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Old 5th December 2005, 10:43 AM   #1
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Default low frequency oscillation in 12B4 linestage

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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Old 5th December 2005, 11:30 AM   #2
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Disconnect the line stage from your main amp and see what happens. That schematic is pretty simple and I don't see any obvious signs. Maybe its a layout problem.
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Old 5th December 2005, 11:31 AM   #3
Zen Mod is offline Zen Mod  Serbia
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just for investigation purpose look if oscilations is there without cathode bypass cap.

if not- just put bigger cap (as Mark and others)

if that is not enough-you must rearrange filter stages in supply,or even-put more muscles in supply
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Old 6th December 2005, 01:43 AM   #4
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That sounds exactly like the problem I had with my 12B4 preamp.
Low frequency noise in a 12B4 preamp
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.
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Old 6th December 2005, 07:12 AM   #5
yagas is offline yagas  Greece
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Decrease the value of the output coupling cap. Of course this is not the best solution but it works. If you want to really kill the problem just regulate !
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Old 6th December 2005, 10:46 AM   #6
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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.
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Old 6th December 2005, 12:15 PM   #7
SY is offline SY  United States
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I've had a terrible time with low frequency noise in 12B4, apparently coming from heater to cathode insulation. Try changing tubes and noting if the amount of noise also changes.
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Old 6th December 2005, 04:48 PM   #8
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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).
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Old 11th December 2005, 12:27 AM   #9
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I increased the size of the cathode bypass cap further until the oscillation went away.
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