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Old 15th December 2011, 03:00 AM   #11
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Thanks George. Funny about the schematic :-) No complains, the important is that it is correct on the PCB.

I'll try the extra cap. I have 15000uF now, from what I measured this cap sees only the 3VAC, so a cap rated 6.3 should do.

I did the trick with the trimmer: I completely replaced R1, and lowered the DC voltage (increased the resistance) until the noise was gone. I ended up with 2.25 V. I hope less reduction would be sufficient. Now it is silent. but I am not sure if I did more damage sound wise.

I was considering the elevated filament voltage, as I read that it provides sonic improvement, but I will trust your judgement and forget about it.

Thanks,

Davide
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Old 16th December 2011, 01:08 AM   #12
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So, I added another 15000uF under the board, and now I manage to go to 2.5 V with no noise.

As I am messing with this build I have two other questions:

1) I have a 100k pot installed now, but I remember you said that it should not be greater than 50k. Is something that is wotrh fixing ?

2) I do not manage to bring the HV of the drivers more than 150 V. I should change the cathode resistors to 680 ohm, or something like this. Again, does this bring any improvement in the real world ?

Thanks,

Davide
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Old 18th December 2011, 12:01 AM   #13
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Make that three people - it's been driving me crazy all afternoon so I went searching the forum and just saw this thread!
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Old 18th December 2011, 11:50 PM   #14
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George,

Any hint on the pot question ? Is it worth to replace the 100K that I have with a 50 K ?

Thanks,

Davide
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Old 20th December 2011, 05:34 AM   #15
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So,

With George advices, I managed to clean the (noise) floor. This is with 1V input sine wave or 10 ohm resistive load.

I still have a small peak at 10kHz, even with input shorted, but we are speaking of not audible things. I will try to get rid of it. To do so I was thinking of bypassing either the filaments and/or the OPT supply with 1uF film caps. Not sure If it will have any effect, I should try first to understand where it comes from.

I have some 5842 and some 45 taken here and there on e-bay, and they can be really different. With 1V in I had the output ranging from 2.4V to 2.9V AC RMS. I managed to get a couple that give me 2.6V.

As you can see in the diagrams, even the noise floor can be different.

If I decide to change the potentiometer, should I go for 50k or something even smaller?

Thanks,

Davide
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Old 20th December 2011, 05:57 AM   #16
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Is Japan still 100V/50Hz? This is the worst possible situation for a line frequency transformer. Instead of tweaking the outputs that are running out of headroom (assuming that no 100V tap is available on the transformer input), another approach would be to hook up a 12V filament XFMR with 1500V isolation with the primary across the input (in parallel with the power XFMR primary). Hook the secondary leads in series with the main power transformer primary - one connection will buck the primary voltage, the opposite will boost. The boost connection should clear up headroom problems.
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Old 20th December 2011, 06:01 AM   #17
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Another thing I noticed is that the square wave has this shape. I remember Morgan John book had a chapter about how to read the square wave, but I do not have it with me.

Davide
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Old 20th December 2011, 06:03 AM   #18
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Quote:
Originally Posted by wrenchone View Post
Is Japan still 100V/50Hz? This is the worst possible situation for a line frequency transformer. Instead of tweaking the outputs that are running out of headroom (assuming that no 100V tap is available on the transformer input), another approach would be to hook up a 12V filament XFMR with 1500V isolation with the primary across the input (in parallel with the power XFMR primary). Hook the secondary leads in series with the main power transformer primary - one connection will buck the primary voltage, the opposite will boost. The boost connection should clear up headroom problems.
I have 100V tap. Painfully custom made transformers.

D.
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Old 20th December 2011, 06:05 AM   #19
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The tilt is most likely from the low frequency BW limit of the output XFMR. This is a typical waveform for a transformer-coupled amp at low frequency, and will straighten out when the square wave input frequency is increased.
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Old 20th December 2011, 06:11 AM   #20
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Thanks !!! Power of the internet.

Here is the theory http://www.kennethkuhn.com/students/...ve_testing.pdf

I have a low cutoff frequency of 24 Hz. Wrenchone, you were perfectly right.

D.
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