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Old 23rd October 2005, 02:20 AM   #1
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Default Help me stop my preamp from farting!

I'm working on the tube (12AX7) overdrive section of the preamp of a Music Man guitar amp. I'm playing with and basically have the sound I want with the hand written changes on the attached schematic. (The entire schematice can be viewed at: http://www.ernieball.com/mmonline/te...ps/1650-rd.pdf) However, when I play low notes, I loose definition and the sound is kind of farty. Not real bad, just bad enough. I want to keep the full tone, without the farts. I've played with different coupling cap values, but then I tend to loose the full tone. I've also played with the cathode paypass caps, not a huge difference, looses a bit of drive as I reduce the value of C10. Any suggestions welcome. Thanks.
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Old 23rd October 2005, 02:25 AM   #2
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Stiffen up your power supply rails. Works as well as Beano.
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Old 23rd October 2005, 04:00 PM   #3
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I have considered beefing up C35, which is a 2uf. I've got a 4uf that I can swap easily. Do you think I need to beef up the 60uf filter caps as well (C33 & C34)?
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Old 23rd October 2005, 04:08 PM   #4
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Basically, you've dropped the rolloff frequency by increasing the coupling caps, so you'll need to decouple the stages better. Think about using 10u or so at the top of each plate resistor. Now, this might change the tone because of slower sag under load, so listen up for that.
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Old 23rd October 2005, 04:15 PM   #5
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Try replacing C6 with something a little larger than 0.047uF, that may help.. Say 4.7uF

The question is what other changes further up the signal chain need to be made. The farting noise you refer to could be a variety of things including saturation of the output transformer. It probably wasn't designed to handle much in the way of low frequencies.

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Old 23rd October 2005, 04:25 PM   #6
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Good point.

It could also be that he's using an EBO bass.
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Old 24th October 2005, 12:01 AM   #7
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Here's what I've done since my post. I did some math and discovered that when I changed the coupling caps to .047uf, I reduced the rollof to about 7Hz. So, I put the coupling caps back to the original value of .01uf, which results in a roll off of about 33Hz. Also, I changed C35 from 2uf to 4uf. Still have good tone, but still have the farts, but maybe a bit less.

What's the purpose of C6? I notice on later versions of this amp, the same value was used, but at 400v rather than 250v as on mine. What should the effect be of changing this from .047uf to .47uf? I thought this affected the high frequencies in the circuit?

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Old 24th October 2005, 12:03 AM   #8
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oops, I see you suggested changing C6 to 4.7uf.
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Old 24th October 2005, 04:21 AM   #9
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C6 is the decoupling cap for the power supply rail feeding the input tube.
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Old 24th October 2005, 01:15 PM   #10
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Ok, I can see that. So, if I increase the value of this, the decouple is greater, if that make sense. Is C6 even necessary?

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