25% (or) 10% Power Switch

jaymenon · August 2023

Is this (resistor-complex) prone to all the problems of resistive (as opposed to reactive) attenuators?

25% Power Switch
The switch places an 8 ohm 25 watt resistor in series with the 8 ohm amplifier speaker (for a total of 16 ohms and a 50% cut in power), then places a 16 ohm 25 watt resistor in parallel to bring the total speaker load back down to 8 ohms and gives another 50% cut in power. The two resistors turn 3/4 of the amp's output signal into heat. With the switch in the "Normal" position the 8 ohm resistor is bypassed and the 16 ohm resistor is disconnected.

10% Power Switch
If you really want near-bedroom volume levels then a 90% cut in power can be attained by using a 16 ohm 25W resistor for the upper resistor and a 12 ohm 25W resistor for the lower resistor (8 ohm output transformer secondary). The 12 ohm resistor will burn most of the amp's output so for anything more powerful than a cathode biased push-pull 6V6 amp like the 5E3 Deluxe you'll need to bump the 12 ohm resistor's power handling to 50 watts to be safe (two 25 ohm 25 watt resistors in parallel will work).

Image: https://robrobinette.com/images/Guitar/5E3P_Build/10_Percent_Power_Switch_Schematic.png

Image: https://robrobinette.com/images/Guitar/5E3P_Build/10_Percent_Power_Switch.png

With the switch set to 10% both jacks will be 8 ohm with 90% of the amp power turned into resistor heat. These 25 watt resistors are adequate for 15 watt amps such as a 5E3 Deluxe or other cathode biased 6V6 push-pull amps.

Expert opinions most appreciated...

ICBM · August 2023

There are no real problems as such with resistive attenuators, some of the best and most highly regarded are (eg Dr.Z Airbrake). If designed correctly they’re largely self-compensating for the difference between plain resistance and the impedance curve of a speaker.

The key is to avoid too low a resistance in parallel with the speaker, which damps it excessively and strangles the tone - this is usually why most attenuators don’t sound good when turned down a long way, rather than their effect on the amp, which becomes irrelevant once there’s any significant attenuation. This design does that, because there’s no parallel resistance with the speaker, it’s actually in series.

(This is not the same problem as generating a line signal from a dummy load, where you do generally need a reactive rather than resistive load, because there is no speaker involved.)

jaymenon · August 2023

ICBM said:

The key is to avoid too low a resistance in parallel with the speaker, which damps it excessively and strangles the tone - this is usually why most attenuators don’t sound good when turned down a long way, rather than their effect on the amp, which becomes irrelevant once there’s any significant attenuation. This design does that, because there’s no parallel resistance with the speaker, it’s actually in series.

Your reply was actually very clear, but just to be absolutely certain:

This design does that…

As in avoiding too low a resistance in parallel with the speaker? Since it’s actually in series…?

ICBM · August 2023

Yes. The more I tinker around with gear, the more certain I am that speaker damping is one of the most - if not *the* most - important and currently overlooked factors in tone, dynamics and ‘feel’. In my opinion it’s why valve amps sound inherently better than conventional solid-state amps, why attenuation has a bad reputation, and why the differences are more noticeable to the player than a listener.

Conventional variable attenuators usually apply a resistance in series with the amp and another in parallel with the speaker, to give a standard power divider - but the problem is that at high attenuation, the resistance in parallel with the speaker is much less than that of the speaker itself, so it damps the speaker heavily and kills its dynamics and ‘open’ tone… hence the reputation they have for producing a flat, lifeless tone. But this design adds a *higher* resistance in series with the speaker - using the speaker’s own impedance as the lower half of the divider - and then corrects the load on the amp with another resistor in parallel with the *amp*, not the speaker.

Does that make sense?

jaymenon · August 2023

Thanks @ICBM ;

So for an 1W amplifier would you recommend a 25% power switch or a 10% power switch?

ICBM · August 2023

It depends how efficient your speaker is and how loud you want it to be.

uncledaddy · August 2023

I did the 1/4 power mod on a 5e3 and it was still loud in a small house, and couldn't get anywhere near breakup at bedroom level- hardly suprising from a non-mv amp. With the help of a TDPRI member (Fenderlover), we came up with the 10% mod, so I built the first version into the same 5e3. Because of the way we hear, and the power vs volume logarithmic scale, 10% output reduced the volume by around 50%, and made the amp much more useable in the house. The 5e3 doesn't really need a standby, so I used the hole for a switch, and mounted the resistors to the back of the chassis. A worthy mod indeed. I build a version into a pedal box too, which was useful.

Winny_Pooh · August 2023

^^^On a low wattage amp you don't even need to build anything.

Here's one built into the back of my tweed deluxe cab. Whisper quiet if need be.

Using one of these:

https://willys-hifi.com/products/l-pad-attenuator-100w-8-ohm-wirewound-volume-control?variant=1194100545

Same as the one used in the Guitarist magazine DIY attenuator project.

pt22 · August 2023

Winny_Pooh said:

^^^On a low wattage amp you don't even need to build anything.

Here's one built into the back of my tweed deluxe cab. Whisper quiet if need be.

Using one of these:

https://willys-hifi.com/products/l-pad-attenuator-100w-8-ohm-wirewound-volume-control?variant=1194100545

Same as the one used in the Guitarist magazine DIY attenuator project.

Ooh nice. I have that project bookmarked so I’ll post it here: https://guitar.com/guides/diy-workshop/diy-workshop-build-your-own-attenuator/

New amp has a great MV though, so I may not even need it. Shame looks like a fun project!

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25% (or) 10% Power Switch

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