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Old 3rd July 2012, 09:47 PM   #1
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Question Q: UL PP Instability - Anode Grid2 Snubber

Hi, folks,

I have stability problems with PP UL amp, 2xKT88, 3.9K transformer, 460V B+.

There is a common trick, described in several sources, connecting RC snubber between anode and grid #2 of the output tube (see reference schematic attached). This trick worked for me, BUT, now I have considerable high frequency roll-off.

The original author of the schematic mentioned above used 5 pairs of KT88, and 1K + 0.001 uF snubber. Looks like he had 1K transformer, and upper frequency limit stated 25 KHz.

Taking into account difference in transformer impedance, I used 4.7k resistor + caps in range of 47 pF - 1000 pF. With caps 500 - 1000 pF high frequency roll off is down to 13.6V (20 KHz) from 20V (2KHz), and with 47 pF down to 15.6V.

Anyone can suggest how to select RC values, maintaining stability, yet without roll off at high frequencies?

Thanks in advance for any suggestion(s).
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Old 3rd July 2012, 10:19 PM   #2
DF96 is offline DF96  England
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NFB means that closed loop HF rolloff is higher than open loop HF rolloff.
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Old 3rd July 2012, 11:48 PM   #3
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I don't think that the output transformer snubbers are a good place to put phase lag. Besides power handling issues it's sensitive to output loading. Not knowing anything else about your amplifier, I can only suggest sticking to the conventional high frequency rolloff further upstream and use the output transformer RC's as snubbers to damp parasitic reactances in the transformer.

That's an old fashioned way to do things, but probably easier to get going well.

All good fortune,
Chris
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Old 3rd July 2012, 11:55 PM   #4
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If you want to get into some theory and maths look here:
Damping of ringing in audio transformers

You may need to navigate around VoltSeconds site a bit to find what you need.

Else - my short cut.

Each side of the push pull is 60% A to G2, 40% G2 to Centre Tap.
So turns anode to anode is 0.3 + 0.2 + 0.2 + 0.3

So you need to shunt 0.3 of total turns. That will look like 0.3 x 0.3 = 0.09 of total impedance.

0.09 of Raa=3900 gives 351 Ohms.

For a Zobel Network you use a resistor value of 1.4 to 2 x the impedance you are shunting. So 491 Ohms to 702 Ohms.

Use say 560 Ohms.
Then increase the cap until it is stable.

Quick calc for a start value.
Choose say 60kHz as the roll off.

Multiply 60,000 by 2 pi, hit the 1 over button and then divide by 560
That gives just about exactly 4n7.

I'd probably try 2n2 to see if that was enough but go higher if required.

Cheers,
Ian
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Old 4th July 2012, 12:22 AM   #5
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Great stuff! If you make it that easy might be interesting to try splitting Zobels between primary and secondary. Just enough on the primary to make open loop scope square waves look pretty and another on the secondary to tame whatever potential stability issues.

Thanks,
Chris
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Old 4th July 2012, 07:12 AM   #6
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Quote:
Originally Posted by gingertube View Post
If you want to get into some theory and maths look here:
Damping of ringing in audio transformers

You may need to navigate around VoltSeconds site a bit to find what you need.
Thanks for everyone who replied !

Dumping ringing is not something I'm looking for. Square wave response at 2 KHz is near perfect at several V.

Amps works fine when load is connected to 8 ohm tap, but unstable if to 4 ohm tap.
With another output transformer it oscillates after certain power level reached no matter of load connection (I have 4 different transformers).

Snubbers between anode and grid 2 terminated instability completely, but introduced high frequency roll off. Quite strange, since in another amp I mentioned there was no such side effect.

Any idea how to avoid HF roll off with anode-grid2 snubbers?
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Old 4th July 2012, 07:29 AM   #7
Yvesm is offline Yvesm  France
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Quote:
Originally Posted by LinuksGuru View Post
. . .
Any idea how to avoid HF roll off with anode-grid2 snubbers?
Go away from UL and feed screens with a clean and stable supply

Yves.
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Old 4th July 2012, 08:38 AM   #8
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by Chris Hornbeck View Post
I don't think that the output transformer snubbers are a good place to put phase lag. Besides power handling issues it's sensitive to output loading.
Are you sure? Oscillation is normally caused by leakage inductance and capacitance, which are more-or-less independent of loading and unique to the transformer.


Quote:
Originally Posted by LinuksGuru View Post
Snubbers between anode and grid 2 terminated instability completely, but introduced high frequency roll off.
At what frequency does it start rolling off?

Last edited by Merlinb; 4th July 2012 at 08:51 AM.
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Old 4th July 2012, 09:42 AM   #9
DF96 is offline DF96  England
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Can we just be clear whether we are dealing with output stage parasitic oscillation or global loop instability? Adding snubbers to the OPT could affect both.

Moving the load from 8ohm to 4ohm tap will reduce the loading on the OPT and hence reduce damping on any resonance.
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Old 4th July 2012, 10:31 AM   #10
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Quote:
Originally Posted by DF96 View Post
Can we just be clear whether we are dealing with output stage parasitic oscillation or global loop instability? Adding snubbers to the OPT could affect both.

Moving the load from 8ohm to 4ohm tap will reduce the loading on the OPT and hence reduce damping on any resonance.
I can check it when return back.

Quite possible stability affected by the very long interconnection wires, I have separate power supply sitting on another side of the table.
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