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Old 23rd July 2012, 07:52 AM   #1
Mnvizb is offline Mnvizb  Lithuania
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Default Attenuator for tube amp

I am planning to build a tube amplifier. I want to have an attenuator in it. At first I was thinking about simple "fixed resistor" design, but now, the ability to control the output seems more appealing.

The L-pad attenuator circuit:

Click the image to open in full size.

I am wondering which one of the resistors should be the variable one. Also, what would be the resistors power rating if my amplifier is going to give 5W. What should the resistance be, if i want to have as low as 1/10th of Watt?
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Old 23rd July 2012, 02:57 PM   #2
Trout is offline Trout  United States
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You might read up on this over at a gear forum, lots of good info here
Aiken's Reactive Dummy Load. - The Gear Page

Some guys just use the instructions that come with the L-Pad on this page
L-Pad 100W Mono 1" Shaft 8 Ohm 260-265
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Old 28th July 2012, 03:19 AM   #3
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by Mnvizb View Post
.
I am wondering which one of the resistors should be the variable one. Also, what would be the resistors power rating if my amplifier is going to give 5W. What should the resistance be, if i want to have as low as 1/10th of Watt?
They BOTH need to be variable. You can buy a pre-made variable l-pad for not a lot of money

L-Pad 100W Mono 3/8" Shaft 8 Ohm 260-262
But these sound bad if you get greedy and try to kill to much power. May as well simply use a pair of 50 cent sand stone resistors

But there is no need for an adjustable l-pad on a guitar amp because:

1) You have a master volume control you can use

2) l-pads make the amp sound bad if you try and cut the sound by more than about 9dB. After that you need a more sophisticated attenuator. The best ones us a speaker voice coil motor as the load. The device is just like a real speaker but with no cone, just the spider. Or you can make a dummy load using an air-core inductor designed for use in a HiFi speaker crossover in series with a resistor. But motors look good (bottom of this page)
https://taweber.powweb.com/amptechtools/truload.htm

Last edited by ChrisA; 28th July 2012 at 03:25 AM.
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Old 13th September 2012, 03:34 AM   #4
Struth is offline Struth  Canada
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Hi Guys

The best way to reduce amplifier loudness without changing tone is to use Power Scaling. The standard kits allow a 44db loudness range, so you can go to "unusably quiet". See the London Power site Tube Amp Kits, Tube Amp Books, Tube Amplifiers by London Power for all the info as they invented it. If your amp has a master volume, that will perform the drive compensation function.

Going from 5W to 100mW will be "quieter", but if you have a speaker that is 100db/1W then the lowest loudness is still 90db - which is not quiet at all. You have to get the power down to milliwatts for truly quiet sound.

With Power Scaling, the amp only produces the amount of power you need for the loudness desired. Attenuators demand that your amp scream out full power - or at least full voltage - which puts the OT at risk.

Have fun
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Old 13th September 2012, 03:51 AM   #5
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IMHO, just take the KISS approach and just a wall volume control. You can get a 30W or 50W model for $10 and that will work easily for a 5W amp.
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Old 13th September 2012, 04:26 AM   #6
Struth is offline Struth  Canada
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Hi Guys

Attenuators can be made to sound okay if you are willing to tweak each setting individually. However, for continuous control you need another pot section that provides something akin to a 'loudness' contour.

There are ways to retain tone and ways to have the easiest approach and these often conflict. I choose tone as the arbiter for sonic solutions.

Have fun
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