Fixing Sneaky Oscillation in P-P Amplifiers

wrenchone · 4th January 2013, 11:09 PM

I have a P-P amp I've been tinkering with that has a snarky burst of oscillation that occurs on only one half-cycle. I recall SY saying in one thread that this could be due to mismatch between transformer primary halves. The problem and its solution are deeply buried in a thread here somewhere, and the problem is frustrating enough that the problem and solution deserve a thread of their own for future frustrated amp builders.
SY, can you restate your suggestion to fix this problem? I think that a series RC across each primary half, or one between plates might be the solution.

astouffer · 4th January 2013, 11:15 PM

If the primary resistance is off you could add a series resistor to try balancing it.

HollowState · 4th January 2013, 11:48 PM

I remember something about this. At least one suggestion was to add grid stopper resistors to the output tubes if not already there. Values around 1K to 1.5K are common. If pentode operated, I'd also consider smaller ones on the screens and perhaps even a small inductor at the plate terminal in series with the OPT.

Wavebourn · 4th January 2013, 11:56 PM

What's the frequency of oscillations?

wrenchone · 5th January 2013, 12:06 AM

A few MHz...

wrenchone · 5th January 2013, 03:45 AM

One of the things I might try before anything else is to increase the output tube grid stopper values to a value similar to that of the Red Light District, with the intent of limiting grid current during positive grid excursions. This may sufficiently squelch the output tubes such that no further action is necessary. The grid stoppers are probably a k or two right now, maybe less. I'd have to look at the half-built carcass to tell, and it's down in the basement, where it'll stay until I get to it...

astouffer · 5th January 2013, 04:56 AM

What does the burst look like?

Wavebourn · 5th January 2013, 05:44 AM

If few MHz it is not transformer ringing. Either grid stoppers would help, or a cap directly from anode to the feedback input, to bypass feedback through transformer. Can be also result of some parasitic feedback, so shielding may help.
If you remove GNFB, does it stop or gets worse?

20to20 · 01:11 PM

What are the lowest and highest test frequencies that it occurs on? Where is the first point in the circuit you can detect it? Any unusual circuit design elements, components? Apart from being a P-P amp, we don't have much to go on.

EDIT: Does it exist at all output power levels? Does it start at the crossover point, the positive or negative half-cycle, top ot bottom of the wave?

LinuksGuru · 5th January 2013, 02:33 PM

Can you please post a schematic and photo of layout?

4th January 2013, 11:09 PM	#1
wrenchone diyAudio Member Join Date: Feb 2004 Location: Silicon Valley	Fixing Sneaky Oscillation in P-P Amplifiers I have a P-P amp I've been tinkering with that has a snarky burst of oscillation that occurs on only one half-cycle. I recall SY saying in one thread that this could be due to mismatch between transformer primary halves. The problem and its solution are deeply buried in a thread here somewhere, and the problem is frustrating enough that the problem and solution deserve a thread of their own for future frustrated amp builders. SY, can you restate your suggestion to fix this problem? I think that a series RC across each primary half, or one between plates might be the solution.

4th January 2013, 11:48 PM	#3
HollowState diyAudio Member Join Date: Aug 2006 Location: Taxland, New Jersey	I remember something about this. At least one suggestion was to add grid stopper resistors to the output tubes if not already there. Values around 1K to 1.5K are common. If pentode operated, I'd also consider smaller ones on the screens and perhaps even a small inductor at the plate terminal in series with the OPT. __________________ "The supercomputer is technologically impossible. It would take all of the water that flows over Niagara Falls to cool the heat generated by the number of vacuum tubes required." ~ Professor of Electrical Engineering, New York University

4th January 2013, 11:56 PM	#4
Wavebourn diyAudio Member Join Date: Sep 2006 Location: Pleasant Hill, CA	What's the frequency of oscillations? __________________ If I disappear suddenly, that means I finally created a time machine and pushed wrong button that brought me to Stalin's Russia. In any experiment any result is the result. Even if it is negative.

5th January 2013, 05:44 AM	#8
Wavebourn diyAudio Member Join Date: Sep 2006 Location: Pleasant Hill, CA	If few MHz it is not transformer ringing. Either grid stoppers would help, or a cap directly from anode to the feedback input, to bypass feedback through transformer. Can be also result of some parasitic feedback, so shielding may help. If you remove GNFB, does it stop or gets worse? __________________ If I disappear suddenly, that means I finally created a time machine and pushed wrong button that brought me to Stalin's Russia. In any experiment any result is the result. Even if it is negative.

5th January 2013, 12:45 PM	#9
20to20 diyAudio Member Join Date: Jun 2010 Location: W-S, NC	What are the lowest and highest test frequencies that it occurs on? Where is the first point in the circuit you can detect it? Any unusual circuit design elements, components? Apart from being a P-P amp, we don't have much to go on. EDIT: Does it exist at all output power levels? Does it start at the crossover point, the positive or negative half-cycle, top ot bottom of the wave? Last edited by 20to20; 5th January 2013 at 01:11 PM.


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4th January 2013, 11:15 PM	#2
astouffer diyAudio Member Join Date: Jun 2004 Location: Pittsburgh, crumbling wasteland	If the primary resistance is off you could add a series resistor to try balancing it.

5th January 2013, 12:06 AM	#5
wrenchone diyAudio Member Join Date: Feb 2004 Location: Silicon Valley	A few MHz...

5th January 2013, 03:45 AM	#6
wrenchone diyAudio Member Join Date: Feb 2004 Location: Silicon Valley	One of the things I might try before anything else is to increase the output tube grid stopper values to a value similar to that of the Red Light District, with the intent of limiting grid current during positive grid excursions. This may sufficiently squelch the output tubes such that no further action is necessary. The grid stoppers are probably a k or two right now, maybe less. I'd have to look at the half-built carcass to tell, and it's down in the basement, where it'll stay until I get to it...

5th January 2013, 04:56 AM	#7
astouffer diyAudio Member Join Date: Jun 2004 Location: Pittsburgh, crumbling wasteland	What does the burst look like?

5th January 2013, 02:33 PM	#10
LinuksGuru diyAudio Member Join Date: Aug 2008	Can you please post a schematic and photo of layout?

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