Curious how to put a volume pedal between tube guitar amp head and speakers of same. - diyAudio
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Old 26th April 2006, 10:32 AM   #1
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Default Curious how to put a volume pedal between tube guitar amp head and speakers of same.

Hi, I'm brand new here so please tell me if this question was already answered. I'm basically looking to reduce the volume after the signal is amplified and distorted using a foot pedal. Same concept as a normal guitar volume pedal, except post amp.
I got the idea from my ernie ball passive volume pedal which you can see here: http://www.analogman.com/ernie.htm The thing works great and has about zero effect on tone, except that when the volume goes down the distortion goes away.
I suspect this requires a vastly different pot than what's used to attenuate an instrument level signal, and I don't know where to begin. Any help or direction is appreciated.
Thanks
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Old 26th April 2006, 01:03 PM   #2
sreten is offline sreten  United Kingdom
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Hi,

you need a high power L-pad potentiometer,

Click the image to open in full size.

http://www.partsexpress.com/pe/showd...number=260-262

/sreten.
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Old 26th April 2006, 01:09 PM   #3
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This isn't a very good idea!, as suggested a massive expensive pot could be used - but it's not the way to do it!.

A guitar amp should have a master volume and an input volume, you should turn the input volume up to clip the front end (giving the distortion) and reduce the master volume to give the loudness you want.

This means the distortion is coming from the front end, and NOT the output stages, guitar amps are designed specifically to be used like this!.

If you're looking for that really overdriven, output clipping, distortion - then you're probably playing heavy metal, and want the volume anyway!.
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Old 26th April 2006, 03:57 PM   #4
sreten is offline sreten  United Kingdom
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Hi NG,

you've obviously never heard of a power soak / brake :

http://www.amptone.com/marshallpowerbrake.html

The point is there is nothing like valve power output stage distortion, and
contrary to your assertion valve guitar output stages are built to distort.

The 100W pot I suggested is under $10, so why consider alternatives ?


/sreten.
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Old 26th April 2006, 04:34 PM   #5
sreten is offline sreten  United Kingdom
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http://www.amptone.com/marshallpowerbrake.htm

link works
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Old 26th April 2006, 04:34 PM   #6
kevinkr is offline kevinkr  United States
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I'm with Sreten on this one, spent four years designing MI electronics before my current gig. Overdriving the pre-amplifier results in an entirely different sonic signature than overdriving the output stage. Powersoaks are commonly used to allow one to get the crunchy overdriven sound wanted without blasting out one's brains doing so, and are frequently used in studio situations as well.

Just make sure that any LPAD you use to attenuate the signal has the required power handling or you might be in for an unpleasant surprise when it fails.. It will need to be well ventilated as well.
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Old 28th April 2006, 09:24 AM   #7
Enzo is offline Enzo  United States
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He wants a pedal. Power soaks are not pedals, and that giant L-pad won't fit in his DUnlop very well. Plus most Lpads don't spin as freely as a pedal pot should.

Power tube distortion is separate from preamp distortion alright, but unless he is playing flat out, he isn;t getting power tube distortion anyway, he just wants an expression pedal.

In front of the amp, a volume pedal will affect the preamp overdrive. But if you run the pedal in the effects loop, then it doesn't affect the overdrive, just the volume. It is in essence a master volume or another post control.

You never mentioned what amp you are running. If it is a 100 watt amp, you don't want a 100 watt Lpad on it, you'd want a good 150 watts anyway. One doesn't run components right at their limits and expect reliability.
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Old 28th April 2006, 02:58 PM   #8
sreten is offline sreten  United Kingdom
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Hmmmm..........

If its just a case of using a normal volume pedal in the effects
loop of an amp rather than than at the input, then I presume
the question has been asked due to a lack of an effects loop.

In this case the answer is to add an effects loop to the amplifier.

/sreten.
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Old 9th May 2006, 05:11 PM   #9
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Please be careful if you want your amp to remain intact!

A power soak is fine, and in principal it could be implemented in a foot pedal, however :

A: You would need a very good quality L-Pad. Most of these 8 and 4 ohm ones are designed to attenuate tweeters in hifi speakers, and not take the full whack of an overdriven tube amp.

B: It would not present the same load to the amplifier as a real loudspeaker, which has much inductance caused by the coil and movement. A purely resistive load such as a simple L-pad will draw more power at lower frequencies, which can stress the OP transformer and cause it to fail.

C: If the L-Pad, Jacks, or any of the wiring were to fail, it could short your output transformer and blow it very quickly.

You could get a real speaker soak with a level control and rig up a mechanism to control it, or rewire the control and install the pot in a pedal. But C would apply here also.

A safer (but much more expensive and bulky) solution would be to use a commercial speaker soak into a solid state power amp. You could pick a 200w one pretty cheap I reckon. Then the volume pedal could go between this and the power amp. Also you get the advantage of putting some effects after the 'speaker' which could give you a wider range of sounds.

Good luck on a noble quest
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Old 30th May 2006, 07:30 PM   #10
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THD has the best power attenuator(imo), and it's small! You could certainly put it on the floor with the rest of your effects rig, but most guys just stick it on top their amp.

http://www.thdelectronics.com/products/hotplate.htm
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