Simple Attenuators - Design And Testing

[Nik Henville]

...With all that said, used properly and as intended, these units are truly worth every penny of that!

Cold
not have
put it any better
than that, my friend.

 

[TonyK]

Nik, I'm basically blind without glasses, long-sighted, so that when my glasses move on my face everything goes in and out of focus all the time! Up close, this can be a nuisance to say the least. I have a neurological disorder that makes my hands shake a lot: annoying as this is when playing guitar, you can only imagine what it's like when holding a hot soldering iron in one hand and trying to hold solder at an electronics joint with the other! Then trying to do all of this in a tiny box... well, you get the picture. But if I was able to build this attenuator -- slowly for sure -- I'm sure that you could manage it. You might just want to consider it and have some fun as well

 

[Nik Henville]

...You might just want to consider it and have some fun as well

Too
darned
busy running
really fast just to stand still financially.
There really just aren't enough hours in my days...
...and those few I do get spare tend to be spent playing guitar loud and proud

 

[mike_lawyer]

Hey guys, I am really impressed by all the work that has gone into this thread! I am planning on building one of the 8 ohm versions, and have a question about parts:

1. I am located in the US, is everyone purchasing wire wound resistors off eBay from China?

2. What size screws do you need to attach the resistors to the chassis?

3. What "thermal grease" is being used between the resistors and the chassis?

Thanks for all your help, I can't wait to start building.

 

[Gene Ballzz]

Hey guys, I am really impressed by all the work that has gone into this thread! I am planning on building one of the 8 ohm versions, and have a question about parts:

1. I am located in the US, is everyone purchasing wire wound resistors off eBay from China?

2. What size screws do you need to attach the resistors to the chassis?

3. What "thermal grease" is being used between the resistors and the chassis?

Thanks for all your help, I can't wait to start building.

Yes, FleBay is the cheapest/easiest source for resistors from Chinesiawanoreanam! You'll likely have to wait a couple weeks or so, unless you want to pay through the nose! Also, it pays off to check out many of the different vendors for variety, etc.

I use #6-32 screws and I drill/tap the fairly thick aluminum boxes I buy and then chop the screws to length. Makes for a fairly neat/orderly build. It's not unusual to have to drill out some of the resistors to fit these screws, but I think you'd not want to use any smaller!

Standard, silicone-based, heat sink compound does the job and is widely/easily available/cheap!

Though you didn't ask, the best selection/source in the U.S. for inductor coils is Madison Speaker Components https://www.madisoundspeakerstore.com

Have A Fun Build!
Gene

 

[JohnH]

Hi @mike_lawyer , great! and good luck!

(Edit: I see Ive crossed with Genes post above. Luckily its all consistent!)

Ive had good experiences with the chinese resistors, and have had no issues with them. At a dollar or two each for me in Aus, they halve the total build cost compared to local or western suppliers, and shipping is free. The orders go through and get sent quickly, but the journey can be slow, sometimes 3 weeks, but there may be a faster shipping option.

They mount neatly to a panel with M3 nuts and bolts. 10mm long is about the minimum but 15mm is less fiddle..

I dont know about the thermal grease products available in the US. So long as you use something intended for that purpose it should be fine. Mostly they are white, containing (I think) micaceous iron oxide. The tube I have now contains a silvery paste, filled out with aluminium dust. Just use the minimum amount for full contact.

 

[JohnH]

Id be interested to find out what toggle switches are being used by others.

My current build uses full-sized toggles, bought locally. They look ok, work fine and have a good spec, but they were suspiciously cheap, less than $3 AUD retail. I found a couple that didn't feel right so I sidelined them. Then, due to a fumble, the box slipped off the amp and hit the floor. Two toggles got pushed in and then didnt feel right. I replaced them, but then opening up the damaged switches I found that the levers had punched through a rather thin internal bakelite panel.

Apart from using a better part, I think some kind of protection to projecting switch levers would be a good idea for units on the road, such as a projecting C shaped handle each side of the switch bank.

