Bridged T Attenuator

[azrael]

I was thinking about build a simple attenuator to use with my Peavey VTM60. I just wanted to get it to work harder at lower volumes. No bedroom volume dreams here! I just want to be able to crank it more at shows and band practice.

I was looking at this...
http://amps.zugster.net/articles/attenuation
Just wondering...What wattage rating would I need for the resistors, for this to safely attenuate maybe 3-6dB for a 60W tube amp?

thanks!

[PRR]

The Bridge-T is a good choice for precision MATCHED systems.

Guitar amps and speakers are not matched. Loudspeaker actual impedance round 300Hz may be a bit less than nominal, rising 2X to 4X at the highest frequencies and maybe 10X at the bass resonance (near your low open string). Depending on details, the amplifier small-signal output impedance may be greater, similar, or less than speaker impedance.

This interaction has a lot to do with amp/speaker tone.

The PV VTM60 is a classic Fender output: output impedance is about twice the load impedance. The speaker loads-down the amp except around 90Hz and above 6KHz.

We can't emulate that at low power (except by dropping B++ from ~~500V to ~~300V).  

3dB is hardly worth doing.

Low-loss pads can be tricky. High-loss pads can cheat a lot.

6dB is not enough for a good cheat.

6dB is simply "one-quarter power". Get four speakers. Wire series-parallel to keep the same impedance. Now replace three of the speakers with silent resistors. Combine multiple resistors, if convenient. The remaining speaker gets quarter-power. It also "sees" a non-zero source impedance similar to how it is with direct connect.



> What wattage rating

The "60W" amp is rated 65W at clipping. These are lab sine-wave measurements. A perfect amp driven with square-waves (or totally clipped) will make twice that power. And PV probably under-states a bit, just to be sure your amp meets the specs.

A recent project on my bench made 13W clean, 16W gitar-rated, but would put out a full 23W of grossly clipped signal power and slow-burn the paint on a 20W resistor.

So your total power "could" be 140 Watts. Maybe your style or finger-strength can't hold gross-clipping for long enough to endanger a large resistor, but let's go with this number.

Since the power is divided in four, each part could get 35 Watts.

Or if you combine the series 8s into one 16, that must take 70W.

These are big resistors. The largest on-shelf value may be 50W. So you are looking for three 8 to 10 ohm 50W resistors.

There's two kinds of hi-power resistors. The ceramic stick will dissipate its own heat. The small bolt-on will NOT dissipate unless you bolt it to a large metal surface (cook-pot). So the ceramic sticks are more DIY. They don't stock 8 ohms; 10 ohms will work the same for practical purpose.

www.mouser.com part VP50K10 is 10 Ohms 50W, 39 In Stock for $11 each. L50J10RE is $8 but only 13 on the shelf this minute, act fast. D50K10RE is $14 and you can adjust the tap for "8 ohms" although the power should be derated to 40W-45W.

Shops like www.apexjr.com/ and www.allelectronics.com/ and www.surplussales.com/ have large resistors at low prices, but odd values (and large 8-ohm resistors sell-out quick).

[azrael]

What about an L-pad design, then?
Parts express has 8 ohm L-pad potentiometers rated for 100W. That should be enough, no?

Hooked up like this:
http://www.forrestwhitesides.com/node/81