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

[Helmholtz]

what kind of ideas where you thinking about for the secondary solid state drivers?

In principle you could use any linear, low output impedance (power) amp. Even an integrated amplifier with tone controls. Just make sure to attenuate the dummy load signal to a level that suits the slave amp's input sensitivity using a voltage divider of total resistance between 1k and 5k. If you attenuate to line level, you may insert line level effect units between dummy load and slave amp, e.g. reverb, delay, chorus, equalizer...or even go quasi stereo with one dry and one wet signal into a dual channel amp connected to guitar speakers.

After I have finished my Marshall PowerBrake attenuator project, I will experiment with my old and trusted Dynaco Stereo 120, which has become part of my lab equipment.


BTW, if you make the capacitor(s) switchable you can emulate the bass response of different speaker/cabinet types. Closed cabinets raise the bass resonance frequency (e.g. vintage Marshall 4x12: fres=120Hz). This would require a cap value of 150µ to 180µ with a 12mH choke.

[SoulFetish]

BTW, if you make the capacitor(s) switchable you can emulate the bass response of different speaker/cabinet types. Closed cabinets raise the bass resonance frequency (e.g. vintage Marshall 4x12: fres=120Hz). This would require a cap value of 150µ to 180µ with a 12mH choke.

Interesting idea, i had thought about the need for an cab/speaker emulator circuit for a possible direct out. But, I like the idea of having Variations in the reflected load. Too bad they didn’t make small variable auto transformers, you could probably use something like that as a variable inductor to dial in a pretty broad range of LC resonance.

[Gregg]

I was going to buy a pair of Jantzen 1031 0.47mH 18 AWG Air Core Inductor for Lh0 & Lh1 and wanted to double check that these are appropriate.
But if I was sourcing out a designated filter choke for the project, can you guys offer up some suggestions to get me in the right direction? I was thinking 4-5A rating (maybe 6?).

I like to insure myself so in the reactive loads I've built so far I was using 1,2-1,3mm thick copper wire (~16AWG) air core inductors. Same for the low freq inductor. For caps my choice was two 100uF/350V motor start caps in parallel (this way they can fit into an 1U rack enclosure). All this is for a load that can handle cranked 100 Watts.
Considering the LF inductor choices you can use a Hammond 159ZJ 10mH/5A choke or go for this one for example:
https://www.parts-express.com/erse-s...-coil--266-946
For those in Europe at Banzai they have the full Mundorf list of caps and inductors.
You can also check Ebay and Aliexpress for 200uf 250-450V CD60 motor start caps (for a 8 Ohm load).
Also I'm running 2 or 3 power resistors in parallel on a massive heatsink fan cooled (Marshall Powerbrake fan circuit) to make sure that if one them fails the amp will have some load left (although with a massive fan cooled heatsink that's very unlikely to happen).

[SoulFetish]

I was also have looking at a 15AWG .47mH jantzen 1518 as well. For a few bucks more, I’ll go with those.

[Helmholtz]

Interesting idea, i had thought about the need for an cab/speaker emulator circuit for a possible direct out. But, I like the idea of having Variations in the reflected load. Too bad they didn’t make small variable auto transformers, you could probably use something like that as a variable inductor to dial in a pretty broad range of LC resonance.

Not sure if we mean the same with "emulate". I meant emulate the bass impedance of different speakers/cabinets. The attenuated and amplified dummy load signal needs to be connected to a guitat speaker.
A direct out for full range reproduction would require additional filtering to emulate the frequency response of a guitar speaker.

[SoulFetish]

Not sure if we mean the same with "emulate". I meant emulate the bass impedance of different speakers/cabinets. The attenuated and amplified dummy load signal needs to be connected to a guitat speaker.
A direct out for full range reproduction would require additional filtering to emulate the frequency response of a guitar speaker.

Don’t sweat it. I knew what you meant.

[SoulFetish]

So, I just got the main reactive components to build this proper.



