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Old 10th November 2010, 08:41 AM   #1
off1 is offline off1  United States
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Default Bypass the tone control

Please guys, bear with me as I'm still learning (a lot, I must say).

In the following circuit:
Guitar Preamplifier Schematic - All Tube - KBapps.com

I want to bypass (actually completely remove) the tone control that sits in between the halves of tube No.2. If I connect pins 1 and 7 directly, do I need to add any resistor/capacitor network to compensate for the removal of the tone control?

Thanks in advance!
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Old 10th November 2010, 08:57 AM   #2
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You'll get more gain, and you'll need a coupling capacitor
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Old 10th November 2010, 02:36 PM   #3
off1 is offline off1  United States
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That's what I thought. What do you suggest to level down the output?
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Old 10th November 2010, 03:44 PM   #4
TheGimp is offline TheGimp  United States
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You have a loss of nearly 20:1 through the tonestack.

There is plenty of gain through the amp as indicated by the resistor divider on the output (22K to 1K pot).

I would remove the the gain stage driving the tonestack, remove the tonestack, and connect the output of the second gain stage to the output stage.

Since there is no feedback you can get away with just connecting the 0.022uF cap that went to the treble wiper to the point that would connect to pin 2 grid of the third tube. I would not mess with the RC netword from gain stage 2.

If gain is now insufficient (not likely with two gain adjustment pots) you can decrease the 22K resistor in the output to the 1K pot to increase output drive.
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Old 10th November 2010, 03:54 PM   #5
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I was going to say "try it like that first", but there's already plenty of gain in that thing
A voltage divider would do the trick, probably best with a small cap in parallel, like the one in front of the third triode, the 470K/470pF-100K arrangement. Perhaps try it with a pot first.

PS I'm not an expert, okay?
Good luck!
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Old 10th November 2010, 06:05 PM   #6
off1 is offline off1  United States
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Like this?


BEFORE - With tone control

Click the image to open in full size.



AFTER - Without tone control

Click the image to open in full size.
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Old 13th November 2010, 03:55 PM   #7
off1 is offline off1  United States
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anyone?
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Old 13th November 2010, 10:43 PM   #8
rman is offline rman  Canada
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I think the gimp meant like this.

Only use one of the coupling caps. (forgot to delete one.)
Attached Images
File Type: gif pre_without_tone_control.gif (8.4 KB, 76 views)

Last edited by rman; 13th November 2010 at 10:45 PM. Reason: made mistake in picture
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Old 13th November 2010, 10:49 PM   #9
rman is offline rman  Canada
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Now my attachment dissapeared, here it is again, and corrected
Attached Images
File Type: gif pre_without_tone_control.gif (8.4 KB, 70 views)
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Old 13th November 2010, 11:30 PM   #10
off1 is offline off1  United States
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you are the rman!!! thanks!
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