Guitar Amp Power Soak

[GRAHAM99]

I have been to a guitar trade fair and got chatting to a chap who makes power soaks.
One of these would be handy for me!
I have done some research and it seems all you need is a L pad attenuator and a box etc?
Might have a go at making one.

[tomas]

It can get surprisingly complicated if there is a lot of power to be soaked up.

This model for example is cooled by a fan that speeds up as more power is being soaked, and the fancy thing is that it runs the fan without any other power source than your amp's speaker output: https://reverb.com/item/3629951-marshal ... -inductive

[ef37a]

I have been to a guitar trade fair and got chatting to a chap who makes power soaks.
One of these would be handy for me!
I have done some research and it seems all you need is a L pad attenuator and a box etc?
Might have a go at making one.

Beware the L Pad power soak! These pots were originally intended for use in speakers and controlled the tweeter level. Thus, a pad intended for use with a speaker rated at 100W might have a rating of 100W or at least 50W. They don't! They are just intended to be used with a 'system' rated at those powers.

A good quality L pad rated at "50W" is probably safe on a 30W valve amplifier and pro rata. But it is much safer to do most of the attenuation with fixed resistors. The 'Alclad' series are very robust and not expensive so you can go nuts and use a 200W resistor for a 50W amp. Then follow it with an L pad to get fine control.

The consequences of these loads going open circuit at full chat are not at ALL good for the amplifier.

How's your Ohm's Law?

Dave.

[GRAHAM99]

Interesting!!
This is the company I was chatting to
https://audiostorm.uk/
They said their units had great big resisters in them.Some models also had inductors.
I would not like to damage my lovely valve Marshall combo but would like to be able to record it without inducing hearing loss!
I am ok with ohms law.Been studying electronics and trying to do some practical projects for some fun!
Do you know of a schematic for a “safe” unit?
I suppose I could just buy one but that would be too easy.

[GRAHAM99]

https://www.esr.co.uk/electronics/loudspeakers10.htm

This company sells a 100w 8 ohm L pad for about £15.
The amp is 22w so I assume this would suffice?

[ef37a]

https://www.esr.co.uk/electronics/loudspeakers10.htm

This company sells a 100w 8 ohm L pad for about £15.
The amp is 22w so I assume this would suffice?

Yes, that would be fine but I prefer a fail safe system. You use two equal 50W resistors in series across the speaker out and that gives you a 6dB (voltage) attenuation. The total load resistance is about twice the speaker Z tapping.
Then, at the junction you connect your L pad equal to speaker Z but that is not critical. Thus, even if the pad goes O/C you still have a load on the amp.

Google will find you dozens of schematics, many of which show a total. full power bypass. Not a good idea IMHO for a home amplifier!

But! You might be disappointed, much more of the classic OD valve amp sound is generated in the pre amp thna people think and with 'classic' Marshall pre amps that is more true than most.

Dave.

[Nazard]

I would not like to damage my lovely valve Marshall combo but would like to be able to record it without inducing hearing loss!
I am ok with ohms law.Been studying electronics and trying to do some practical projects for some fun!
Do you know of a schematic for a “safe” unit?
I suppose I could just buy one but that would be too easy.

Schematics only tell you part of the story, because the most important aspect of a project like this is the mechanical build, heatsinking, excellent soldering and all components and wiring able to deal with the power being dissipated, with a large margin of safety built in.

I'm not sure it's the best project to start for some fun, but it's for you to decide.

[BigRedX]

But! You might be disappointed, much more of the classic OD valve amp sound is generated in the pre amp thna people think and with 'classic' Marshall pre amps that is more true than most.

Add once you start seriously driving the power stage of a valve amp a lot of the character of the sound produced will be down to speaker break-up. Putting an attenuator between the amp and speaker will negate all of this.

[ef37a]

I would not like to damage my lovely valve Marshall combo but would like to be able to record it without inducing hearing loss!
I am ok with ohms law.Been studying electronics and trying to do some practical projects for some fun!
Do you know of a schematic for a “safe” unit?
I suppose I could just buy one but that would be too easy.

Schematics only tell you part of the story, because the most important aspect of a project like this is the mechanical build, heatsinking, excellent soldering and all components and wiring able to deal with the power being dissipated, with a large margin of safety built in.

I'm not sure it's the best project to start for some fun, but it's for you to decide.

All very true but if the OP makes sensible resistance tests and ensures there is no condition where there is a open or short circuit he should be OK. Over-engineer the power ratings? Abso bloody lutely! As I said, those ally clad Rs are really quite cheap so he can go nuts. For the loads I made up I always made the main 'dissipator' two resistors in parallel, i.e for 8R 2X 22R 50W each for a 50W amp. (yes, I know that is 11 Ohms! But amps are not fussy to an Ohm or three and there is some evidence that soaks sound better if the amp is loaded a bit above nominal Z. In any case , speakers are FAR from resistive and only ever go UP)

The "speaker breakup" idea is one that is rather hotly disputed, even Celstion say you don't need to drive the nuts off their speakers. In any case, 4 times V30s can sound great with a 10W amp that's 2.5W per cone!

THE biggest problem with power soaks is that if you smash a 50W amp down to 100mW you lose all the dynamics. And you DO need to get down to mWs if you are not to wake the chavvy.

Dave.