 

[mike_lawyer]

Thanks for all the replies, much appreciated! Does anyone have a favorite chassis for this project? I would like something aluminum and sturdy, but not too large.

 

[JohnH]

Aluminium is a good material, easy to drill and it has much better thermal conductivity than steel, plus it tends to be thicker.

My tendency, every time I build, is to end up wishing I had a bigger case to work in. Mine is the full Attenuator M circuit, and its crammed into a 170x120x55 diecast Al case. It relies on the depth so that the coils can sit on the base, with the resistors mounted to the top and ends, jacks at the back, switches at the front. It would have been a more ideal case size for a 50W lite version.

But, once built, its all fine and barely heats up cranking a 50W VM.

 

[Gene Ballzz]

Here's the ones I use:

https://www.ebay.com/itm/Aluminum-DIY-Project-Box-Enclosure-Case-PCB-Amplifier-Electronic-200-105-55mm/142615103868?ssPageName=STRK:MEBIDX:IT&_trksid=p2057872.m2749.l2649

For a pic of how I laid it out, go to post #178. I have since pulled the one pictured apart, but general layout is still similar.

Hope This Helps,
Gene

 

[tmingle]

Id be interested to find out what toggle switches are being used by others.

My current build uses full-sized toggles, bought locally. They look ok, work fine and have a good spec, but they were suspiciously cheap, less than $3 AUD retail. I found a couple that didn't feel right so I sidelined them. Then, due to a fumble, the box slipped off the amp and hit the floor. Two toggles got pushed in and then didnt feel right. I replaced them, but then opening up the damaged switches I found that the levers had punched through a rather thin internal bakelite panel.

Apart from using a better part, I think some kind of protection to projecting switch levers would be a good idea for units on the road, such as a projecting C shaped handle each side of the switch bank.

https://www.mouser.com/ProductDetai...ha2pyFadujQBe3n7F%2B9Vvv1RYtAuHIjZfaEGn/ttN0=
I always purchase high quality switches. I also generally over spec them. I've replaced too many to remember at my job. I have a scrap microscope light that I am going to re-mount my build inside.

 

[mike_lawyer]

Here's the ones I use:


For a pic of how I laid it out, go to post #178. I have since pulled the on pictured apart, bu general layout is still similar.

Hope This Helps,
Gene

Thanks a bunch, I just ordered one of those boxes. Looks high quality and durable.

 

[mike_lawyer]

Here's the ones I use:

For a pic of how I laid it out, go to post #178. I have since pulled the on pictured apart, bu general layout is still similar.

Hope This Helps,
Gene

Gene -

I looked at your post 178, it said you were having some serious heat issues with your build. Did you get that squared away?

Mike

 

[Gene Ballzz]

Gene -

I looked at your post 178, it said you were having some serious heat issues with your build. Did you get that squared away?

Mike

Mike,
I'm not sure I'd call them "serious" heat issues, and even though it can get pretty hot, that is when I'm "severely brutalizing" the whole rig, much more than I would normally do in practicality! While it can heat up pretty well, it gets to a certain point and stays there, without getting any hotter. A couple thoughts for you:

> It would be a good idea to read at least a few of the posts following that #178, as there are some points fairly pertinent to the build you seem to be shooting for. One of those points is that in my future builds, I will be mounting the resistors on the inside of the top, instead of the bottom and that should improve the cooling substantially.
> I don't think you mentioned what amp you intend to use this with? It might help us help you, if we know this?
> As with all projects, you really want to embrace the "Measure Twice, Cut Once" concept here! Think really hard about where your switches and jacks want to be and especially whether you want it mounted into an amp or cabinet. For example, my next one will be intented for mounting inside my DSL20CR, at the bottom and I want the switches accessible through the slot at the bottom. This will require my switches to be on the "sides" as opposed to the ends. Of course then, the next one will be intended for installation in one of the Studio Series heads (not yet sure which one) and that will likely require some rethinking of the enclosure that I use and a redesigned rear panel for the amp!
​

And if you don't mind me asking, what general part of the planet are you located in?

