Homebrew Attenuator

Almach · October 2017

Seems to work OK.

ICBM · October 2017

Looks good, but one important thing you need to do - put shakeproof washers on the jacks, if you're relying on them to complete the ground connection. You should anyway really, even if you're not. Alternatively connect the grounds of the two jacks together with a wire - which may be the better solution. Or both! It's especially important here because it's the input jack which could come loose, which will leave no load on the amp. (Or even worse, an intermittently contacting one.)

I would also be careful about the '100W' rating - that's electrical power, not a cranked 100W amp which can put out up to double that.

Almach · October 2017

Good point about the earthing , thanks. The resistors are 50W vice the 25W shown in the diagram. It will run all day on a Deluxe Reverb but 10 minutes on a Supersonic 60 and it is smoking hot. I need a bigger box I think.

ICBM · October 2017

Yes, I would have thought that a cranked Supersonic will be putting out around (or just over) 100W. If it's the thermal dissipation of the enclosure that's the limiting factor, and you don't mind it being a bit ugly or awkward to carry, you could bolt some large heatsinks to the outside over the resistors - since you've used countersunk screws they will go on tightly. Heatsink paste also a good idea there too.

clarkefan · October 2017

Nice

I fancy having a go at something like this, did you buy the parts all in one go or did you make it from stuff you had? Any tips?

ecc83 · October 2017

clarkefan said:

Nice I fancy having a go at something like this, did you buy the parts all in one go or did you make it from stuff you had? Any tips?

I have built a number of load boxes up to 0.5kW continuous rating and there are few wrinkles. Give me a PM if you like and I might save you a bit of time, cash or burnt pinkies (or even worse? Something of 'er's indoors!)

Dave.

clarkefan · October 2017

Thanks Dave, very generous!

I've been looking around the net at different L-pad schems, trying to work out why I need big resistors rather than just the actual L-pad when I only want to build a simple 8 ohm jobbie for my 50w Marshall (no bypass switch per ICBM), but when I get my head around it I may well be in touch, cheers!

ICBM · October 2017

clarkefan said:

I've been looking around the net at different L-pad schems, trying to work out why I need big resistors rather than just the actual L-pad when I only want to build a simple 8 ohm jobbie for my 50w Marshall (no bypass switch per ICBM), but when I get my head around it I may well be in touch, cheers!

It's because the L-Pad is not capable of taking 100W, despite that being the supposed rating. Their original design purpose was as a treble attenuator in hi-fi type systems - where 100W is the *system* power. In that application the L-Pad won't have to take more than about 25W continuously at most, because in a mixed music programme the average power is never more than half the peak power (usually less), and the treble frequencies use no more than half the available power either.

Added to that, a fully distorted guitar amp will put out much more than its rated clean power, usually up to about twice, although probably not quite continuously - so really, you can't use a "100W" (ie 25W) L-Pad with much more than about a 15W amp safely since it can be putting out around a continuous 25W on average.

Hence to build a 100W attenuator you need three 25W resistors and and a "100W" L-Pad, and even then it will be close to the limit with a 60W amp as Almach has discovered, even using 50W resistors (although not very heatsinked).

This is why commercial attenuators are big, heavy and expensive - there really isn't an easy way around dissipating this sort of power, at least not if you want to have the output controllable.

clarkefan · October 2017

So would three 50w resistors be a better way forward than three 25w resistors? Plus big heatsink?

This would be just an experiment, but at the same time I have zero interest in bollixing my amp

ICBM · October 2017

clarkefan said:

So would three 50w resistors be a better way forward than three 25w resistors? Plus big heatsink?

Yes, although the L-Pad will become the limiting factor at anything more than that.

clarkefan · October 2017

Thanks as always @ICBM

NPP · April 2018

I'm dragging this old thread back up because I'm tempted to build something like that.

I recently acquired a Tweed Champ style amp built by @4114Effects ;that sounds sweet and is very responsive to pick attack and to the volume pot but obviously benefits from being turned up a little more than I can justify at home. With my previous amp (Blues jr.) I'd just stick a pedal in front of it for hair or drive but with the new one this feels like taking away the magic.

So what would I need to build to get the sound of this little 5w amp at 5-7 on the dial, but with the volume it has on 2?

I haven't built pedals before but do my own guitar wiring so I assume I should be ok on that front.

ICBM · April 2018

NPP said:

So what would I need to build to get the sound of this little 5w amp at 5-7 on the dial, but with the volume it has on 2?

Assuming the amp is a 4-ohm output like a standard 5F1, you need an 8-ohm L-pad in parallel with an 8-ohm resistor of at least 5W, preferably 10W - although 25W would be even better, since then it could be used with up to about a 20W amp if you ever want to, and they're not that expensive that you really need to economise.

Those in a simple metal enclosure with two jacks should work fine.

If the amp is an 8-ohm output then the L-pad alone will be enough.

NPP · April 2018

thanks @ICBM . Can you recommend a supplier for the components? Or are these things available in kit form, perhaps even with a wiring diagram?

Edited to add: I suppose this: http://www.theguitarmagazine.com/diy/diy-workshop-build-your-own-attenuator/ takes care of the wiring diagram and instructions

4114Effects · April 2018

It's an 8 ohm output :-)

4114Effects · April 2018

Happy to build you one if you need

normula1 · April 2018

One of these http://www.bluearan.co.uk/index.php?id=MONAT62H

NPP · April 2018

4114Effects said:

Happy to build you one if you need

thanks Dan @4114Effects ! My first thought was 'no, I want to give this a go myself', my second one was that if you do it will probably get done quicker, and it might actually work. Send me a message!

ICBM · April 2018

If it’s an 8-ohm amp then all you need is that L-pad, two jacks and a metal box - with three holes drilled in it and four pieces of wire. Simples!

MartinB · April 2018

I should knock up something similar for my old DIY Princeton Reverb some time. It sounds best turned up to about 6, which is a bit much for some situations even with a small amp like that.

clarkefan · April 2018

I looked into this a couple of months back and eventually thought I'd end up spending like 30 quid and maybe damaging something expensive, or end up with a crap sound. Changed my mind and bought a Palmer PDI06 from a fellow fretboarder which works brilliantly with my DSL 50w. They're about 120ish on the bay, works way better for me, safety first

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