Question power resistors Airbrake attenuator

[guile]

I posed this question under de Hotrod Trainwreck discussion, but Im not sure it will get noted so dare I ask my question here?

I want to build the Airbrake and I happen to have 30 Ohm instead of the required 25 Ohm power resistors.
Would this be a problem for the Airbrake?
Please teach me.
Thanks

[roberto]

Which resistor in which position?
A schematic would help giving a reply.

[guile]

Please see attached file. There are two power resistors.

[pompeiisneaks]

Is there a schematic? The layouts like that can be tricky for me at least to read.

~Phil

[guile]

Please see attached

[guile]

Is this sufficient?

[martin manning]

Do your 30 ohm resistors have adjustable terminals? If so just set one at 25 ohms on both of them.

[guile]

Thanks. Good idea. I only have one adjustable one.
My question is basically if overall higher resistance could hurt the amp. I don't understand the calculation of resistance of this circuit, so hopefully someone can shed a light?

[roberto]

Basically R1 is in parallel with the composite load of R2 (and R3 in highest attenuation) plus the speaker.
R2 makes a voltage divider with the speaker, reducing the signal hitting it, while R1 keeps stable the impedance seen by the amp. Very simple.

Do your own math with both values and see if it works.

[guile]

Thanks!
Could it be in any way harmful for the amp?

[roberto]

You mean except throwing it to the amp?

(have you tried to do the math on how the impedance would change?)

[guile]

(I have no idea how, to he honest

[martin manning]

If you don't have the adjustable terminals, I don't see how you can build it as shown. Are you intending to build some reduced version?

If you build with 30 ohm instead of 25 ohm resistors the difference is not significant until you get to the extreme settings. The first four will be very close, and the last two will be a few ohms higher than intended. In any case, the Airbrake does not maintain a constant load, it increases with higher attenuation settings. It may be that an increased load at higher attenuation settings sounds better, but I would treat an Airbrake as an 8 ohm load on the first three (loudest) settings, and a 16 ohm load on the last three (most quiet) settings, adjusting the amp's impedance selector as required. The combination of running a high load (more than 2x the design value) and high amp power settings could lead to problems, including damage to the output tubes and other components.

[romberg]

The amp will see a load which is R1 (the first power resistor to ground) in parallel with R2 (the switched/variable power resistor plus your speaker impedance). So, the amp load would be:

rl = 1 / (1/r1 + 1/(r2 + rs))

I wrote a short python program to generate some numbers for this circuit (attached below) using 4, 8 and 16 ohm speaker loads and both 25 and 30 ohm power resistors. The results of running it were:

==== rp: 25 ===
--- rs: 4 ---
r2: 4 rl: 6.3
r2: 8 rl: 8.6
r2: 16 rl: 12.0
r2: 25 rl: 14.7
--- rs: 8 ---
r2: 4 rl: 8.6
r2: 8 rl: 10.4
r2: 16 rl: 13.3
r2: 25 rl: 15.7
--- rs: 16 ---
r2: 4 rl: 12.0
r2: 8 rl: 13.3
r2: 16 rl: 15.5
r2: 25 rl: 17.3
==== rp: 30 ===
--- rs: 4 ---
r2: 4 rl: 6.3
r2: 8 rl: 8.6
r2: 16 rl: 12.0
r2: 30 rl: 15.9
--- rs: 8 ---
r2: 4 rl: 8.6
r2: 8 rl: 10.4
r2: 16 rl: 13.3
r2: 30 rl: 16.8
--- rs: 16 ---
r2: 4 rl: 12.0
r2: 8 rl: 13.3
r2: 16 rl: 15.5
r2: 30 rl: 18.2

As martin points out, the difference between 25 and 30 ohm R1 and R2 is minor. However, the numbers (as martin also says) do show that for lower impedance speakers, this attenuator varies the amp load by a significant amount. Using a 4 ohm speaker, with R2 full up puts a load on the amp of 16 ohms. This is more than a 2x mismatch. So, the moral of the story is that this type of attenuator would probably work best with 16 ohm speaker cabs and the amp set at 16 ohms.

Mike

[guile]

Martin and Mike, impressive stuff.
Thank you both very much for all this knowledge!