# 5E3 1/4 power switch?

Discussion in 'Shock Brother's DIY Amps' started by Uncle Daddy, Jun 13, 2018.

1. ### timewaster1700Tele-Meister

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Ok I did the math for anyone that's interested. My guess is I made it too complicated but here we go:

Here's my makeshift circuit diagram without the switch for simplicity:

>------^^R2=16ohm^^----^^R3=8ohm spkr^^------>GND
| |
|_________^^R1=12 ohm ^^________________|

Here's my math:
We want an equation such that the speaker power is some fraction of the total power: P3 = (1/n)*Ptotal
where we can define n to be 4 for quarter power, 9 for 9th power, etc.

P3 = (1/n)*(P1+P2+P3)

We can rearrange this to be: 1/n = P3/(P1+P2+P3) ;

P1 = V*I1 = V^2/R1

P2 = V2*I2 = V2^2/R2

V2 = (R2*V)/(R2 + 8)
therefore,

P2 = (R2*V^2)/(R2+8)^2 ;

P3 = V3^2/8

V3 = (8*V)/(R2+8)^2
therefore,

P3 = (8*V^2)/(R2+8)^2;

Now if we plug P1, P2, and P3 into our original equation we get:

1/n = [(8*V^2)/(R2+8)^2] / [(V^2/R1) + (R2*V^2)/(R2+8)^2 + (8*V^2)/(R2+8)^2]

The voltage terms cancel out and we can multiple the numerator and denominator by (R2+8)^2 to get:

1/n = 8/[(R2+8)^2/R1 + R2 +8]

If we take the reciprocal of both sides we get:

n = (R2+8)^2/(8*R1) + R2/8 + 1

Now lets solve for R1 in terms of R2 and n:

(n - R2/8 - 1) = (R2+8)^2/(8*R1)
then
8*R1 = (R2+8)^2/(n-R2/8-1)

which simplifies to our final equation:

||| R1 = (R2+8)^2/(8*n-R2-8) |||

If we want 9th power (n=9) and we make R2 = 16 as @FenderLover suggested we can enter those numbers into this equation to solve for R1 and we get:

R1 = (16+8)^2/(8*9-16-8) = 12

One last thing though you also have to keep the total resistive load on the output transformer 8 ohms so you have to also fulfill a second equation at the same time:

8 ohms = 1/[1/R1 = 1/(R2+8)]
which is easier to think about like this:
1/8 = 1/R1 + 1/(R2+8)

Maybe the best procedure would be find a bunch of R1 and R2 values that work for the
above equation then plug them into this equation to get your fractional power value:

1/n = 8/[(R2+8)^2/R1 + R2 +8]

Last edited: May 3, 2019
2. ### timewaster1700Tele-Meister

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Ok I wrote a little LabVIEW program with the above two equations and got the following resistor options:

You can see the standard 16 ohm and 8 ohm for 1/4 power and then @FenderLover 's 1/9th power suggestion of 12 ohm and 16 ohm.
Looks like 1/25th power can be had with a 10 ohm and 32 ohm resistor.

I tried up to 1000 different combinations of R1 mixed with 1000 combinations of R2 and these are the only ones that nail the 8 ohms exactly.

If I loosten the tolerance for the load to be between 7.968 ohms and 8.032 ohms it opens up the possibilities a little more:

I can expand the tolerances if someone is interested. I have no idea how close the impedance of the load needs to be to the ideal 8 ohms. I'm guessing I could open up the tolerances and be fine.

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Last edited: May 3, 2019
3. ### timewaster1700Tele-Meister

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Here's another run with the tolerances opened slightly and a 4th column that shows the total impediance the output transformer will connect to using the resistor combination and an 8 ohm speaker assuming everything was ideal and perfect values (which of course is not true).

Last edited: May 3, 2019
4. ### mcentee2Tele-Meister

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From what I have read speaker rating is "nominal" anyway given its variance with an AC signal/frequency.

I don't know how tolerant it is though, but I suspect an "ohm" either way won't break anything.

Maybe pick out the pairs with common values, easily purchased?

5. ### timewaster1700Tele-Meister

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I opened the tolerances a bit more

Looks like 10 and 33 (both available from mouser) would give 1/25th power. I'm tempted to try this and then the 1/9th power and compare.

6. ### timewaster1700Tele-Meister

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Here's my hand sketch for an attenuator wiring diagram that uses an ON/OFF (SPDT) switch and a LOW/HIGH ATTENUATION (DPDT) switch. I think I'll give this a shot.

robrob likes this.

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Good luck fitting that lot into a 5e3 chassis

8. ### timewaster1700Tele-Meister

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I should have mentioned the plan was to put it in a pedal box. Either the small A size Hammond or the larger B whichever works.

robrob and Uncle Daddy like this.
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