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Old 19th December 2004, 04:08 PM   #11
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The parts sheet calls for 6) 100mfd/450V caps. I have them below the board.
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Old 19th December 2004, 04:13 PM   #12
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Default ling stage board off from ovrseas.problems

I see the bypass caps on the schematics but not on the picture of the board. When bypass cap dry out or fail,you'll get hum
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Old 19th December 2004, 04:16 PM   #13
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Quote:
Originally posted by burnedfingers
Jan

The parts sheet calls for 6) 100mfd/450V caps. I have them below the board.
OK, I see. I thought they were different ones, the board looked low on the sheet in the pic.
How loud is the hum? Where is your volume control, before the preamp? Did you try with the input shorted?

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Old 19th December 2004, 04:18 PM   #14
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Any comments on the feedback change I made?
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Old 19th December 2004, 04:29 PM   #15
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Well, the 330pF with the 100k will roll your freq response off starting at 5 kHz or so. This will defenitely sound dull -
In fact, this may cause the ramping, what is the ramp frequency?

The 1pF is not unreasonable for ultrasonic roll off. Try the smallest cap you can find, like 5 pF.

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Old 19th December 2004, 04:39 PM   #16
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So I must have had a high frequency osolaton with the 470K resistor in the feedback loop?

Am I more in line with the different feedback resistor value?

Can't I leave it off (1P cap in feedback loop)since the tube amp should cover the low frequency roll off?
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Old 19th December 2004, 04:43 PM   #17
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Volume control is before the preamp. Tried with input shorted and still hum. Well, it(hum) wasn't super loud but you could tell it was there.
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Old 19th December 2004, 04:44 PM   #18
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The 1 pF cap in the feedback loop is for high frequency stability. It's easiest to use a gimmick cap, which is just two one-inch lengths of wire twisted together. Heck, the feedback resistor probably already has a pF across it just in stray...
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Old 19th December 2004, 04:48 PM   #19
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Sy

Would you suggest trying it without the cap? I have never seen a
gimmick cap so I'm not sure on making one.

What are your feelings on the resistor change I made? R102 470K to 27K?
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Old 19th December 2004, 04:52 PM   #20
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If I may (sorry SY), why did you make the change, and why that value?

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