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Old 10th December 2010, 07:43 PM   #11
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is this a 60hz hum or 120hz hum, or a wining noise?

do you have a 2 prong or 3 prong power cord on your amp and power amp? don't know why, but sometimes hum is caused by using a 3 prong cord in the amp or even preamp. I chased many ghost thruout many power amps, only to find the noise did not originate in the amp. for testing purposes, you could down convert a 3 prong, to a 2 prong using that grey plug addatper with the green wire.

another hum ghosts was scared off by changing driver tube type....had 12aT7's, put in 12hb7's, had 6414's, put in 6829's, etc.

also, have you tried metalized film by-pass capacitors? the frequency you are talking is uneffected by eletrolytic capacitors, you need a small value metal or poly film cap.

good luck, keep us posted
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Old 11th December 2010, 12:40 PM   #12
DF96 is offline DF96  England
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Hum is very much affected by electrolytics. It is unaffected by small value bypasses - whatever they do is at much higher frequencies.
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Old 11th December 2010, 02:53 PM   #13
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Originally Posted by DF96 View Post
Oscillation can sometimes happen with UL. If not already present, try adding grid stoppers to the output control grids. Then g2 stoppers (say, 470ohms or 1K 2W wirewound). Finally, you may have to add snubber networks from each anode to g2.
Confusing ? See pic.
The principle applies whatever the poweramp rating esp when the UL tap is a greater part of the primary. In UL designs using high gm tubes and wide bandwidth transformers which may appear stable without this Zobel, there are other advantages in implementing it.

richy
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Old 13th December 2010, 10:27 AM   #14
vlucian is offline vlucian  Romania
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Sorry for this long silence. I take back home the amplifier, so no more Mr. Oscilloscope! I tried again with the old transformers (those without UL) and ... the hum wasn't there anymore! So I tried with the new OTs but using this time the same penthode mode and miracle! the hum disappeard! I must recognize I didn't try putting snubber networks as DF96 told me but I had grid stoppers in g2. Now I am in penthode mode and I have some other problems with anodic current but tonight I'll put some bigger values for those potentiometers in the negative bias area. Now I know the amplifier could be dead-silence. I still have some doubts: penthode, triode or UL? Anyway, I let know about the results.
thanks again.

Last edited by vlucian; 13th December 2010 at 10:31 AM.
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Old 13th December 2010, 02:26 PM   #15
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Originally Posted by vlucian View Post
Sorry for this long silence. I take back home the amplifier, so no more Mr. Oscilloscope! I tried again with the old transformers (those without UL) and ... the hum wasn't there anymore! So I tried with the new OTs but using this time the same penthode mode and miracle! the hum disappeard! I must recognize I didn't try putting snubber networks as DF96 told me but I had grid stoppers in g2. Now I am in penthode mode and I have some other problems with anodic current but tonight I'll put some bigger values for those potentiometers in the negative bias area. Now I know the amplifier could be dead-silence. I still have some doubts: penthode, triode or UL? Anyway, I let know about the results.
thanks again.
Get that scope back again and examine the output stage....Tell-tale signs.....My guess is if anode current varies for no other reason, then output stage is oscillating into HF.......Do Not connect speaker, use a dummy load instead.

richy
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Old 14th December 2010, 10:36 PM   #16
vlucian is offline vlucian  Romania
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The scope was an old, huge Tesla and the friend don't have a high voltage probe.
Anyway, good news: I think I have a resonable hum. I'm sure it's induced by power transformer. But! If I connect only one RCA input connector. If I connect both, then the noise goes a bit up only in one channel. Somehow there is a ground loop that get closed through the signal cables, right in the cd player. But why is it humming only one channel? Another strange thing on the other channel: even if the volume is at minimum there I can stil hear the music playing on cd (very very low level, but it's there). Just to have the entire picture: yesterday I had another strange situation - slowly turning the volume, at one point there was a small crack noise only in one channel and the anodic current was going nuts: on that channel by doubling (from 45 to 100 mA) on the other channel dropping from 45 to 35 mA. I changed the value of NFB resistance from 10k to 30k and this behaviour ends.
I made a primitive sketch of my cables connection, I can't attache it.
Thanks.
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Old 15th December 2010, 10:16 AM   #17
DF96 is offline DF96  England
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That 'sounds' like an ultrasonic oscillation around the global feedback loop in one channel.

You now need to adjust the compensation to get it stable. A capacitor across the feedback resistor, with or without a resistor in series, and/or a lead-lag CR network at the inout stage anode. The circuit shown in post #13 has both of these, but you need to adjust the values to suit your circuit/OPT/layout.
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Old 15th December 2010, 12:01 PM   #18
vlucian is offline vlucian  Romania
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Quote:
Originally Posted by DF96 View Post
That 'sounds' like an ultrasonic oscillation around the global feedback loop in one channel.

You now need to adjust the compensation to get it stable. A capacitor across the feedback resistor, with or without a resistor in series, and/or a lead-lag CR network at the inout stage anode. The circuit shown in post #13 has both of these, but you need to adjust the values to suit your circuit/OPT/layout.
You mean by 'that' the strange modification of the anodic current? If so, then this is solved (I hope) after modifying the value of NFB resistor (as I said).
Now the problem is with that ground loop. I read this post Help! ground hum and I'll try to do the same.
About the other thing with the sound keep going even with the volume at minimum, I think I must check the potentiometer maybe is something wrong there.
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Old 15th December 2010, 06:06 PM   #19
DF96 is offline DF96  England
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The change in the NFB resistor will have increased the voltage gain by about 3, which may or may not be what you want. You will get a bit more distortion and a narrower bandwidth, but better than oscillation!

Some volume pots don't quite go to zero at the bottom, but it could be a grounding problem.
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Old 15th December 2010, 07:35 PM   #20
vlucian is offline vlucian  Romania
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Originally Posted by DF96 View Post
The change in the NFB resistor will have increased the voltage gain by about 3, which may or may not be what you want. You will get a bit more distortion and a narrower bandwidth, but better than oscillation!.
Maybe I should try with something like 20K.

Quote:
Originally Posted by DF96 View Post
Some volume pots don't quite go to zero at the bottom, but it could be a grounding problem.
I tried another pot with the same result. So it's a grounding problem... but where? What do you suggest to look for? I really don't have the nerve to start again.
On the other hand now I only have the induction hum from the power transformer... it start immediatelly after powering on and remains the same. I did what Tom suggested at page 4 in Help! ground hum and apparently that was all.
Thanks!~
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