resistor wattage for an airbrake?

[arjepsen]

Hey.
I'm thinking about building an attenuator, and have been looking at the airbrake.
However, I can't easily get those big resistors, but thought that I might put something together using some available power resistors instead.
I think I might not use all options, but just the following:
13R to ground - 25R signal.
13R to ground - 10R signal.
25R to ground - 0R signal.
I think for the 13R to ground, I could just parallel to 25R - that would help with the wattage, but what wattage do these, and the remaining resistors need to be?
I plan on using it on 15-20 watts amps (marshall 18 watt, matchless lighting, fender princeton and deluxe.)

Regards
Anders

[vibratoking]

The wattage depends on what amp you intend to attenuate. Here is ONE link from the TWreck files section:

https://tubeamparchive.com/viewtopic.php?t=8733

[arjepsen]

Yeah, I saw that link - but as I mentioned, it's a bit difficult and expensive to come by those particular resistors where I live.
Also, I plan to use it with about 15-22 watt amps.

[xtian]

Where do you live? You can order online:

http://www.parts-express.com/cat/dummy- ... stors/1535

[romberg]

It is not totally clear exactly what circuit you are referring to. But in general you want a resistive load to be able to handle twice the rated power of the amp. Amps are generally rated as RMS power clean right before clipping. Clipped (square waves) have roughly twice the RMS power. So, I think you are looking at an attenuator that can dissipate 50 watts.

Mike

[arjepsen]

Sorry, I'm looking at the "attenuator III" found in here:
https://tubeamparchive.com/download/file.php?id=8823

Ok, so aiming for around 50 watts.
Do both of the resistors in the schematic above dissipate the same amount of heat? I think I got it into my head, that R2 would dissipate less...??

[romberg]

It depends on the setting of the rotary switch. All the way "down" you have 26 ohms in series with your 8 ohm speaker. This is then in parallel with 13 ohms for the first resistor. 9.4 ohm load to the amp. Roughly 70% of the power is being dissipated by the 13 ohms and 30% by the leg with the speaker.

On the third click down you have 21 ohms in parallel with 4 + 8 (spkr). 80 plus percent of the power is going down the speaker leg. So, now the other resistor is dissipating more than the first.

If you really wanted to, you could do all the math and find out the maximum power dissipated at each setting. If it were me, I'd just get two 50 watt ones and call it good.

Mike

[arjepsen]

Thanks for the info.
So I'm thinking about the following arrangement.
5 possible levels:

1:
R1=22R/50W & 33R/25W parallel giving 13,2R;
R2=12R/20W + 15R/20W in series giving 27R

2:
R1= like 1 - 13,2R
R2= 27R from 1 with a parallel 33R/20W giving 14,8R

3
R1=like 1 - 13,2R
R2= 12R/20W & 33R/20W parallel giving 8,8R

4
R1=22R/50W
R2= 12R/20W & 15R/20W & 33R/20W all three parallel giving 5,5R

5
Full level.

How does this sound? I think it's a bit different from the non-rheostat airbrake, but would it work ok, or would any of these values be too far off in some direction?

Regards
Anders

[romberg]

The math works like this (unless I made an error I am assuming you are plugging this into an 8 ohm load.

> 1:
> R1=22R/50W & 33R/25W parallel giving 13,2R;
> R2=12R/20W + 15R/20W in series giving 27R

Version III of this schematic puts R2 in series with your speaker and R1 in parallel with that. So, in case 1 there is 27 + 8 ohms in parallel with 13.2. Your amp sees a 9.6 ohm load. Not ideal but probably ok.

Rload = 1 / (1/13.2 + 1/35) = 9.6

The relative power going through the parallel load (I'm calling R1) vs the speaker load (R2) is from there ratios:

P1 = 1/13.2 / (1/13.2 + 1/35) = .73
P2 = 1/35 / (1/13.2 + 1/35) = .27

So, 73% of the power is taken up by the "R1" leg and 27% by the one your speaker is on. Your R1 leg is really two resistors in parallel. So, the 22 and 33 ohm resistors dissipate:

P22 = 1/22/(1/22 + 1/33) = .6
P33 = 1/33(1/22 + 1/33) = .4

This is a fraction of the 73% down this leg. So, Total dissipation is:

P22t = .6 * .73
P33t = .4 * .73

Down the speaker leg, everything is in series so the powers are:

Pspkr = 8/ (8 + 12 + 15) = .23 * .27 = .06
P12 = 12 / (8 + 12 + 15) = .34 * .27 = .09
P15 = 15 / (8 + 12 + 15) = .43 * .27 = .12

So, on setting 1 each component will dissipate this percentage power:

33R: 44%
22R: 29%
12R: 9%
15R: 12%
Spkr: 6%

At 50 watts:
33R: 22 watts
22R: 15 watts
12R: 4.5 watts
15R: 6 watts
Spkr: 3 watts

And 3 watts into a healthy cabinet will still have folks telling you to turn it down .

Mike

P.S. No I'm not doing this for the other positions. I'll leave that to you

[arjepsen]

P.S. No I'm not doing this for the other positions. I'll leave that to you

That's great - thanks a lot!

[arjepsen]

Now I just need to get my head around making the connections, either with a 2pole 6position switch, or maybe with two dpdt's