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Old 15th February 2006, 04:20 PM   #1
wboyd is offline wboyd  United States
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Default Removing tone controls - Need a little help

After doing a little tune-up on my recently aquired Akai M-7 tube amps (from the Akai reel to reel deck) I am pretty happy with the sound. I love the EL34 tube sound - smooth, seems to have plenty of bass (more than enough), good soundstage, etc.

I have been thinking about removing the tone controls, but have no idea how to go about it. Can I just remove them all together, or do I need to replace the pot with some type of 'bypass' curcuit, or do I need to add some type of jumper?

I have seen a few articles on removing tone controls from the PAS 2/3x, but nothing really generic...

BTW, the tone controls on all amps (I have 6 monoblocks), are fairly 'scrathy' anyway, and I really don't feel like they are needed in the first place....atleast for my application.

Thanks,

Wayne Boyd
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Old 15th February 2006, 05:17 PM   #2
SY is offline SY  United States
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Tone controls are generally configured as feedback networks. If you remove them, you'll need to replace them with flat feedback to keep the gain reasonable and distortion low.

In some cases, a tone control stage can be bypassed completely. You'll need to study the schematic, determine the gain structure, then figure out if that stage can be clipped out.
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Old 16th February 2006, 07:28 PM   #3
wboyd is offline wboyd  United States
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If it's any help....the tone control pot is 30K.
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Old 16th February 2006, 07:50 PM   #4
bob_v5 is offline bob_v5  England
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if you measured the tone controll resistance when its in the position you like you could just replace it with a resistor of that value. Not the best solution but it should work.
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Old 16th February 2006, 08:30 PM   #5
wboyd is offline wboyd  United States
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Thanks, excellent idea. I may try that tonight.
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Old 16th February 2006, 10:13 PM   #6
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you can take out the tone controls and if its a passive config replace with a volume pot or trim pot if the gain is too high
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Old 17th February 2006, 02:46 PM   #7
bob_v5 is offline bob_v5  England
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no problemo
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Old 17th February 2006, 08:08 PM   #8
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As Sy pointed out, if the tone control is in the feedback loop things can get a little sticky, BUT, if it's a passive circuit its a snap.

Look about halfway down this post and you will see the tone circuit of my Fisher 800C. I simply shunted the pins numbered 4 & 7 together, and voila', tone is bypassed, and I got around 6dB of extra gain.

Casey
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Old 17th February 2006, 09:07 PM   #9
wboyd is offline wboyd  United States
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Ok, here is a partial of the schematic. The tone control is noted as VR2. It seems to be part of the preamp curcuit. I have not discovered, nor can I tell if it is passive or not. If someone wanted to take a look at the whole schematic, that would be great...I did not want to upload the whole thing and take up to much space.

Thanks,

Wayne Boyd
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Old 18th February 2006, 02:31 AM   #10
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Looks like your in luck

The 50p C5 in parallel with the 100k R8 tilts the highs up, this feeds the junction of C5-C6. C5 and Vr2 is a low pass that tilts the highs down to flat when Vr2 is in its mid position, further cutting the highs as it gets lower in resistance, and "boosts" the highs as its increased...tone control.

Remove the 50p (C4) and the the .01uF (C5). This will kill the tone control, and raise the gain. If the gain is to high, put a resistor from ground to the C6-R8 junction, making a voltage divider. If it were me, I would connect C6 directly to the grid of V2a, bypassing the play/record switch..a notorious point of failure.

If Iv'e made a boo-boo in my analysis I hope one of the more learned people on this forum will correct me.

Casey
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