DIY Power Attenuator for my Valve Jr.

[mac]

I need to reduce the output volume of my VJ, and I found this simple attenuator:

http://www.bcae1.com/lpad.htm

I've read some ago that there is a chance of blowing the OT when using a power attenuator. Is this true?

I had another idea. I have a 8 ohm, 25 watt speaker that can be sacrificed. I wonder if I can remove the cone and use just the coil, in series with a 8 ohm cabinet. A total of 16 ohm --> to VJ 16 ohm output. Looks dangerous?

Please tell me what do you think.

mac

[ddr]

These are about L-Pad style:

    http://www.regiscoyne.com/ampwell/

    http://www.tedweber.com/understandingattenuators.html

    http://www.tedweber.com/atten.htm

    http://amps.zugster.net/articles/attenuation

And here are schematics for ones using a de-coned speaker:

    http://freestompboxes.org/viewtopic.php?f=2&t=2287

I don't have one at all, although I have used L-Pads for my in wall stereo speakers and like them for that.  My Bugera V5 had one, but I removed it when I replaced the Output Transformer.  I am on the lookout for a sacrificial speaker to build one of the MASS Lite units in the last link.

[Pushtone]

+1 for Adam Amps Bridged T attenuator.

http://amps.zugster.net/articles/attenuation


I've built several after comparing the B-T to the L-pad. The B-T sounds a lot better IMHO and easy to make for amps 50 watts and under.
You can use basic ceramic power resistors (25watt) for a five watt amp like the VJr or aluminum clad ones for 25-50 watt amps.

I've build fixed and dual attenuators (dual meaning I used a simple toggle switch to switch between two attenuation levels).
I like -14dB for moderate and -24dB for extreme (bedroom level) amounts of attenuation for a 5 watt amp.
I had to go to -27dB to tame a 20 watt amp (2xEL84) down to bedroom level and that one gets hot!

Even a -9dB to -12dB attenuator can be helpfull to take a bit off the top and allow you to krank an amp that sound great at that level but is just a little too loud.

If you have your heart set on an attenuator with many steps of attenuation then I suggest you have a look at the Weber rotary switch they use in there amp kits for impedance selection. Finding a rotary switch that can handle 2-5amps of current is tough unless you want to pay 35$ for it.
The Weber rotaries are the best price I can find in a high current rotary. That why I use toggle switches. They are rated at 250V 15amps for a couple of bucks.
https://taweber.powweb.com/store/sword.htm

Keep in mind the more you attenuate the more the amp will not sound like itself with an attenuator.
I think this is because of less speaker movement and interaction rather than the attenuator changing the sound.

I'll see if I can dig up my spread sheet that calculates the resistor values for different levels of attenuation. Although all the info you need is on Adams page.

[daveh222]

Could you please repost your link for the de-coned speaker attenuator schematic? It looks like an incomplete link.
Thanks!!
And anyone who has made or tried one - how do they sound compared to a resistive attenuator?

[DougH]

If you register at amp garage, you can get the plans for an airbrake. I built one in about a half hour and love it. Can handle up to a 100W amp and 4/8/16 ohm impedance. It's a very useful "all in one" tool for all of my amps.

[mac]

My neighbours will be pleased!

Doug, I have to take a look at the airbrake.

I thought of another solution... to sell my apt and buy a house!

mac

[mac]

I found a speaker magnet. I can buy wire and make a 8 ohm coil. I guess that a 0.2mm wire is ok. Enough turns to have a decent inductance.
0.4mm wire might work with my LC30 but a lot of turns needed to get 8 ohm and maybe too much inductance.
I have to do some math.

mac

[DougH]

Here's another idea: http://eminence.com/fdm.asp

[DougH]

I thought of another solution... to sell my apt and buy a house!

mac

Yeah, apartments suck for guitar amps. I usually suggest to apt dwellers just to get a POD (or something similar) and a good set of headphones.

[frank_p]

If you register at amp garage, you can get the plans for an airbrake. I built one in about a half hour and love it. Can handle up to a 100W amp and 4/8/16 ohm impedance. It's a very useful "all in one" tool for all of my amps.

You don't even need to register Marcello:
http://ampgarage.com/forum/viewtopic.php?t=2402&highlight=airbrake

It's cool to have a line out on attenuators, you can plug all your amps in series  , or put an amp in front of your pedalboard.  Seriously, what I like is putting a graphic eq at the output and run it in an other amp.  Great for shaping the distortion because at high level of attenuation, the sound is not always that good.

[mac]

Thanks Frank!

... I wonder if the attenuator will work on my girl friend as well...

mac

[eleanor296]

Why not go with Variable Voltage Regulation (VVR)?

There's some threads on Ampgarage about it, and I put it in my heavily modified Valve Junior when I had one... and it sounded great in that it didn't sound any different other than a decrease in volume.

[DougH]

VVR is an interesting idea. But my problem with it is that once you implement it, you've only dealt with the issue with one amp. And it has issues of its own too. People who try these schemes usually find they need a master volume to "scale" the preamp drive too etc etc... You can really end up chasing your tail with some of this stuff. An external "all in one" attenuator that can also serve as a load box with a line out is probably the most useful IMO. With a line-out you can run into another amp at any volume, or go direct through a speaker sim for recording and etc. And this works with all your amps, not just one.

[mac]

The BT is atractive, a few resistors and switches to choose levels of attenuation and impedance.
But a 8 ohm coil is more natural, and I could use it in parrallel with my LC30 and in series with my VJ.
I'll try both.
I'll wire a coil, 6 ohm, and I'm going to calculate the min inductance needed to let lows pass, same as OT, 80hz. This will give me an idea of the number of turns and the best wire diameter.

A nice starting reading about coils and speakers,
http://www.epanorama.net/documents/audio/speaker_impedance.html

[.Mike]

VVR is an interesting idea. But my problem with it is that once you implement it, you've only dealt with the issue with one amp. And it has issues of its own too. People who try these schemes usually find they need a master volume to "scale" the preamp drive too etc etc... You can really end up chasing your tail with some of this stuff. An external "all in one" attenuator that can also serve as a load box with a line out is probably the most useful IMO. With a line-out you can run into another amp at any volume, or go direct through a speaker sim for recording and etc. And this works with all your amps, not just one.

With VVR on the VJr, you have two options:

1. Scale the whole amp, including the power and pre. Because this scales both at the same time, the preamp doesn't slam the power tube and create excessive distortion. The distortion, for the most part, scales with the amp (obviously, speaker distortion is reduced with volume). The parts for this cost around $5.

2. Scale just the power amp. If you scale just the power amp then yes, you would need to add a master volume control, else the full gain of the preamp slams the scaled power tube. Add $2 for a pot and a knob.

The problem with VVR, to me, is that as you scale back the power, you also scale back the treble. This is easily fixed with a simple tone control or a bright cap.

One benefit of VVR over attenuators, as far ask I can gather, is that tube life is extended (tubes running well below the their maximums), whereas with attenuators the tube life is reduced in comparison (tubes running full blast).



Mike

[mac]

I have just built the BT with (3db, 2.5w) and (6db, 1.25w) attenuation levels. I used 5w resistors, they get a little warm, can be mounted inside a pcb box. I forgot to buy an extra double switch to make the bypass :S
Sound pretty decent at 1.25w. I use my Red Fuzz to cut highs and lows, nice tones, although still loud but my neighbours can live with it... or move to another flat!
I´m going to try a coil soon.

mac