low frequency oscillation in 12B4 linestage

Original Burnedfingers · 5th December 2005, 10:43 AM

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.

astouffer · 5th December 2005, 11:30 AM

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.

Zen Mod · 5th December 2005, 11:31 AM

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

greyhorse · 6th December 2005, 01:43 AM

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.

yagas · 6th December 2005, 07:12 AM

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 !

Original Burnedfingers · 6th December 2005, 10:46 AM

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.

SY · 6th December 2005, 12:15 PM

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.

kevinkr · 6th December 2005, 04:48 PM

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).

Original Burnedfingers · 11th December 2005, 12:27 AM

I increased the size of the cathode bypass cap further until the oscillation went away.

5th December 2005, 11:31 AM	#3
Zen Mod diyAudio Member Join Date: Jan 2003 Location: ancient Batsch , behind Iron Curtain	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 __________________ my Papa is smarter than your Nelson ! tnx to clean thread ; Cook Book ; PSM LS Cook Book ; Baby Diyaudio FORUM ; BAF Forum & Gallery;I'm dumb

6th December 2005, 07:12 AM	#5
yagas diyAudio Member Join Date: Sep 2004 Location: Athens	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 ! __________________ Mic

6th December 2005, 12:15 PM	#7
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	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. __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache

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5th December 2005, 11:30 AM	#2
astouffer diyAudio Member Join Date: Jun 2004 Location: Pittsburgh, crumbling wasteland	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.

6th December 2005, 01:43 AM	#4
greyhorse diyAudio Member Join Date: Feb 2002 Location: Ontario	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.

6th December 2005, 10:46 AM	#6
Original Burnedfingers diyAudio Member Join Date: Nov 2002	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.

6th December 2005, 04:48 PM	#8
kevinkr diyAudio Moderator Join Date: Sep 2004 Location: Boston, Massachusetts Blog Entries: 6	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).

11th December 2005, 12:27 AM	#9
Original Burnedfingers diyAudio Member Join Date: Nov 2002	I increased the size of the cathode bypass cap further until the oscillation went away.

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