Have Fun!
Gene

 

[mike_lawyer]

Mike,
I'm not sure I'd call them "serious" heat issues, and even though it can get pretty hot, that is when I'm "severely brutalizing" the whole rig, much more than I would normally do in practicality! While it can heat up pretty well, it gets to a certain point and stays there, without getting any hotter. A couple thoughts for you:

> It would be a good idea to read at least a few of the posts following that #178, as there are some points fairly pertinent to the build you seem to be shooting for. One of those points is that in my future builds, I will be mounting the resistors on the inside of the top, instead of the bottom and that should improve the cooling substantially.
> I don't think you mentioned what amp you intend to use this with? It might help us help you, if we know this?
> As with all projects, you really want to embrace the "Measure Twice, Cut Once" concept here! Think really hard about where your switches and jacks want to be and especially whether you want it mounted into an amp or cabinet. For example, my next one will be intented for mounting inside my DSL20CR, at the bottom and I want the switches accessible through the slot at the bottom. This will require my switches to be on the "sides" as opposed to the ends. Of course then, the next one will be intended for installation in one of the Studio Series heads (not yet sure which one) and that will likely require some rethinking of the enclosure that I use and a redesigned rear panel for the amp!
​

And if you don't mind me asking, what general part of the planet are you located in?

Have Fun!
Gene

Hi Gene, thanks for the info! I am located in Virginia here in the good old USA. I am going to be using this unit primarily with a 6V6 Plexi that I built about a year ago. I plan to use it beside the amp head, and not built into an amp.

I like the idea of having the resistors on the top half of the chassis, that will allow for better circulation and cooling. I think I am going to place all of the switches on one end, with the input/output jacks on the other side.

I purchased the chassis and ordered the resistors from China. I ended up ordering all 100W resistors, which is probably overkill, but they were about the same price anyway. I still need to purchase the switches and the inductors. I am going to use that source in the US for the inductors.

Mike

 

[JohnH]

Hi Mike - all sounds good. what kind of power does your amp put out?

Its R1 and R2 that do most of the work in absorbing power, so to help spread heat, you can place them not next to each other, maybe with a less driven one in between. With all 100W resistors, they will add a lot of extra thermal mass which is good. it will take longer to heat up as so the average temperature that it all reaches will be over a longer time, so less max. Also, allow for lots of holes in top and bottom.

 

[mike_lawyer]

Hi Mike - all sounds good. what kind of power does your amp put out?

Its R1 and R2 that do most of the work in absorbing power, so to help spread heat, you can place them not next to each other, maybe with a less driven one in between. With all 100W resistors, they will add a lot of extra thermal mass which is good. it will take longer to heat up as so the average temperature that it all reaches will be over a longer time, so less max. Also, allow for lots of holes in top and bottom.

My 6V6 Plexi puts out about 22 watts, but it is the loudest 22 watts I have ever heard. That is great advice to spread out R1 and R2 to help dissipate heat.

I would like to build a 1987 50 watt clone at some point, so this attenuator will come in really handy.

 

[matttornado]

Does anyone know or have an idea how many watts a full cranked metal panel 1959 Superlead actually puts out? It's over 100 watts, correct? I'm sure it varies from amp of the same model. Mine is a 1974 modified for EL-34s with 480 V on the plates.

 

[matttornado]

Check out this 100 watt/ 60 ohm resistor I got from China compared to the Arcol brand (16 ohm)! I'm a bit skeptical that these are actually 100 watt resistors. More like a 50 watt sized package, not 100 watt.

If I have time, I'm going to pump 100 watts into these Chinese resistors (two 60 ohm in parallel = 30 ohms) here in the lab at work and see how hot they get. I wish I had a data sheet to go with these Chinese resistors....

A good Arcol brand resistor can cost an arm & a leg , especially for the power requirements we need for a 100 watt amp. One was Like 145 dollars from DigiKey.

 

[JohnH]

Ive not had any thermal issues cranking a 50W amp through this. The 50W spec , if running at 50W, puts 28W into R1, so there's a factor of about 3.6 against the 100W component spec. Yours is the same ratio. Itll need all the case contact for cooling though!