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Old 10th August 2006, 05:31 AM   #11
LuisMCP is offline LuisMCP  Spain
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Quote:
Originally posted by danFrank
Make sure your rectifiers are good also. In a push pull amp even 100uf in the power supply is more than enough to get the hum down into the mv range, so I don't think adding more uf will help much. .
Daniel
Hi danFrank,

I used UF5408 diodes (ultra-fast) with 0.1 uF mkp in parallel with each diode.

I agree with you that 200 uF could be enough for low noise.

Quote:
Originally posted by danFrank
Check the diodes to make sure they aren't passing AC
I suppose you want say DC.

mmm... interesting. I will test it.

Quote:
Originally posted by danFrank

Thinking about it some more, I would thnk your problem lies with a ground loop. Usually hum doesn't increase that much when the volume is turned up with a power supply problem; it's VERY loud all the time!
Daniel
Surely. At the moment, I replace some GND points (RCA and Volume POT) and noise is lower and is the same when Volume POT is moved.
This was a huge improvement.

I think the same, noise is greatly caused by ground-loops or a bad 0V reference point, when hum arises with volume turned up, things seems clear to me that there is an earthing issue.


cheers,

Luis.
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Old 10th August 2006, 05:44 AM   #12
LuisMCP is offline LuisMCP  Spain
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Quote:
Originally posted by bembel
Hi, to me 6Vpp is a lot too !
Jed's remarks are very pertinent (is your CD player three or two connectors plug to mains ? ----> ground loops)
Have a look at this, it may be helpfull ---> http://www.tone-lizard.com/High_Resistance_Ground.htm
Do you see any difference tightening inputs/outputs cables together? (thus reducing ground loops on interstages cables)

Hope it may help.
Great article !

thanks.
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Old 10th August 2006, 05:58 AM   #13
LuisMCP is offline LuisMCP  Spain
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Quote:
Originally posted by rwellerson
Hi Luis

Try this: from B+ make a voltage divider to get around 40 to 45 V.
Place 1 microF x 250V poliester from this point (40~45V) to ground.
Connect that point to the CT of the heaters (and of course disconnect the CT from ground...)

And say good bye to hum.... (well: in 99% of excessive hum instances it will be gone...)
And it doesn't hurt to use DC for the heaters' supply to the input stages....

Buenas sortes
Ricardo

Hi Ricardo,

Ouch! I forgot the good old methods.

I have not a voltage divider in my PSU, but I have cathode bias at EL34 (with a resistor), with 33 VDC at cathodes and I could tie CT filaments at this point.

It seems a good advice.

Thanks Ricardo!

Luis.
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Old 22nd August 2006, 05:49 AM   #14
LuisMCP is offline LuisMCP  Spain
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Default Re: hum

Quote:
Originally posted by LuisMCP
Hi all,

I have some problems with hum in a tube amp.

Hum is bigger when I turn volume pot to max volume, and when I plug a source in the RCAs input.

Hum is always seen as a 100 Hz signal in scope.

Ripple in PSU output is small, about 6V peak to peak.

Some help is welcome.

regards,

luis.
Hi all,

Further investigation about the matter give me a new point of view, I saw that my amp had three kind of noise:

1) Ground Loop hum.
2) High frequency noise
3) Some noise of ripple from PSU.


1) Was eliminated first day, when i reconnect RCA grounds.
2) This noise is done in PSU and snubbering is a must. I use an RC network to lowering this noise.
3) Was located at phase splitter stage, and mismatched output tubes makes things worse; it could be fixed with more filtering in
voltage amplifier stages or using a voltage regulator. I have to try these in next days.

In the meantime, I am not shure how good mains transformer is, when I have measured these numbers:


Ls (leakage inductance at secondary) = 23 mH
Cs (leakage capacitance at secondary) = 1.21 nF
Total Copper Losses: 16 watts.
Total Iron Losses: 9 watts.
Efficiency: 90%


I would like to know if you have measured your mains transformer, for comparation.
It seems that Ls and Cs are very large.

Regards,

Luis.
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