Inexpensive Attenuator

[Joe6v6]

A lot of times when playing an amp wide open I wish I could get that sound & feel without the extream volume. My route 66's wide open are Led Zep Heaven. Ive done the VVR thing with cathode biased amps & honestly for me its just not the same, I dont want or need wisper quite volume just a lot less than full volume with the same feel & drive. So I stared looking at Attenuators, Mass, hot plate, schematics for airbreak etc. All of which can add up to some cash. . I ran across this on the Bay- http://www.bogen.com/products/pdfs/specialelectronicspdfs/AT10As.pdf  & bought one for 18 dollars to my door, 35 watt model, not knowing if it would work at all. I got it today & made an enclosure for it and tried it. I gotta say I just spent a half hour with a route 66 wide open & this thing set on 5 and it works great, same tone same feel same drive,crunch & dynamics- maybe a touch of highs rolled off(very little) and half the volume-exactly what I wanted. After half an hour it was not even warm.

So does anyone think I am destroying something by using this? Power tubes maybe, output trans? the unit looks like a decent size transformer with a bunch of taps coming out of it. Ill take a pic if someone wants to see it.

                                  Joe

[alerich]

Here's the manual for the device:

http://www.bogen.com/products/pdfs/specialelectronicspdfs/AT10Am.pdf

Seeing the speaker in the diagram with its own transformer makes me wonder about proper impedance matching in the application for which you are using it for. It sounds like some sort of step down transformer from your description but I am not familiar with 25v/70v audio networks. I have an old Knight PA I rehabbed into a guitar amp. It had 25V and 75V taps on the OT for this type of system. I suppose if I connected this device to that amp in that fashion I might be okay with it. Connecting it to a standard 4/8/16 ohm OT and then driving the amp really hard... I dunno about that. Waiting to see what the others think.

[Joe6v6]

Yea I hadent seen that install manual but the instructions that came with it show a "Matching" transformer. Ive seen the 25 & 70volt taps on Bogen PA & stereo amps that ive scavenged but I dont know what its all about. I do know this unit is doing exactly what I wanted it to do for a very low price when compared to say a THD Hot plate. I just dont know what price the amp is paying (if any) and what the long term Ramifications may be. I know on Webers site they talk about tube life being shortend in proportion to the amount of attenuation ie- If you dime your amp and attenuate it down to talking volume your power tubes will pay the price & upon their demise other componets may suffer coleratial damage.   . ..    Joe

[alerich]

Any attenuator that allows you to run the amp hard into an external load that you then dial down to your preferred volume is going to add to wear and tear on the tubes. It's a trade off along the lines of setting your bias hotter. If you like the tone great - it just means you'll be replacing your output tubes more often. Same as an attenuator. I wouldn't call that damage so much as increased wear and tear as long as the attenuator load is properly matched to the output impedance of the amplifier.

[Joe6v6]

as the attenuator load is properly matched to the output impedance of the amplifier.

The instruction sheet did say 8 ohm speaker & thats what im using so at least I know im good with that part of it.

[PRR]

When this autotransformer is turned-down, the actual load on the amplifier goes up. At "5", an 8 ohm speaker becomes a 253 ohm load on the amplifier.

PA amps (and most hifi amps) are designed to take any load higher than rated minimum load. Guitar amps often are not. At minimum, the tone changes. Worst-case, the OT kickback punctures the OT insulation.

I would at least add a load resistor, speaker impedance and at least full rated amplifier power, on the amplifier side of the attenuator. This "should" be switched-out when att is turned to "10"; might be simpler to jam a pebble in the "10" click so you can't go there, and add a bypass switch (or re-plug) for full power.

[PRR]

> instruction sheet did say 8 ohm

The link cited in your first post does not say ohms. It says 35W on either 25V or 70V systems; this works out to 17 ohms or 140 ohms.

There's no real issue using it as 35W 8 ohms. Output will be a bit lower than marked, but you *want* "lower" and are setting by ear.

[Joe6v6]

 

Worst-case, the OT kickback punctures the OT insulation

. Not good - I dont like this, sounds like burned up output transformer

jam a pebble in the "10" click so you can't go there, and add a bypass switch (or re-plug) for full power

. Yes- 10 is equal to unpluging the speaker, again not good

There's no real issue using it as 35W 8 ohms.

This is what I want to hear
Here are the instructions I refered to.

[imaradiostar]

Why not just install a large value resistor across the input and let it ride? I'm thinking 16 to 40 ohms, a big somewhat inductive ohmite thing. That should protect all but the most under screen-resistored under hipotted amps out there while being mostly transparent to an actual 8 ohm audio load.

The other option is to make your own L-pad/airbrake kind of thing. I built one for a friend and he loves it. It's loosely based on Ken Fischer's design but I got rid of the goofy bedroom level thing and calculated all my own resistance values to give the amp a little more consistent load while making the volume steps more consistent -3dB values. I used 100 watt wirewound resistors and bought extra taps from mouser. In fact I think I saved it as a mouser project if you want to see the parts list.

jamie

[samato]

I know this is an old thread but I just saw it and thought I'd post a link to my DIY Attenuator build:

http://www.tdpri.com/forum/shock-brothers-diy-amps/223421-diy-attenuator-pro-jr.html

I'm not sure if the pics will show up if you're not a member of that forum.  I can find the pics and post them here if anybody wants me to.

This thing works pretty good and doesn't change your tone much other than a slight loss of highs at the highest setting.  It's still not the same as actually running your amp loud though.  I'm convinced there is no substitute for that.  Unfortunately a lot of us just can't or don't want to be loud enough to make our amps work really hard so we make these "compromise" devices.  It's probably better to just find the amp that sounds the way you want at the volume you want.

Still, I thought this might be helpful for someone out there.