L-Pad Attenuator Help

[avenger7326]

I have a little 5W tube amp running into a 10" 75W 8 Ohm speaker.  Hoping to put this L-Pad circuit between the amp out and speaker in with the result being a simple three-way switch selecting between a bypass, attenuation to approximately 1W, and attenuation to approximately 0.1W. (at least that's close to what i got with my calculations.)

First question, is this circuit correct for what I want and if not where have I gone wrong? (I'm hoping that I'm at least in the ball park.)

Secondly, assuming the circuit is correct, what would the wattage rating for each resistor need to be?

Thanks a lot!

[thermionix]

I don't actually know the right way to do it, but I don't think this is it.  In position 1 your amp now sees a 4 ohm load, where it wants 8 ohms (unless you switch it to a 4 ohm tap).  For use with a 5W amp, 5W resistors should be plenty, but 7 or 10W would give you a little more margin, though there's no scenario where a single resistor gets the full power of the amp.

Somebody smarter than me will chime in.  I know Nathan has recently built an attenuater so he probably knows the best way to do it.

[PRR]

Welcome.

> a little 5W tube amp

In position 3 your tube amp sees 114 Ohms, MUCH higher than designed, which can be a bad idea for tube amp transformers.

[PRR]

You want to keep the impedance seen by the amp in a 0.5X-4X zone for happy loading and low stress.

You also want to control the impedance seen by the speaker. Probably not "zero" as they do in hi-fi. Probably about equal to the speaker, or some higher. But not 10+X higher as in your lo-power setting.

I have a feeling you can't get arbitrary selection of impedance and loss with a 1-pole switch.

Also note that the low-price rotary switches are rated 50mA, 0.050A, and 5W 8r implies 0.78A. If switched repeatedly while WAILing I bet you could burn the switch -or- the output transformer.

Here's one approach to the individual networks. Finding a workable switching system may be tough.

[avenger7326]

Hmm, so you think its doubtful to create a circuit that can incorporate a switch? (assuming a switch that can handle the current can be found.)

The idea for this came when I was trying a Bulgera V5 Infinium at GC.  On the rear of the amp there is a switch to attenuate between 5W, 1W, and 0.1W.  My plan is to try and replicate that.

Also would all resistors needed to be rated 5-10W?

Thanks alot!