Noise problem in 572-10 PP amplifier - diyAudio
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Old 10th March 2010, 07:40 PM   #1
santa is offline santa  United States
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Default Noise problem in 572-10 PP amplifier

Hi,
Recently I built this amplifier and I have a problem with noise that I can't seem to get rid of.
This is the power supply,
Click the image to open in full size.

I'm using 300 uF for C101 and C102, 5H, 250 mA choke, 470uf for C103 and C104. Power trans is Hammond 1000VCT.

Amplifier schematic,
Click the image to open in full size.

I'm having a problem with this noise spike.
This is taken on the speaker output, 2mS per division about 100 mV in amplitude.

Click the image to open in full size.

I'm using a DC supply for the front end filaments and it is free of noise.
Using AC on the power tube filaments.

No spike on the 150 V supplies but it is present on the plates of V1B and V2B.
I can hear a slight buzz from the PT as the supply comes up to full power.
I have tried another pair of diodes but there was no change.

Click the image to open in full size.

What should I look at next?

Thanks,
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Old 10th March 2010, 07:56 PM   #2
TheGimp is offline TheGimp  United States
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That looks like may be line frequency related. Check it with respect to the ac out of one transformer winding to see if it is locked to the line freq. If so, I'd look at the Lamda power supply next to see if it is kicking noise on the line. Check the line to see if there is another source causing noise on it.
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Old 10th March 2010, 08:13 PM   #3
santa is offline santa  United States
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Sorry I was not too clear, not using the Lambda's.
AC on the 572 filaments with balance pot and transformer/full wave rectifier/cap type linear supply on the 6BM8's.

I turn the lights off in the shop for the scope shot.
No other devices are powered up at the time.

So I should measure the frequency of the incoming line, and each of the secondaries to the CT to ensure the same frequency?

Could frequency be changed on one half of the secondary?

I have an RF filter on the AC input, I may try bypassing that.
I'll report back later.
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Old 10th March 2010, 08:43 PM   #4
llwhtt is offline llwhtt  United States
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The spikes are a tad over 8ms apart, that's 120HZ. That means it's on a full wave rectified DC line. Odd for a tube rectified PS. The 150 Volt supplies are half wave and you verified there's no noise there, it would be 60 Hz rate anyway. I'm building guitar amps that use 2 x 5U4s and big caps also, no spikes that I've seen. I've had problems using solid state diode noise getting into the heater windings and then into the cathodes of the input tube and had the exact same output. A separate heater transformer for the input tubes cured that. My point is to scope your input tube heater voltage and see if there is a spike. In fact try turning the input heaters off to see if the spike goes away.

Craig
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Old 10th March 2010, 11:29 PM   #5
santa is offline santa  United States
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Ok, line freq is 60.00 hz, both secondaries are solid at 60.00 hz.
533 VAC from each half of the secondaries.
The spike is present at the R25/C14 junction so it looks like it is on the main supply.
The filament supply does not show the spike.
The +/- 150 VDC supplies do not show the spike.
Bah, Think I'll just turn it up for a while and rock out..
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Old 12th March 2010, 10:44 PM   #6
santa is offline santa  United States
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About the only other thing I can do is to put snubbers on the choke as recommended in Morgan Jones's "Valve Amplifiers". Waiting for parts...
Any other suggestions?
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Old 12th March 2010, 10:50 PM   #7
kevinkr is offline kevinkr  United States
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Where is the high voltage center tap of your supply connected to? Generally I find that restricting the capacitor charging current loop to the first filter cap is a good idea, i.e. connect the center tap to the first filter cap and from there go to the star ground. Might or might not help in this case.
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Old 12th March 2010, 10:57 PM   #8
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by santa View Post
Ok, line freq is 60.00 hz, both secondaries are solid at 60.00 hz.
533 VAC from each half of the secondaries.
The spike is present at the R25/C14 junction so it looks like it is on the main supply.
The filament supply does not show the spike.
The +/- 150 VDC supplies do not show the spike.
Bah, Think I'll just turn it up for a while and rock out..
Actually what you said is not necessarily true, is it also present on the center tap of your OPT? or the other side of R25?

If you disconnect the feedback from the output of the amplifier does the spike at C14 go away? I'm wondering if you have magnetic coupling between the choke and the OPT or an issue with the way you have configured your internal grounds.

With the feedback disconnected check all stages for presence of those spikes. You may be picking it up somewhere else.

Can you post pictures of the internal layout and wiring? Something might be more obvious them..
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Old 12th March 2010, 11:00 PM   #9
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Looks like the diodes are causing switching glitches.
I had the exact same problem and scope picture on my valve pre amp.
Cured it with a capacitor across the supply after the diodes.
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Old 12th March 2010, 11:19 PM   #10
santa is offline santa  United States
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Lets see,
The CT is grounded to the chassis and negative buss at the PT, one of the next components along the buss is the first cap.
I did not probe the high side of R25 or the CT as I am leery of blowing up my scope.
Can I put a cap of sufficent voltage rating in series with my scope probe to ensure DC blockage when measuring the B+?

I can't work on the amp until tomorow night but
I'll snap a few pics tonight and post them tomorrow morning.
I did put another cap just past the diodes as suggested but no improvement was found.
I have used these diodes before at higher voltage and current and have not had this problem before so I'm thinking induction or a ground issue.

Thank you guys very much...
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