was wondering if anyone knew where exactly this filter cap should connect (im guessing somewhere on my B+ near the trafo, but im just guessing and I don't want to blow this thing up being so close)

Thanks in advance!

 

[JohnRoberts]

When you made the wires neater is there any chance you have inadvertent coupling between high impedance wires that is causing the instability?

JR

 

[joaquins]

I'd put the 0.1 cap bypassing last big cap in PS but as near to the tube generating the problem as possible.
I'd guess the double triod of the sidechain, from top (supply side) of anode resistor to ground. Another one near the audio path won't harm.

I've seen used little caps in parallel with rectifier diodes in order to supress sparks from rectification in tube circuits, this will make it act as slower diode (.002 1kV in a fender twin)

JS

 

[sr1200]

Ive swapped out the tubes with my working unit and have the same issue.  After reading more in the all things la2a thread, i ordered some 60v zener diodes to maybe use instead of the neon.  My only thought it since the wires are now really neatly bent and placed, i'm thinking maybe too many wires are on top of one another and bound together with zip ties.  Only real difference between the 2 units.  The voltages in general are about 2v max off my other unit (but in this circuit, thats kind of to be expected i think)

 

[sr1200]

Just put the unit on the scope... im getting a NASTY sharp spike around 40K and as i bring the GR knob up the  spike travels lower until it gets into the upper audible range. Gonna try that filter cap tonight and see what that does.

 

[JohnRoberts]

Maybe clip a few zip ties and move the wires around... see if that makes a difference.

JR

 

[pucho812]

be careful what you move the wires around with, I suggest the chopstick method i.e. use a chopstick or other similar device that will not conduct electricity so you do not shocked...

 

[sr1200]

The only wire that i can see effecting the spike of doom is the cap coming off the T4B that runs over to the 6AQ5A.  Moving it changes the frequency of the spike and even lowers it a bit, but not seemingly enough to do anything useful.  I tried moving the Ne. out of the way of any wires as well, and that didn't even make the spike wiggle.  Possible its just an issue with the PS?  Gonna try and get that cap in tonight.  My session last night had be up till 2am. ugh!