Best way to variably attenuate speaker level outputs?

I’ve got a small 5-10 watt tube amp (old pa) that sounds pretty good as a guitar amp.
I’d like to use it to drive a couple different speakers via a speaker selector switch, but I’d also like to be able to variably attenuate the speakers after the speaker switch.
Any ideas on a cheap and safe way to do this?

A high-power L-Pad works, but keep the input power to 1/10th of the L-pad's rated power, or less. (I once saw smoke rise from a "100W" L-pad driven by a cranked-up Fender Pro Jr.!). An even better arrangement is a stepped L-pad (built with high-power resistors) for coarse adjustment, followed by a continuously-variable L-pad for fine adjustment.

Here's an example of the kind of L-pad I'm talking about:
http://www.partsexpress.com/pe/showdetl.cfm?&Partnumber=260-262

To reiterate, I do not recommend these devices for higher-powered amps.

Yeah and don't run it with the volume way down a lot. we get smoke once a month from headphone boxes that the kids run at 8o'clock. they run the mix real loud and turn the phones way down. run them at least 12 o'clock and turn the send down.

Probably of little consequence for your purposes, but also be aware that you could be reducing damping factor appreciably with L-pads, unless the amp has a comparable output impedance used directly. In this case of your little tube amp this could well be, so you probably won't notice much change in the speaker response using the pads.

Were it a solid-state amp things could be a lot different.

I just recently picked up one of these to use with my 5 watt Epiphone Valve Jr....

http://www.radioshack.com/product/i...ll+volume&kw=in+wall+volume&parentPage=search

Works great.

if you want a t.v listening level you almost have to get it down to lessor than a watt, it's easier to isolate in another room than a big amp but over the years i still keep learning that nothing sounds the same as a loud amp , you know the liveness & interaction , exciting the room as well .

let us know what works best for you , regards Greg

I've used a Calrad Stereo L-pad since the early '80s. It's a 100w stereo one that I wired in series. I started out using it for an extension Bandmaster 2x12 cab I had hooked to my Pro Reverb (40w). The cab was way louder because it had JBL's in it, so I wanted the L-pad to balance it with the two 12" Jensens in the amp.

I still use it these days (just tonight) with my AC30, at around 6db cut for pub gigs.

I did replace the L-Pad once because it started feeling a little crunchy when I turned it, but no catastrophic failures or amp troubles with it.

Whichever route you go, make sure you get it right, and make sure you don't switch loads while there is signal; tube amps don't like open or improperly loaded outputs.

Whichever route you go, make sure you get it right, and make sure you don't switch loads while there is signal; tube amps don't like open or improperly loaded outputs.

Click to expand...

That's the funny thing, I've had this old, trash-picked tube PA hooked up to all kinds of different speaker combos, and I've manually switched loads and removed loads while it was running, and it still works fine. I hooked the multi-tapped output transformer up to a 6 way rotary switch and switched output impedences without problems too. I don't see how this thing was ever effective for public address, but it sounds good with a guitar plugged into it. I'm planning on using it to drive a couple of spring reverb tanks and a couple of homemade Cooper Time Cube copies (speakers attached to garden hoses with lavalier mics at the other end). I'll have all the outputs running to an old Bogen mixer, should allow me to blend several analog reverbs/delays to use as send/returns to my main mixer.

I ordered a few 100 watt L-Pads, I'll let you know how it goes.

[quote author="bcarso"]be aware that you could be reducing damping factor appreciably with L-pads[/quote]

If this is important to you, Id suggest building a fixed value L-pad, that way you at least have a fighting chance of getting the impedances you want at that one point. even if you try to (resistively) match the input and output impedances so that to the amp the pad looks like the speaker, and the to speaker it looks like the amp, youll still be changing it. a speaker is a reactive load, and to a lesser extent so is an output transformer. you might well be able to get a BETTER match with the pad in, but the sound will be *different*. does this matter? probably not unless you are taking measurements and trying to correlate them with the pad-less setup. infact you might experiment with adding series resistance to the amp output (or the L-pad) to delibrately lower damping factor, you might get more "tone" or you might hear more of the speaker's sound you don't like.

mike p

Using a dual-ganged L-pad with the two sections in cascade is a great idea. I'm going to order one soon. As it is, for low-volume rockin' in my apartment with my 2-watt amp, I have to run my (single) L-pad very low. Volume adjustment at that end of the control range is finicky, to say the least.

Inserting a pad between speaker and amp does affect the damping factor and sometimes the tone as well, but not necessarily for the worse. My 2-watt amp actually sounds better with the L-pad than without. Of course, we're talking guitar applications here, where you're not after absolute fidelity but rather a certain "character."

yeah - this is just the ticket. i'll grab the 50W 8ohm mono l-pad from jaycar tonight after work.

chris
mfdu

Get a 100W L-pad if they have one. I've found the power ratings on those L-pads to be somewhat optimistic. As I mentioned before, I really like the idea of using the two sections of a stereo L-pad in cascade; I've recently ordered one and I'll be trying it soon.

even for a 5W valve junior?

do you recommend i use a ganged pair for impedance? impedance isn't a problem - i have 4, 8 and 16 ohm outputs from the head. (the speaker is 6.7ohm . . .)

would a ganged pair make adjustment a bit too fine? what is the purpose of using a ganged pair?

chris
mfdu

Try this! It's ok for a small watt amp.
http://i144.photobucket.com/albums/r190/hasseand/volreduc.gif

Looking at Bryson's message again, I'm not sure if he was talking about literally wiring the two L-pads in series (using, say, a stereo 4-ohm L-pad with an 8-ohm load) or, as I interpreted it, in cascade (8-ohm stereo L-pad, 8-ohm load). Either way, thanks for the idea. The cascaded dual L-pad works great.

mfdu, the advantage of using the stereo L-pad with the sections in cascade is that it gives you a steeper slope of attenuation, which yields more control if you're trying to knock the output of your 5-watt amp down to "bedroom volume." With a single L-pad, most of the action will be in the lowest portion of the rotation and fine adjustment is difficult. I noticed a dramatic improvement in control after going to the dual L-pad.

Using a dual L-pad also splits the attenuation between the two sections, although the lion's share of the power dissipation is still handled by the first L-pad in the chain. FWIW, I wired up the rear section to be first in the chain since it's a bit better-ventilated than the front section.

aaah - i'm hearing you then.

unfortunately, i have gone and grabbed a mono lPad before i read your message. i'll advise once i've installed it.

chris
mfdu