Airbrake Bedroom Control Mod

[Littlewyan]

I’m not sure about others, but I never found the bedroom control on my Airbrake to be of any use. It always sounded way too fizzy. I really wanted to play my larger amps at home, so I started researching more into attenuators to fix this and came across this thread.

https://www.marshallforum.com/threads/s ... ing.98285/

This guy did a lot of research into making a good attenuator and found that not only does the amp need to see the correct impedance, but so does the speaker. These two things were key.

Taking this into consideration I looked at my Airbrake and found that the speaker can see as much as 188ohms when using the bedroom control! To bring this down, I stuck an 18R 7W resistor across the speaker output that only comes on when the bedroom control is in use and also removed the 0.1uF bypass cap. The result, my Airbrake now gives a good tone on every setting! Earlier today I was playing my JTM50 at talking volume and it still sounded great! So I highly recommend this mod to anyone who has an Airbrake with the rheostat. For me it was a revelation!

[Littlewyan]

Just some extra info....The higher the impedance the speaker sees, the brighter your tone gets. Almost as if you’re giving the speaker a longer leash. Unfortunately, the correct output impedance depends a LOT on the amp you’re using. Amps with NFB will have a much lower output impedance than those without. So if you use a non NFB amp you might find that this mod makes the tone dull. I suggest you experiment to see what sounds best to you. For me it was an 18R resistor, but I haven’t any non NFB amps to test this with. I’ll do some simulations in LTspice at some point to see if it makes a big difference.

As for how I switch it in and out, I’m only using half of the Lorlin rotary switch that gets used in Airbrakes, so I used the unused half to switch it in when the bedroom mode is selected.

Edit: I changed the 18R to 40R (22R and 18R in series). 40R sounds better to my ears, sounds less restricted at lower volumes.

[dorrisant]

Good info. Thank for posting!

I'll have to build one of the M-4s when time permits.

[Littlewyan]

No worries! It was a revelation for me so I had to share it!

If you go to page 56 or 57 then there is an updated simplified M2 design. I was thinking of building one using Ohmite resistors, but it may required a hefty chassis! I'll draw it out when I get a chance.