DIY Attenuator

[caspercody]

I finally was able to buy my very first tube amp. I bought a Marshal DSL 40C. Now I would like to use a attenuator to be able to use this in the house.

I bought a L pad, it is a 16 ohm 100w version. It works, but after doing some reading the L pad might do damage to the amp.

So is there a easy DIY project to do a attenuator that will allow me to turn it down to low volume and not hurt the amp?

Thanks
Rob

[Fender3D]

If your amp's OT runs on a steady impedance (your pad's 16 ohms) you won't ruin anything until you broke/smoke said pad...
Just in case you may tack one 100 Ohm from OT output and GND

[caspercody]

The L pad did smoke a little when I first used it, but does not anymore.

What does OT mean?

[Fender3D]

output transformer.

The L pad did smoke a little when I first used it, but does not anymore.

then I would check its value, look for ohms not THC 

[blackieNYC]

Output transformer
And they can easily be fried by running the amp with no load, among other things. 



Edit - wow! spellcheck turned Fried into Tried. Whooooole different sentence.

[caspercody]

Is the L pad considered a load?

It is a 16 ohm L pad (Parts Express 100 watt 16 ohm)

From a video I watched, these L pads are just a resistive thing and the amp is also looking for a inductive thing.

Trying to find a DIY project that includes a 16 ohm L pad, with maybe a inductor...If this is what is needed.

Thanks
Rob

[PRR]

> the amp is also looking for a inductive thing.

No!

It may sound "different" with pure resistance or resistance plus inductance. But the amp certainly does not "want" inductance.

Put your ohm meter on the input of this L pad, with your speaker connected on the output side. Turn the knob all the way each way. If this is supposed to be a 16 Ohm circuit, it should not go outside 8 Ohms to 32 Ohms at any point in rotation. If so, you are good.

> Marshal DSL 40C

That amp is rated 40 Watts "sorta clean". Such an amp can make 80 Watts of pure distorted heat, Your 100 Watt pad is a bit marginal for heavy abuse. Keep a nose and finger handy to monitor how hot it is getting. Some pads are not good for rating unless mounted to a substantial metal panel. If you do burn the pad "open", and strum hard saying "huhhh?", the amp will die quickly.

[p_wats]

I recently built a simple attenuator as a test using a 100 watt 8 ohm l-pad. I only have low-wattage amps to use it with (15-20 watts max), but it has been working very well with zero heat issues (doesn't even get warm, really).

I haven't tried it for very long periods of time--only about 20mins here and there.

[caspercody]

Sorry, instead of inductive I should have wrote reactive.

Looks like using a speaker voice coil might be the best way to go. But how does one determine what speaker voice coil to use?

Is there a way to use a speaker voice coil, and a L-pad together?

Has anyone tried the EMINENCE REIGnmaker speaker?

Thanks
Rob

[Fender3D]

Looks like using a speaker voice coil might be the best way to go...

a speaker voice coil, without magnet and cone, is basically a wire resistor...
I don't know why don't just use a "real" resistor as your L-pad, if it's rated 100W it should not have problems with 40...

As I said above, if your concerned about L-pad sudden death, you can also add 1 resistor on speaker output jack...

[caspercody]

I am trying to understand the best way to do a attenuator.

I do have the L pad, and it works. But as I read more on these attenuators, sounds like adding the speaker coil with magnet (to the L pad) will give me better results.




Looking at figure 1 is kind of what I was looking at doing. Making the L pad (16 ohm 100 watt) the pot in the schematic. Using a speaker coil/magnet rated at 16 ohms, and 75 watt (or higher).

Is there any extra benefits to this as opposed to just using the L pad by itself?

Do you have a drawing showing the one resistor you mentioned?

Thanks
Rob

[Fender3D]

First:
what are "better results"?
Your speaker will sound as it's supposed to do only when your L-pad is @ max.
Otherwise it will be always more or less "influenced" by the added resistance.
(You may add impedance and/or cap-resistor net(s) to "equalize" it, of course).

in your first post you said you were looking for an attenuator to be able to use the amp in the house, and I assumed you just want to lower its sound pressure, then L-pad is enough, just follow PRR's advices above.
If you're after a "Marshall" sound, remember most of what you hear in records comes from "watts-hard-pushing-cones".

resistor:
you should never leave an output transformer in no-load condition.
Then 1 resistor connected at OT output, or at speaker input, or your L-pad, or at anything connected to amp's output, will grant some kind of load for OT protection in case anything connected after that, dies.

schematic:
look at your figure 3 and sub "MASS Motor" with "100 ohm resistor"

[caspercody]

Thanks Fender3D for all your help!!!!

I think I will stop, and just continue to use the L pad. You are right just trying to get the volume down to bedroom level. Your comment about "watts-hard-pushing-cones" finally made sense.

If you do not mind a couple of questions:

About the 100 ohm to protect the OT, looking at figure 1, if one was to replace the MASS motor with the 100 ohm resistor, remove R1, and use the L pad for R2, would this be what I would need?

What size wattage should the 100 ohm resistor be? I guessing same size as speaker.

Do you have examples of impedance and/ or cap-resistor nets?

Thanks
Rob

[Fender3D]

About the 100 ohm to protect the OT, looking at figure 1, if one was to replace the MASS motor with the 100 ohm resistor, remove R1, and use the L pad for R2, would this be what I would need?

Yep

What size wattage should the 100 ohm resistor be? I guessing same size as speaker.

This resistor is needed in case L-pad or speaker dies... you'll clearly notice the "lack" of sound...  said that, I guess 5W may suffice for short periods of debugging (you know, sudden silence, strum guitar: no sound, raise volume and strum again harder: no sound again, and so on untill you realize L-pad (or speaker) is dead  )

Do you have examples of impedance and/ or cap-resistor nets?

That's what the schematics you posted are for...