Attenuators between a tube amp and the guitar speaker: some measurements and theory

[Helmholtz]

What do people use to physically mount these things?

I would think of cable ties or a nylon center bolt and plastic washers. Or epoxy potting them into suitable small enclosures. Maybe it's enough to hot-glue them to a board.
But first I would do some tests to check if wire temperature stays uncritical at full power.

I am not saying that metal mounting hardware can't be used but influence on inductance is hardly predictable. OTOH, variation up to say 10% shouldn't be problem as speaker inductances also vary.

I decided for a different approach, using a single choke with a heavy steel center bolt as core. This construction took me some time to optimize, but now emulates the high frequency impedance of a speaker very well. Only drawback is that the steel core will have to dissipate up to 25% of total amp power.

[Gregg]

I got my inductors from the local transformer shop. Here's some observations.
The lamination core inductor measured ~9.5mH out of the enclosure (1.2mm black steel) and ~11mH when installed. There was no room for experiments because it was an 1U rack enclosure and I had other stuff right next to that. Using stainless steel bolts and brass or plastic standoffs for mounting didn't affect the inductance any further.
The air core inductor would buzz at close to full power. Also at close to full power the bigger inductor made the enclosure vibrate so in the end I had to use some 15mm thick soft rubber in order to eliminate most of those vibrations but that power has to go somewhere, right?

[SoulFetish]

Both the nylon washers and bolts, and potting sound viable. If I decide to pot the, I have some small steel powr Transformer covers I could use.
On the other hand, the copper temp in these coils concerns me a little. At 18AWG, these are a bit underrated compared to the other parts.

[catalin gramada]

For an project potting and encasing will be the last think I'll do. It is very easy to effective modify the value of a coil take off or add some turns if necessary to adjust. You can made further some additional taps for experiments.Sometimes few turns can be equivalent with a coil of same size if need to add.(the impedance of a coil raise slow at beginning of turns and very fast at the end).
And be aware, under rating prone to saturation (just for iron cores at certain freq.) so take this in consideration aso.

[SoulFetish]

Update.. on the race to take the most time to complete an attenuator project. Not for nuthin', but I feel like I'm kicking ass in this race.

In reality, I'm fixating on the enclosure design, and haven't been able to settle on anything yet. But while I sort that out, I found a great score for the main dissipation resistor over the weekend (I was going to use several equal parallel resistors to get the 5-6Ω I needed).
I found a nice aluminum Dale 250W resistor on ebay for around $16 shipped.



I think he still has one left if anyone wants to jump on it.

https://www.ebay.com/itm/Dale RH-250 5.6Ω 1% 250W

[SoulFetish]

I would think of cable ties or a nylon center bolt and plastic washers. Or epoxy potting them into suitable small enclosures. Maybe it's enough to hot-glue them to a board.
But first I would do some tests to check if wire temperature stays uncritical at full power.

I am not saying that metal mounting hardware can't be used but influence on inductance is hardly predictable. OTOH, variation up to say 10% shouldn't be problem as speaker inductances also vary.

I decided for a different approach, using a single choke with a heavy steel center bolt as core. This construction took me some time to optimize, but now emulates the high frequency impedance of a speaker very well. Only drawback is that the steel core will have to dissipate up to 25% of total amp power.

Can you post some pictures of the inductor you constructed? Also, what did you go with for the single inductor value? and finally, was your choice to use a steel core inductor because of eddy current losses(that would probably explain the heat dissipation)?

Gregg, a question for you:
did you pot your air core inductors? what would you recommend for damping in the construction if you weren't limited to the rack space requirements in your existing build?

[HaroldBrooks]

I use a speaker attenuator every time I gig. I use a Weber with the fancy speaker motor and stuff. I have the footswitch option so I can turn it off and on at will. It’s great for bringing lead volumes up with the amp saturated. It has some treble cut/boost switches on it I rarely use. It does depend on the cab. AND I’m not squashing the output.. just taking the “edge off”. Honestly though, as I mentioned in another post, for my last 20 watt build I simply added a $12 100 watt L-Pad from all parts on the back. I really don’t hear a huge difference in tone when used in that manor. Everyone thinks it’s magic in the little studio where I use it.

I too have used an L-Pad just as an experiment, and I agree that if you don't shave off too much wattage (about a 6DB drop or less), they work quite well !

