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Old 24th March 2012, 06:02 PM   #11
avp1 is online now avp1  United States
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Originally Posted by LinuksGuru View Post
Thanks, avp1. I have reduced idle current of output tubes tenfold. Oscillation continues, down from 12 - 20 mV to 4 - 8 mV.
If you can't close them completely, take them out.
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Old 24th March 2012, 06:11 PM   #12
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Ground loops, definitely ground loops.
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Old 24th March 2012, 06:23 PM   #13
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Originally Posted by avp1 View Post
If you can't close them completely, take them out.
I removed 6N6P phase splitter - oscillation stops. Is this enough to determine 6N6P as source of oscillation? I tried to take an oscillogram from the grid of output tube, looks like my oscilloscope have problem with DC (from BIAS supply).
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Old 24th March 2012, 06:51 PM   #14
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I have soldered one leg (2nd go to oscilloscope probe) of 0.33 uF cap to R23 - grid stopper of output tube in order to isolate DC out of oscilloscope probe.

After all, its seems 6N6P long tail phase splitter is a source of spurious oscillation at at 2 frequencies.
Turning off amp cleans noise on oscilloscope screen - so this is oscillation from amp, not noise from any other device.
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File Type: jpg Noise-from 6550 Grid n2.jpg (32.9 KB, 34 views)

Last edited by LinuksGuru; 24th March 2012 at 07:05 PM.
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Old 24th March 2012, 08:01 PM   #15
DF96 is offline DF96  England
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Add g2-anode snubbers on output stage? UL often needs these.

LTP phase splitter can oscillate if input and output too near each other as one output is in phase with the input. Physically large caps make this worse.
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Old 24th March 2012, 08:32 PM   #16
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Originally Posted by DF96 View Post
Add g2-anode snubbers on output stage? UL often needs these.

LTP phase splitter can oscillate if input and output too near each other as one output is in phase with the input. Physically large caps make this worse.
Thanks, DF86.

What cap and resistor value would you recommend for anode-g2 snubbers?

Is there any trick to suppress oscillation at LTP?
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Old 24th March 2012, 09:30 PM   #17
DF96 is offline DF96  England
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I seem to recall 1k-few k plus 1-few nF. Generally, the largest resistor and smallest cap which kills the oscillation.

I cured LTP oscillation by inserting a grounded screen between in and out. The screen was just some bent thick copper wire. I did this because I had no scope to increase separation, which is always the first thing to do. Next time I use big caps (on the LTP output) I will allow more space for them!
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Old 24th March 2012, 09:43 PM   #18
MelB is offline MelB  Canada
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Your negative bias circuit...does it really not have any capacitance??? (Capacitors)
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Old 24th March 2012, 09:52 PM   #19
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Originally Posted by MelB View Post
Your negative bias circuit...does it really not have any capacitance??? (Capacitors)
Power transformer, rectifiers and filters for HV B+ and BIAS are not shown (schematic is too obvious). BIAS supply have 500uF electrolyte and 0.22 uF film filter caps.
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Old 25th March 2012, 11:05 AM   #20
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Originally Posted by avp1 View Post
First increase R1 to 10-15k. Then try 330pf and 10k between plates of VL2. Then 1000pf capacitor with 1.5k resistor from B1 to plates of each output tube.
AVP1, are these mods to reduce phase shift between input/output signal and make GNFB more stable? Is there any formula in order to exactly determine these values of R and C?

Large transformer have quite high stray capacitance, so actually increasing it by 1000pF may lower resonant frequency of primary winding of the output transformer. Or am I wrong here?
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