Attempting to build my own attenuator

[El_Diablo572]

After looking at the guide which i'll post below, i've decided to go ahead and build one.

http://www.guitar-bass.net/diy/diy-workshop-build-your-own-attenuator/

With my build, i'm going to omit the on/off switch (if i dont want it in my signal chain i'll just unplug it) and the 8/4 ohm switch also, because my amp has a 4 ohm output, therefore i dont need it to be switchable. I am however going to keep the treble lift facility. Both my guitars are humbucker equipped solid bodies, one is a baritone, and i play lots of down-tuned, high gain stuff, so this seems logical to me.

I have two simple questions:

1: can anybody see any obvious problems with this guide? All seems pretty legit to me, but my knowledge of electronics is extremely limited...

2: any idea what gauge of wiring i should be using here? The guide is pretty concise but theres no detail given as to what a suitable thickness for the wiring is

[Ciaran Haslett]

Looks good.  2 things come to mind.

I hate the idea of a switch.  It leaves open the possibility of the output transformer being completely disconnected from ANY load albeit for a very short time.  I come from the thought of "Never run a tube amp without a load."  

If it were me, I'd remove the switch altogether.  I can't really imagine a situation where I'd want to quickly switch to attenuated output where I wouldn't have the 2 minutes it would take to just power off and connect the thing...or I'd just turn the l-pad all the way down and get "close enough."

This is also a purely resistive load and not a more realistic reactive load from a real speaker.  Might be worth taking a look at some reactive load circuits.

As for the wire...just use whatever gauge wire came with your speakers.  Stands to reason that if you have a 100 watt amp, the speaker wire it came with must be rated to handle at least that.

[El_Diablo572]

Yeah, i'm not bothering with the switch, for the reasons you stated, and because it seemed like extra expense and effort for, well, nothing, really...

When you say wire that "came with my speakers", i have absolutely no idea... my amp is a marshall combo, had it 15 years from new and i looked after it, therefore i've never modded it or changed anything other than the pre-amp valve, because to be perfectly honest, i haven't needed to, it's built like a tank...

[Travis]

Since you're using a Valvestate amp with a solid state power amp, what is the point of using an attenuator in the first place? Why not just use the master volume???

[squirrels]

I used an L-Pad with both my old Dark Terror and OR15 for years with out issue. Make sure the L-Pad is rated three times higher (I think) than you need.  They can get very hot with too much volume and too much attenuation. I mostly used it to help out the master volume since it was very sensitive when playing at low volume. I prefer the volume pot in the loop trick now though. I live in a condo so I don't get to play loud too often and attenuators help, but when I do get too crank the volume it's worth having a nice amp.

[PMowdes]

Anyone come across the alex's attenuator or the scumback attenuator??

[El_Diablo572]

It's a 50w combo, and i live in a house with 4 other people, anything past 1 on the master volume control makes windows rattle, but the valve pre-amp doesnt really do much until you turn it past about 2 or 3, meaning that it sounds like shit at bedroom levels.

I've done a lot of research on these resistive attenuators and it seems to me that most problems caused by resistive circuits are a result of either bad design/shoddy workmanship/cheap components, or because people use it as an excuse to turn everything up to eleven, because, well, they can, which is gonna put strain on your amp no matter how you do it, with or without a resistor network.

All i want is usable volume at a respectable level, which an attenuator should provide quite easily, without straining anything else. I'm not planning on abusing my amp, and if i want it loud i'll just remove the attenuator and turn it up....

[squirrels]

The one I built was copied from a Bitmo 10-Uator (minus the line out). I turned the amp up until it was a little too loud and used the L-Pad to back the volume down until it was usable. I'm thinking if building another for my new Vox AC10.

[El_Diablo572]

I've got most of the components i need now, except the wire and that giant resistor, so i'll be setting to work on it later this afternoon, i'll keep things updated

[El_Diablo572]

Its all looking good, i'm quite pleased so far. I went with gold to try and match the face of the amp but it's not as close as i'd hoped, not sure what the laquer will do to the sheen yet though.

