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Old 3rd February 2012, 04:51 AM   #1
cancon is offline cancon  Canada
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Default OT Master volume

In my past build (5w powered preamp) I installed my master volume pot across the OT secondary and ground, with the wiper feeding the output. There is an 8ohm 5w resistor across the pot/output. (8 ohm because of the OT secondary impedance and because a speaker may or may not be hooked up).

What would be the safe practice for accomplishing this trick with a higher wattage amp (speaker permanently connected), say 40w?
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Old 3rd February 2012, 05:25 AM   #2
chrish is online now chrish  Australia
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Why would you want to do that?
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Old 3rd February 2012, 09:31 AM   #3
DF96 is offline DF96  England
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Guitar FX?
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Old 3rd February 2012, 03:47 PM   #4
cancon is offline cancon  Canada
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The original design was based on the herzog (guitar FX...single ended 6V6) but i've also been using it as a combo and for driving a tube Leslie speaker - It turned out really well.

I wanted to mesh the design with a Deluxe or Bassman style push-pull 6V6 or 6L6 output but I've read about people experiment with adding Masters to these style of circuits without outstanding results. My latest compromise was to build both designs into one box separated by that master volume. I know the herzog isn't widely enjoyed, but the circle I run with really digs the sound.
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Old 6th February 2012, 03:59 PM   #5
cancon is offline cancon  Canada
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Okay okay, so I take it there was little interest in this idea... I have since had another one:

Build the fx unit as one channel; input/preamp, 6V6, OT, which is then fed into CH 2 (which is the regular guitar amp clone) via my master vol and dummy load for the first OT. That way I can get the Master vol in the position that I love so much, and don't have to mess with a tried & tested clone circuit that wouldn't like a master volume much anyway. The Fx can be bypassed via footswitch or with a switched input jack.

I have a cheap source for the fx OT's, so not factoring cost, am I going the long-about way of building this?

Thanks!
adam
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Old 6th February 2012, 04:28 PM   #6
kevinkr is offline kevinkr  United States
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Posting to the right forum where the guitar guys hang out would probably help, and since it is forum policy I'll move it there.
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Old 8th February 2012, 08:07 AM   #7
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Quote:
Originally Posted by cancon View Post
In my past build (5w powered preamp) I installed my master volume pot across the OT secondary and ground, with the wiper feeding the output. There is an 8ohm 5w resistor across the pot/output. (8 ohm because of the OT secondary impedance and because a speaker may or may not be hooked up).

What would be the safe practice for accomplishing this trick with a higher wattage amp (speaker permanently connected), say 40w?
Put a resistor in series with the pot (signal - resistor - pot - ground) with the wiper still as the output.

A 40 watt amp will have a signal of about 18Vrms at full tilt, so a 150k in series with a 10k pot will give a little over 1Vrms max on the pot as a starting point.
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Old 12th February 2012, 04:21 PM   #8
cancon is offline cancon  Canada
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Thanks for your reply. Here is the output section of the champ clone:

Click the image to open in full size.


In the 5W amp, this works wonders, I find.
Would this work in a higher-power, push-pull application? Would the method you describe accomplish the same thing? Would the only difference in my version need to be a (i.e. 30W amp) a 30W resistor?

Thanks!

Last edited by cancon; 12th February 2012 at 04:27 PM.
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Old 21st February 2012, 08:13 AM   #9
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Here is what I meant
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Old 22nd February 2012, 08:36 AM   #10
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for some reason the pic didn't appear. try this

Click the image to open in full size.
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