# RobRob's 10% power switch in a 5F6A- Values?

Discussion in 'Shock Brother's DIY Amps' started by TelZilla, May 1, 2019.

1. ### TelZillaFriend of Leo's

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I just got hipped to this very cool mod: 10% Power Switch. Rob's diagram:

I'm in the middle of a homebrew 5F6A, and I'm interested in incorporating it. I want to see if I've got the right values for the chassis mount resistors.

Relevant details:
• It's a 2 x 10" (built in a BFDR sized cabinet)
• 2 8-ohm speakers run in parallel, so I'm using the 4 ohm tap on my OT
Since my amp is 2 x 6L6, I'm just going to use 50 Watt resistors. I believe I should use 8 ohm for the "top" resistor, since I'm using an 4 ohm tap, not 8. But for the resistor going to ground, should I stick with 12 ohm? Divide by 2 and get 6 ohm?

2. ### robrobPoster ExtraordinaireAd Free Member

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Yes, you would cut both resistors' value by 50% so 8 ohm and 6 ohm.

3. ### TelZillaFriend of Leo's

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Thanks!

4. ### FenderLoverFriend of Leo's

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It might be fun to note that the 16 ohm/12 ohm values work well for a Universal Half Volume attenuator. Into an 8 ohm tap, a 4 ohm load comes to 7.5 ohms net, and a 16 ohm load comes to 8.7 ohms net. Practically fool proof. It's the Great Equalizer because the network is 90% resistive. As a stand-alone unit, there is no need for a switch. When not attenuating to Half Volume, just plug your load into the correct tap on your amp. Chance are good that you will be using it or not, and not switching it in and out.

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I built this into a 5e3 earlier this year, and it's a great mod. There is a little perceived treble loss, but that's easily fixed by diming the tone knob. At 8 ohms, it really gives a significant drop in volume.

6. ### muswell_hillbillyTele-Meister

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I was intrigued by this post and the underlying question about power reduction level, resistors and the nominal output speaker load.

So I spent some time (probably too much) going through the math, and believe I've settled on closed-form expressions for R1 & R2 in @robrob's mods for power reduction (please correct me if it's wrong).

For the equations below, R1 is the parallel resistor to ground, and R2 is the resistor in-series with the speaker load (Rs). "p.r." is the power ratio, so 25% = 0.25, 10% = 0.1 :

So the resistor values change linearly with respect to the nominal speaker impedance, as @robrob pointed out in his response. If you drop the speaker load from 8 ohm to 4 ohm (as TelZilla did), drop R1 & R2 from @robrob's example by half. If you go to 16 ohm, double the values. The important take-away for this mod is that the values of R1 & R2 must be chosen such that the the overall load presented to the secondary winding matches the nominal load of the speaker!

Below is an Excel spreadsheet graph for these equations. Notice that R1 heads toward infinity (open circuit) and R2 heads toward zero (short circuit) as the power ratio approaches unity, which is the unmodified design!

In the interest of time and not transcribing my chicken-scratch math, I only included the final results above, using a really cool online equation editor:

https://www.codecogs.com/latex/eqneditor.php

If anyone has other recommendations for free equation editors, please share! I could also scan my chicken-scratch math, if anyone is interested

Cheers!

Last edited: May 2, 2019
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7. ### mcentee2Tele-Meister

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My similar journey is in a thread in the Amp forum, this post sort of sums up a bit of the maths as well, with a link back to the electronics of LPads and “direction”:

5E3 1/4 power switch?

On that link page, the equations for the Lpad looking from the load backwards to the OT look “similar” to yours, but probably from a different direction and set of variables.

Last edited: May 2, 2019
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8. ### rogbTele-Afflicted

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You can build a great attenuator that will work up to 50w no problem, I use it with my JCM800. It is slightly more beefy than using the little 25w resistors alone. I built mine in a little metal box and drilled a bunch of holes in it. I used an 8r 25w resistor in place of the rheostat.

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9. ### muswell_hillbillyTele-Meister

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Thanks for the reference, @mcentee2! I'd seen that thread a while back, but hadn't made the connection between the two.

I believe the equivalent circuits for these 2 attenuation methods are slightly different:

So now curiosity will get the best of me, and I'll need to grind through the math on the LPad version. Thanks? lol

Last edited: May 2, 2019
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10. ### mcentee2Tele-Meister

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I hope you get to the bottom of it!

My gut tells me, as I have said in the other threads, there is something that a standard LPad does re the orientation Vs the Fender (speaker array replaced wirh resistors) that results in so many bad reviews of them as attenuators.

11. ### TelZillaFriend of Leo's

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Another question about a different mod- this time the humdinger heater hum pot.
Yeah, that one is definitely the Dr. Z airbrake. Might look into that one instead.

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12. ### rogbTele-Afflicted

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This is how they look if you ditch the rheostat

13. ### TelZillaFriend of Leo's

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Thanks for the Pic- you really did drill a lot of holes. A couple questions:
• Why'd you ditch the rheostat? Price?
• What do you miss out on by not using the rheostat?
• I notice you omitted the 0.1uF/400v cap. What effect would that have?
• How do you decide where to put the extra lugs on the bottom resistor? Does lug to the right= more attenuation or less?

14. ### muswell_hillbillyTele-Meister

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Well, for whatever it's worth (granted, probably not much), I derived similar equations for the L-Pad attenuator, again in terms of the desired power ratio (p.r.):

$image=https://latex.codecogs.com/gif.latex?R_%7B1%7D%20%3D%20R_%7Bs%7D%5Cleft%20%28%201%20-%20%5Csqrt%7Bp.r.%7D%20%5Cright%20%29&hash=a336524ed6d01bc7671362012e5e8bef$

$image=https://latex.codecogs.com/gif.latex?R_%7B2%7D%20%3D%20R_%7Bs%7D%5Cleft%20%28%20%5Cfrac%7B%5Csqrt%7Bp.r.%7D%7D%7B1%20-%20%5Csqrt%7Bp.r.%7D%7D%20%5Cright%20%29&hash=98fdc409cefc0903726e832696e72327$

and the Excel graph:

I put a few examples into the Weber L-Pad calculator and got the same results, so it seems to be correct.

Haven't given it any thought beyond the math!

15. ### rogbTele-Afflicted

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That's not mine, it was quicker to find a pic online than open up and photograph mine.
I don't need bedroom volume in the unit as I use MVs and PPIMVs so no rheostat.
The cap is a bright cap, you may or may not need it.
You measure the resistance to place the lugs according to the values given.

16. ### muswell_hillbillyTele-Meister

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At son's basketball tournament all day yesterday, with hours between games! So I took the opportunity to capture my scribbled notes on these resistor calculations...

#### Attached Files:

• ###### SwitchPowerReduction.pdf
File size:
293.6 KB
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