[El_Diablo572]

Christ on a bike, i didn't realise how dirty my amp was til i saw these photos, disgraceful!!!

[El_Diablo572]

Finally got all the parts i needed to finish this after a fuck up at the post office...

 Before i put it back together and fire it up, could somebody check over my circuit and confirm that it's right and that it should work?

I don't want to find out that this is wrong the hard way, i cant afford another amp and i dont think i'm smart enough to fix it if she blows...





Thanks in advance for the help guys

[El_Diablo572]

Okay.... I just attempted to test this with my multi meter before i went ahead with anything else. My concern is that my amps output and speaker are rated at 4 ohms. I attempted to test the impedance of my attenuator using the following method:

Connected my speaker to the attenuator output, and a loose cable into the input (the one that would connect to the amp) after this i simply tested the tip and the ground of the cable with my multi meter.

Is my method correct? Assuming so....

on the lowest ohm setting i get a reading of 06.2. This seems a bit off to me. I'm aware that a multimeter testing resistance isnt a very accurate way of doing this, but my understanding is that testing a speaker i should be looking for the following

4ohms - somewhere between 2 and 3.5 ohms
8ohms - between 5 and 7.5

If all of this is correct (if not please say so) the that mean i got something wrong.....

Help/advice/ideas?

If i am wrong about any of this please don't hesitate to inform me how much of an idiot i'm being, i'm sure i've probably made a glaring error somewhere, you only get smarter by learning from your superiors!!!

[El_Diablo572]

Ignore my last, it was late in the evening, and i'm extremely dumb....

Managed to accurately (and safely) test this out and had it verified by a mate who's an electrical engineer (he also fixed my thinking with a quick slap to the back of the head) and... Oh my god!!!

I love it. It works perfectly. The gain is maxed out and the volume is happily sitting at 12 o'clock, and all my stuff is still standing on the relevant shelving where it should be. The attenuation is perhaps not quite as profound as i'd hoped (if i was to guess i'd say it cuts the volume by roughly a third) but apart from that i couldn't be happier.

Played for about half an hour straight with the above settings, no problems at all, very minimal heat build up, mostly in the area where that giant resistor is mounted, and to my ear no real difference in sound quality either, even at low levels with the treble booster switched off, it's extremely transparent. It just fucking works, plain and simple.

The only small issue i've noticed, which is a component problem rather than something i've done, it that if you use the l-pad to turn the volume right down, it will continue rotating into a "off" position, which obviously is bad because the amp will effectively be disconnected from the circuit. Easily fixed, i'll just glue a block of plastic or something in there as a stopper so that it cant rotate round into the "danger zone", but i thought this was worth a mention.

Also worth noting, i decided to make a new patch cable to run between the amp and attenuator, i reasoned that a regular instrument cable wasnt in the least bit suitable for the power of the amp, so i bought a length of professional quality speaker cable designed for PA systems, rated at 1kw. I bought a 1 meter length and used one half for the hookup wires inside, and used the other half with some plugs i had lying around to make the cable.



More than happy with it.

[IvIark]

Excellent, looks great.  I've got an Lpad knocking around somewhere which I bought a couple of years ago but haven't boxed up yet, so you've inspired me to pull my finger out.  At the moment the only attenuator I have only has two fixed resistances and so it will be nice to have something variable.

[El_Diablo572]

Yeah, i was looking at fixed resistance methods when i came across the guide in my first post, all i really did was copy what the dude did there, apart from eliminating two of the switches which i thought were un- necessary, so the credit goes to him really.

Also, i noticed after reading the comments in the guide, one bloke there also picked up on the issue with the L-pad (i also chose to use the same one), his description is much better than mine, so maybe check that out. I fixed mine by gluing some plastic pieces in there to stop it rotating into the dead zone like i said in my last post