When I had the speaker grills off I noticed some some low frequency oscillation in my 12B4 linestage that I never noticed before. I am getting very slow movement of the low frequency drivers in and out with no signal. I have tried other tubes and the problem persists.
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That sounds exactly like the problem I had with my 12B4 preamp.
http://www.diyaudio.com/forums/showthread.php?threadid=35012&highlight=12b4
In my case, I ended up tearing the amp apart and building a power amp with the tubes so I never got around to tracking down the problem. All I know was that it was a power supply problem, because the noise that was causing the problem remained even after replacing the tubes with resistors. Best of luck with the fix.
http://www.diyaudio.com/forums/showthread.php?threadid=35012&highlight=12b4
In my case, I ended up tearing the amp apart and building a power amp with the tubes so I never got around to tracking down the problem. All I know was that it was a power supply problem, because the noise that was causing the problem remained even after replacing the tubes with resistors. Best of luck with the fix.
I decreased the value of the output coupling cap with no change.
I increased the size of the cathode bypass cap with no change.
I powered the unit with a regulated supply with no change.
I tried it without a cathode bypass cap with no change.
I added larger decoupling caps prior to the plate resistor with no change.
The linestage is using the same power supply that powered my 5687 linestage and it had no problems.
I increased the size of the cathode bypass cap with no change.
I powered the unit with a regulated supply with no change.
I tried it without a cathode bypass cap with no change.
I added larger decoupling caps prior to the plate resistor with no change.
The linestage is using the same power supply that powered my 5687 linestage and it had no problems.
Try raising the filament supply above ground about 20 - 30V. (Just divide down your plate voltage supply with resistors and a filter cap - connect one side of your filament supply to this point so that it floats on this divider network.
A very large value cathode bypass capacitor may help. Say 1000uF.. 1/F noise..
Provided the larger cathode bypass cap helps and if you want to avoid that huge cathode bypass cap you could try fixed bias operation with the cathode right at ground - can't make it quieter than that.. A single 9V battery for grid bias would suffice. (cap and resistor also required obviously.)
Also check your filament supply voltage and make sure it is stable and within specification.
Kevin
Edit to add cathode, cap value, fixed bias and fix grammatical error(s).
A very large value cathode bypass capacitor may help. Say 1000uF.. 1/F noise..
Provided the larger cathode bypass cap helps and if you want to avoid that huge cathode bypass cap you could try fixed bias operation with the cathode right at ground - can't make it quieter than that.. A single 9V battery for grid bias would suffice. (cap and resistor also required obviously.)
Also check your filament supply voltage and make sure it is stable and within specification.
Kevin
Edit to add cathode, cap value, fixed bias and fix grammatical error(s).
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