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Noise problem in 572-10 PP amplifier
Noise problem in 572-10 PP amplifier
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Old 12th March 2010, 10:22 PM   #11
nigelwright7557 is offline nigelwright7557  United Kingdom
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Quote:
Originally Posted by santa View Post
Lets see,

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...
It needs something like 100nf to quench the spikes.
An electrolytic is too inductive to do it.
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Old 12th March 2010, 10:30 PM   #12
santa is offline santa  United States
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I think I have a 0.68uF in that position, not sure of the material. I think it is met. polypro. Should that be a smaller value?
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Old 12th March 2010, 10:32 PM   #13
nigelwright7557 is offline nigelwright7557  United Kingdom
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Originally Posted by santa View Post
I think I have a 0.68uF in that position, not sure of the material. I think it is met. polypro. Should that be a smaller value?
That should work ok but you need one in each leg.
Each diode needs a capacitor across its output to ground.
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Old 19th March 2010, 03:24 PM   #14
santa is offline santa  United States
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Family issues have kept me away from the shop until yesterday when I had a few minutes to probe the circuit.

I made a small coil and connected it to the scope and was able to see the spikes are from the diodes conducting.
As the diodes have top cap I placed a 0.1 uf at the junction where the wires join at the main fuse.
This seems to quiet the amp down a bit, for example I could hear switching noise from my computer PS before, none after the cap was added.
Also the spike is now cleaner, less ringing and a fast recovery
I realize that the HV wire from the top cap is an aerial, I could use a shielded wire but the spikes are radiated right through the glass envelope.

I then probed the circuit and find that the power supplies do not show the spike.

Looks like kevin kr was correct, the feedback wire is carrying the spike back into the front end. I think the noise is coupled into the wire as I ran it too close to the rectifiers.
First I'll open the loop and hopefully things will quiet down. Then relocation to a quieter location in the chassis and I should be set.

Here's a pic of the underside. Originally I had it buttoned up nicely but troubleshooting and adding caps has made things a mess.
I plan to eliminate the noise, clean things up and fab a botton cover.

Click the image to open in full size.
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Old 19th March 2010, 04:42 PM   #15
santa is offline santa  United States
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almost forgot,
I wanted to ask,

If I reduce the value of the first cap in the PS by say 50% would that reduce the spikes?

I modeled this in duncan PSU and did not see a change but I am not sure if the is is the case in reality.
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Old 19th March 2010, 04:48 PM   #16
kevinkr is offline kevinkr  United States
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Noise problem in 572-10 PP amplifier
Very crowded inside that chassis.. In looking I also noticed that there are a lot of excessively long wiring loops - filaments, drive to the final grids, etc. - you really need to shorten up these lines, and dress them close to the metal chassis plate. One grid line in particular is looping all over the place..

I'd also recommend using tie strips or similar rather than the many floating connections I see whether insulated or not.

There are so many transformers scattered around the chassis hopefully none of them are inducing into the OPT.
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Old 19th March 2010, 08:00 PM   #17
santa is offline santa  United States
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yes, its pretty ugly.
Things were neater before I started "troubleshooting"
I'll clean things up and report back.
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Old 20th March 2010, 01:06 AM   #18
Michael Koster is offline Michael Koster  United States
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I can't see all of your layout but your schematic shows the 6D22 heaters floating. I wonder if somehow the heater-cathode voltage is being exceeded. The heaters should be connected to the cathodes.

Cheers,

Michael
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Old 21st March 2010, 10:24 PM   #19
santa is offline santa  United States
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I hsve never had the need to bias the heaters in my previous SE build.

Last night I was probing and pushing wires around and noteed that the feedback pot seemed especially sensitive to noise like a magnet for noise
actually.

Today I rewired the amp and the noise problem is gone, vanished completely.
I still have a few things I want to fix but today I am a happy camper indeed.
Lessons learned (again)...

Pics forthcoming
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Old 21st March 2010, 10:53 PM   #20
kevinkr is offline kevinkr  United States
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Noise problem in 572-10 PP amplifier
Quote:
Originally Posted by santa View Post
I hsve never had the need to bias the heaters in my previous SE build.

Last night I was probing and pushing wires around and noteed that the feedback pot seemed especially sensitive to noise like a magnet for noise
actually.

Today I rewired the amp and the noise problem is gone, vanished completely.
I still have a few things I want to fix but today I am a happy camper indeed.
Lessons learned (again)...

Pics forthcoming

Sounds like all good news.

I do completely agree with Mike about the rectifier filaments - it is a good practice as it minimizes cathode to filament insulation stress. In the event that you don't trust the filament windings to handle the full B+ voltage you can often use a resistive divider and reference the filaments to that with the proviso that the voltage after division should be within the insulation capability of both the filament/cathode and the transformer winding insulation. NOTE: This obviously only applies to rectifiers such as indirect heated types without a common filament/cathode connection..

The exception to this rule is in lower voltage applications where you wish to use the same 6.3V filament supply to heat a bunch of tubes in which case the filament supply is usually referenced to some % of filtered B+ that is safe for all tubes used. Needless to say this is not a very good idea in applications where very high voltages are present on the cathode of the rectifier. (Say anything over 350V or so) 6V4, 6CA4, 6AU4, 6BY5 are all types where this may be done safely at reasonable voltages.
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Last edited by kevinkr; 21st March 2010 at 10:59 PM.
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