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Old 16th March 2013, 12:54 AM   #21
Gilgy is offline Gilgy  United Kingdom
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Can't see how the schematic doesn't make sense!

There's nothing there that really stands out as a source of hum. Preamp tubes and phase splitter have DC heaters with a single supply shared (I assume) across two monoblocks.

I'd be looking more at the placement, shielding and orientation of the transformers.
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Old 16th March 2013, 01:06 AM   #22
AJT is offline AJT  Philippines
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in my amp builds i locate the input jacks as far away from any traffos as i can get away with....though some find it ugly....
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Old 16th March 2013, 06:49 AM   #23
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I had a friend over to the house tonight to hear my system. He's heard a lot of tube amps in his time.

"That's nothing.", he said. "Forget about it."
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Old 16th March 2013, 08:21 AM   #24
hpeter is offline hpeter  Europe
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Quote:
Originally Posted by TheGimp View Post
Wrong on the voltage rating.

Inductors are specified for Maximum DC voltage rating.

They are not guaranteed not to break down above the rating. While most will be tested at higher ratings, it is a bad idea to ignore the specifications.

Check the Hammond site for an example:

Hammond Mfg. - D.C. Filter Chokes - (153 - 159 Series)
i saw a schematic where chokes were used between minus poles of capacitors (last cap minus was also GND point), plus pins were just connected together
this way, choke will stay at zero potential
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Old 16th March 2013, 10:41 AM   #25
12E1 is offline 12E1  United Kingdom
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I suspect that the other noises being heard (other than hum) could well be due to some RF instability. Unless the layout and circuit has been altered in this area, I would say that the grid stopper resistors (R9 and R10) are in the wrong place. Firstly they are on the PCB and really should be as close to the tube socket as possible. Second, they should be between the grid bias resistor and the tube, rather than having both the bias and stopper resistors joined to the tube. Having the two components on a tracked PCB connecting to the grid on the tube rather than just a single directly wired resistor significantly reduces the effectiveness of the grid stopper.

So, to try this - remove R9 and R10. Replace each with a very short insulated wire. Then connect the removed resistors in place of the blue wires that connect from the PCBs to pin 5 on the EL34 tube sockets. It should not make anything worse, and it may very well help. You should, of course, apply the modification to both channels - doing just one may be tempting but if there is a problem here, if one channel is still unstable it could also affect the other.

It will not cost anything to try this, and if the current draw is more stable it may also lessen the need to tweak the power supply.

Last edited by 12E1; 16th March 2013 at 10:46 AM.
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Old 16th March 2013, 02:25 PM   #26
MelB is offline MelB  Canada
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See if you have room to enlarge C12. Something like this:
ESMH100VSN473MQ50T United Chemi-Con | 565-2601-ND | DigiKey

48000uF @10v might fit right in there.....could lower your humm a lot for $5.
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Old 16th March 2013, 03:46 PM   #27
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Thanks a lot guys!

I love to tweaak things, and any reason to heat up the iron is reason for me to get excited. Sounds trivial, perhaps, but it is so much fun for me.
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Old 16th March 2013, 05:23 PM   #28
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Default Update

I turned the amp on this morning and after about 2 minutes of listening I started getting considerable buzz-crackle in the left channel, with corresponding drop in music volume. Right away I knew this must be a bad solder joint related to my capacitor upgrade.

I was right. I reflowed the left channel cap joints and bingo, fixed.i

A side effect is that the intermittent buzz-sizzle seems to have been eliminated. I'm guessing that was the issue all along.

Might look at upgrading C12 and re-arranging R9 and R10 connections at a later date. Just happy to have it running right. Got my solder fix for the day.

Note: I figure it worked perfectly last night for the listening session with my friend because it had been on all day so it was nice and hot. Overnight it cooled and the solder joint returned to its default dodginess.

Last edited by cogitech; 16th March 2013 at 05:39 PM.
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Old 17th March 2013, 01:22 AM   #29
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Quote:
Originally Posted by cogitech View Post
I turned the amp on this morning and after about 2 minutes of listening I started getting considerable buzz-crackle in the left channel, with corresponding drop in music volume. Right away I knew this must be a bad solder joint related to my capacitor upgrade.

I was right. I reflowed the left channel cap joints and bingo, fixed.i

A side effect is that the intermittent buzz-sizzle seems to have been eliminated. I'm guessing that was the issue all along.

Might look at upgrading C12 and re-arranging R9 and R10 connections at a later date. Just happy to have it running right. Got my solder fix for the day.

Note: I figure it worked perfectly last night for the listening session with my friend because it had been on all day so it was nice and hot. Overnight it cooled and the solder joint returned to its default dodginess.
The Chinese amps I've worked on had that crappy lead free solder. When you use leaded solder you need to remove all the lead free crap. They have different melting points which causes cold solder joints and the situation you mentioned. I'd go back to all the solder joints you touched and give them a touch up at least. Last time I had to work on one with lead free solder I just broke down and removed all that lead free solder and replaced it with leaded silver. Every one of those lead free solder joints looks suspicious to me, probably the way that stuff is supposed to look like but I cant get used to the look not shiny and actually the solder looks like it needs more heat.

Cheers,
Bob
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Old 19th March 2013, 03:44 PM   #30
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Update: I was wrong. It isn't fixed

3 times now I have gone under the hood to reflow solder joints in the left channel and I am beginning to get pretty frustrated. The symptoms are now very consistent:

1) When I first turn the amp on both channels work fine

2) Some time within the first five minutes or so, the left channel begins intermittently dropping out partially and there is obvious distortion or "scratchiness"

3) This continues on and off over the next 20 to 30 minutes, and finally it settles down and starts to function normally again

I could solve the problem by simply turning the amp on 30 minutes before using it (as many people do religiously) however I would much rather solve this issue as I suspect it will just get worse over time.

What are the usual suspects, based on the symptoms? Solder joints? Socket contacts? Tube pins?

Seems obvious that it is temperature related, so "some connection" is dodgy enough to be affected by heat expansion only at a certain temperature range...

Last edited by cogitech; 19th March 2013 at 03:53 PM.
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