For the main dissipation resistor(s), I have 55Ω/50W resistors I can run 10 in parallel. Or, I can buy a 1 or 2 higher power resistors at some point.
But I'm looking for an aluminum enclosure I can use to house these components--tall enough to mount the caps and install a fan, and enough real estate to install an active speaker driver.
Any suggestions?

[Helmholtz]

But I'm looking for an aluminum enclosure I can use to house these components

Some remarks:

- A steel housing could be expected to provide better shielding of the air chokes' magnetic fields and prevent coupling with the PUs. Aluminum needs to be very thick for similar results.

- Directly mounting air core chokes to aluminum can decrease their inductance by 20% and more. There are several influencing factors and I recommend measuring inductance.

- Closely mounting 2 air chokes also changes inductances.

[catalin gramada]

Some remarks:

- A steel housing could be expected to provide better shielding of the air chokes' magnetic fields and prevent coupling with the PUs. Aluminum needs to be very thick for similar results.

- Directly mounting air core chokes to aluminum can decrease their inductance by 20% and more. There are several influencing factors and I recommend measuring inductance.

- Closely mounting 2 air chokes also changes inductances.

Completely agree. This was a reason to mount my coils on the wood blocks otherwise coupling derive can be( see my attenuator pic below). Also the position into chassis (eg. mounting into a corner) can do a difference. The inductance should be measured with inductors mounted into chassis.

[catalin gramada]

So, I just got the main reactive components to build this proper.

[ATTACH=CONFIG]54967[/ATTACH]

For the main dissipation resistor(s), I have 55Ω/50W resistors I can run 10 in parallel. Or, I can buy a 1 or 2 higher power resistors at some point.
But I'm looking for an aluminum enclosure I can use to house these components--tall enough to mount the caps and install a fan, and enough real estate to install an active speaker driver.
Any suggestions?

Take a look at Hammond dealers for case.

[SoulFetish]

Some remarks:

- A steel housing could be expected to provide better shielding of the air chokes' magnetic fields and prevent coupling with the PUs. Aluminum needs to be very thick for similar results.

- Directly mounting air core chokes to aluminum can decrease their inductance by 20% and more. There are several influencing factors and I recommend measuring inductance.

- Closely mounting 2 air chokes also changes inductances.

Thanks. I hadn’t considered those factors.
I was thinking about aluminum for the heat dissipation and weight. I suppose I could just use a steel chassis.
Initially i was going to mount them (air coils) on two small computer fan grates(grills)

[SoulFetish]

Do you think it would be better to mount them with internal screening (inside the enclosure), so not to radiate into any drive circuitry I may include?

[Helmholtz]

Do you think it would be better to mount them with internal screening (inside the enclosure), so not to radiate into any drive circuitry I may include?

Sure, if you can find some small steel enclosures for the air coils, that will help. But keep some distance between steel surfaces and chokes as otherwise inductance will increase.

[SoulFetish]

What do people use to physically mount these things?

[Chuck H]

When I built my phony speaker load for my own attenuator I used air coils and had significant feedback with single coil pickups within about nine feet of the unit at full attenuation.

I've tried copper shielding inside an aluminum enclosure. Didn't help much (grounded or otherwise). What worked for me was to make the inductor sandwich. Two coils in series, out of phase, stacked one on the other with two thin insulating plates and a copper plate in between those. BARELY extending beyond the coils. The goal was to create a RADIATED anti phase EMF. It worked with respect to mitigating interference with the guitar or any amplifier components, but...

I didn't test for preserved coil inductance. I could hear that there was a reduction of the original value. How the anti phase EMF works as a sort of reverse humbucker shouldn't be sensitive to the value of the inductors though. So if actual inductance testing of the assembled "inductor sandwich" was done then the values of the individual inductors necessary for the assembly to get a desired 'in circuit' inductance could be determined.

JM2C (based on my own experience)