They also add some audio compression, but when you really squash the output to nighttime bedroom levels, your amp will sound sickly thin, but hey that's too big a change IMHO, to expect everything to be ok. I've NEVER heard an amp played super low that I liked as good as when it is cranked. Part of that has to do with the non-linear frequency and amplitude response of the human ear itself. Not an easy thing to replicate just with simple electronics, and I'm sure it even varies from person to person and is in the realm of psychoacoustics.

Your better off IMHO of starting off with a lower watt amp that you think sounds good, and then doing what you do by using an L-pad to shave off just a bit of the volume as and when needed, and not get into too large a change. I'm sure reactive load boxes work better, but sometimes splitting hairs to get a small improvement becomes very expensive, and unnecessary if you plan well in advance of your performances based on logistics, positioning, and sometimes muting just a bit.

The best sounding rock amp ever IMHO is the Marshall Plexi, and I am talking the non-master volume variety. Unfortunately I've never heard one with a power soak that sound and feels like an unbridled one run flat out. Part of that is do to what's happening to to greenback speakers when the are on the verge of destruction. It's awesome, and rather unpredictable, a howl hear, a growl there, a scream or two. When you cut the wattage, a lot of that goes away. Oh well...

[Helmholtz]

Can you post some pictures of the inductor you constructed? Also, what did you go with for the single inductor value? and finally, was your choice to use a steel core inductor because of eddy current losses(that would probably explain the heat dissipation)?

Here you are:






Started out as a 0.33mH aircore choke (DCR: 0.51 Ohm)
Completed it measures (Ls: effective serial inductance = reactive part of impedance, Rs: serial AC loss resistance = real part of impedance):

100Hz: Ls = 1.3mH/Rs = 0.9 Ohm
1kHz : Ls = 0.7mH/Rs = 2 Ohm
10kHz: Ls = 0.43mH/Rs = 12 Ohm
100kHz: Ls = 0.15mH/Rs = 100 Ohm

Eddy and magnetic skin effect cause the desired frequency dependence. Had to try different screws and arrangements as magnetic steel properties vary.

[Gregg]

Gregg, a question for you:
did you pot your air core inductors? what would you recommend for damping in the construction if you weren't limited to the rack space requirements in your existing build?

Yes, I did, but at higher volumes they would "sing" anyway

[Chuck H]

Yes, I did, but at higher volumes they would "sing" anyway

I know this was already covered in this thread, but I had trouble with air core inductors in my own build when using single coil pickups. My solution was to run two inductors of half value in series/out of phase and I put a copper plate (barely larger than the inductors) between them, like a reverse humbucker. Most of the EMF would cancel and no more whistling. Of course there will be some interaction between the inductors even with the copper plate so it would be best to test for that and adjust the values. The attenuator sounds just fine so I haven't done that refinement yet.

[SoulFetish]

I know this was already covered in this thread, but I had trouble with air core inductors in my own build when using single coil pickups. My solution was to run two inductors of half value in series/out of phase and I put a copper plate (barely larger than the inductors) between them, like a reverse humbucker. Most of the EMF would cancel and no more whistling. Of course there will be some interaction between the inductors even with the copper plate so it would be best to test for that and adjust the values. The attenuator sounds just fine so I haven't done that refinement yet.

Small update and advise needed–here is my 390µF capacitor board. nice and compact, with a small physical footprint.



But, I'm still looking for some advice on a shielding solution for the air coils inside the enclosure. I'm looking for something making as efficient use of space as possible that would fit the two coils (something cylindrical, ideally).
I inquired about buying two mu-metal enclosure in which to house and pot the coils.. until I heard back from the manufacturer. - $50 a piece. So, now I'm looking to go another direction. Even modestly permeable steel would probably be okay? Any suggestions on where to get a shielded enclosure maybe used for a small toroid or something?
Also, how effective would Permalloy Shielding tape be, provided it is cheap enough?

[SoulFetish]

I found it! (the air coil shielding solution)
Fits all the criteria.
- should house both inductors, making efficient use of space.
- easily mounted
- ported to run wires in and out of the enclosure
- 360˚ of shielding.



It was right in front of me the whole time. Well, not in front of me; at the bench next to mine. My colleague who handles the turntable/keyboard/tape machine type repairs had a burned out teac motor(several) in a box of debris. I wasn't sure how difficult the disassembly would be, but the enclosure came apart easy peasy.
It's got enough clearance in it's inner diameter in case I decide to wind some inductors using 16AWG enamel wire to get the inductance correct.