Old amp hum repair - Page 3

Feagil · 2nd April 2012, 11:28 PM

You guys are right. I will replace the power supply caps and try that signal trick then get back to you.

Thanks

Electron Ron · 3rd April 2012, 03:22 AM

If you can determine that it indeed is 60 Hz or whatever your mains frequency is, and the amp is quiet when the volume control is all the way down; then it's probably a poor circuit ground to chassis ground on your input jack itself. This should be the only connection between the circuit ground and the chassis. Check to see that it's not oxidized and is making a good connection.

KatieandDad · 3rd April 2012, 01:23 PM

Quote:

Originally Posted by Alexontherocks

I told the poster to parallel twp 20uf new caps if he can't gind a single 40uf new one

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Sorry, I was referring to hos post #15 not yours.

Alexontherocks · 3rd April 2012, 02:35 PM

Sorry... Thought you referred to my post.

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Feagil · 9th April 2012, 12:42 PM

Hey everyone

Due to the Easter weekend and the fact that high voltage caps can be a bit tricky to find in SA I havent had a chance to swap out the caps yet. I have a 1000uF 350V Siemens cap from an old professor friend of mine. Could I risk using this cap? The caps in there are obviously much smaller, 20uF each but there are a few in parallel. Should I try that cap or rather wait till I can source the ones with the correct ratings.?

KatieandDad · 9th April 2012, 12:44 PM

I would use caps as close as possible to the originals if I were you.

M Gregg · 9th April 2012, 12:50 PM

Quote:

Originally Posted by KatieandDad

I would use caps as close as possible to the originals if I were you.

+1

At least untill you know what your dealing with extra uF can lift B+ and destroy old components..Coupling caps etc..

Regards
M. Gregg

Alexontherocks · 9th April 2012, 02:02 PM

If higher b+ is a concern you can always tune it down with a resistor after the cap. Either that or place multiple identical high uf value caps in series. Cspacitance will be reduced greatly.

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Electron Ron · 9th April 2012, 10:19 PM

Replacing the original 40uf cap with a 1000uf cap is not a good idea for two more reasons than the reason previously posted by M Gregg and they are: 1 If the old professor's 1,000uf cap hasn't been charged up in several years it may be no good or it may need to have it's plates reformed, either way you will destroy your rectifier if it's a 5U4 or 5Y3 or similar rectifier. The second reason is :Tube rectifiers aren't made to handle such a low reactance load as a 1,000uf cap presents and a 5U4 or 5Y3 will not be long lived if it's connected directly to a 1,000uf cap. Back to your original hunch that it's ground related. Did you say the amp is quiet with the volume all the way down? If so re-read my earlier post.

Alexontherocks · 10th April 2012, 08:08 AM

Cap loading a tube rectifier is always a bad idea. Tubes are prone to arc under the wrong conditions. A small choke is always the best way.

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2nd April 2012, 11:28 PM	#21
Feagil diyAudio Member Join Date: Apr 2012	You guys are right. I will replace the power supply caps and try that signal trick then get back to you. Thanks

3rd April 2012, 03:22 AM	#22
Electron Ron diyAudio Member Join Date: Feb 2011 Location: N Y	If you can determine that it indeed is 60 Hz or whatever your mains frequency is, and the amp is quiet when the volume control is all the way down; then it's probably a poor circuit ground to chassis ground on your input jack itself. This should be the only connection between the circuit ground and the chassis. Check to see that it's not oxidized and is making a good connection.

3rd April 2012, 02:35 PM	#24
Alexontherocks Account disabled at member's request Join Date: Jul 2009	Sorry... Thought you referred to my post. Inviato dal mio GT-I5800 usando Tapatalk

9th April 2012, 12:42 PM	#25
Feagil diyAudio Member Join Date: Apr 2012	Hey everyone Due to the Easter weekend and the fact that high voltage caps can be a bit tricky to find in SA I havent had a chance to swap out the caps yet. I have a 1000uF 350V Siemens cap from an old professor friend of mine. Could I risk using this cap? The caps in there are obviously much smaller, 20uF each but there are a few in parallel. Should I try that cap or rather wait till I can source the ones with the correct ratings.?

9th April 2012, 12:44 PM	#26
KatieandDad diyAudio Member Join Date: Nov 2011 Location: UK	I would use caps as close as possible to the originals if I were you.

9th April 2012, 02:02 PM	#28
Alexontherocks Account disabled at member's request Join Date: Jul 2009	If higher b+ is a concern you can always tune it down with a resistor after the cap. Either that or place multiple identical high uf value caps in series. Cspacitance will be reduced greatly. Inviato dal mio GT-I5800 usando Tapatalk

9th April 2012, 10:19 PM	#29
Electron Ron diyAudio Member Join Date: Feb 2011 Location: N Y	Replacing the original 40uf cap with a 1000uf cap is not a good idea for two more reasons than the reason previously posted by M Gregg and they are: 1 If the old professor's 1,000uf cap hasn't been charged up in several years it may be no good or it may need to have it's plates reformed, either way you will destroy your rectifier if it's a 5U4 or 5Y3 or similar rectifier. The second reason is :Tube rectifiers aren't made to handle such a low reactance load as a 1,000uf cap presents and a 5U4 or 5Y3 will not be long lived if it's connected directly to a 1,000uf cap. Back to your original hunch that it's ground related. Did you say the amp is quiet with the volume all the way down? If so re-read my earlier post.

10th April 2012, 08:08 AM	#30
Alexontherocks Account disabled at member's request Join Date: Jul 2009	Cap loading a tube rectifier is always a bad idea. Tubes are prone to arc under the wrong conditions. A small choke is always the best way. Inviato dal mio GT-I5800 usando Tapatalk