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PostPosted: 21 Nov 2018, 02:38 
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Joined: 24 Oct 2010, 07:05
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Danny wrote:
The noise stops instantly when I turn the power off. However, when the B+ kicks-in (after the time delay) it takes a few seconds for the noise to come-up to full level.
Could you turn the power off while the amplifier plays music? It came to my mind that you may have too low capacitance in the power supply filters to support the necessary current after powering off. By the way it (low capacitance) may be the source of the noise, but in my opinion in this case the noise should be rather close to the doubled mains frequency, not the wide band noise you have.


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PostPosted: 25 Nov 2018, 17:35 
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Joined: 29 Jun 2016, 15:59
Posts: 60
Bruce & Poty,
The latest findings:
1) placing a short between the grid of the lower triode section of the driver stage and ground, did not make a difference. The noise is still there and at the same level.
2) same for disconnecting the LED illuminating the driver tube.
3) turning the power off while the amp was playing music resulted in the audio slowly coming down. It takes about 15 seconds for the audio to dissipate completely. Don’t think the problem is low capacitance. I am using the 5/6/15 unified schematic for OddBlock amp power supply schematic.

Can you elaborate on the switching noise from the rectifiers issue? And how would I go about resolving it?

On a separate topic, I believe I read that I can use the lower current rating (60 ma) with the KT88s. What would be the pros and cons?

Thanks,
Danny


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PostPosted: 26 Nov 2018, 04:27 
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Danny wrote:
3) turning the power off while the amp was playing music resulted in the audio slowly coming down. It takes about 15 seconds for the audio to dissipate completely.
OK.
Danny wrote:
I am using the 5/6/15 unified schematic for OddBlock amp power supply schematic.
You were speaking of
this? Do you use LR8 for the driver section or not?
Danny wrote:
Can you elaborate on the switching noise from the rectifiers issue? And how would I go about resolving it?
To be short: there are several points where rectifiers can add noise to the power line. You can mitigate such noise by choosing a fast and ultrafast rectifiers, use dampers in the first and secondary transformer windings, bypassing the electrolytics in filters with film capacitors and so on. Most of the measures are already in. But usually it does not bring any problem to the output stage and the driver stage is heavily filtered (or even stabilized by LR8). In your case the driver stage gets something from the power line, so I'm trying to understand what you are using here.
What power transformer do you use?
Wild guess: the design depends on the safety ground wire in mains. Is it possible you have problems here?


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PostPosted: 26 Nov 2018, 12:26 
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Joined: 29 Jun 2016, 15:59
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Poty,

Do you use LR8 for the driver section or not?
Yes.

You were speaking of this?
Yes. Except that this is the newer PS schematic which adds a 1 uF cap between the LR8 and GND (I do not have this additional cap). Also, the amp section has an additional 1 uF cap at the top driver tube's anode (I do not have this additional cap also).
Should I add these 2 caps? Would they make a difference?

What power transformer do you use?
The one made specifically for the KT88 Odd Blocks - the Edcor XPWR264 Power Transformer (120V, 60Hz. or 240V, 50/60Hz. line to 360V (180-0-180) at 350mA center tapped and 10.9V at 2A.)

the design depends on the safety ground wire in mains.
I use a line filter whose earth's GND tab is connected to the aluminum plate and to the amp's GND through a 120 ohm and 0.1uF cap in parallel.

Thanks,

Danny


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PostPosted: 27 Nov 2018, 02:54 
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Danny wrote:
Should I add these 2 caps? Would they make a difference?
They could make a difference, but I think the source is somewhere else. Adding the capacitors could improve current situation, but the situation should be good even without it.
In an other thread there is a report for similar problem, I even pointed to the discussion here from the thread. While I never heard of such behaviour of LR8, the user states that it oscillates in his circuit. Some messages earlier Bruce mentioned the possibility of oscillation as a source of the problem, but from another part - NFB. Could you try to temporarily add a capacitor 100uF 400V in parallel with a 0.1-1uF film capacitor between the anode of upper driver tube and the ground (mind safety!!!) and listen to the result?
Danny wrote:
the design depends on the safety ground wire in mains.
I use a line filter whose earth's GND tab is connected to the aluminum plate and to the amp's GND through a 120 ohm and 0.1uF cap in parallel.
You are describing the input section of your block (it seems correct), but I asked about the safety ground of your mains - does it really exist and properly connected to your electric outlet?


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PostPosted: 28 Nov 2018, 11:28 
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Joined: 29 Jun 2016, 15:59
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Poty,
Thanks.
I would have to buy the 100uF & 1uF caps to try your temporary test (to install between the anode of the upper driver tube and ground). What would this prove?

The mains safety ground is good. Checked with an AC plug tester.

Also, it seems that I am stuck unless I purchase an oscilloscope to check the B+ or the output.
Do you have any recommendation on what to buy? Hopefully not too expensive?
Thanks,
Danny


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PostPosted: 29 Nov 2018, 20:05 
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Joined: 29 Nov 2011, 01:02
Posts: 18
Location: Doha, Qatar
Danny,

Maybe you could post a few pictures of the interior build of your amp. They say a picture paints a thousand words. I have built several of Bruce's amps and have never had the problems that you are having.

Regards

Greg

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PostPosted: 30 Nov 2018, 18:58 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, Yes post some photos. These amps are very stable and very quiet. So something is unusual about yours in either layout or build. There can be faulty components, but it doesn't sound like any situaltion I have encountered before.

Good listening
Bruce

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PostPosted: 02 Dec 2018, 11:03 
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Team,

Here's 3 pictures. Let me know if you need other more specific pictures. Thanks again,

Danny


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PostPosted: 03 Dec 2018, 16:41 
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Location: Arizona, USA
I don't see anything all that unusual. I will have to copy the images to a program I can really look closely though. Two thoughts occur to me. Both easy. Try grounding to the signal grounds the metal cases of the coupling caps. One at a time. They can pick up stuff from the environment and possibly be the source of the noise. If it doesn't make any difference then temporarily attach a wire from the grid of the slave tube to the signal ground. You may need to re-balance the tubes. See if this fixes the problem. Let us know what happens.

Good listening
Bruce

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