The guitar tone of Brian May is legendary. The common folk lore is that he used a treble booster into the Normal channel of an AC30. The AC30’s Normal channel is very dark, especially in comparison to the Brilliant channel, so combining it with a treble booster makes a lot of sense. Reportedly Brian used a Dallas Range Master treble booster early in his career, and then later switched to a custom built booster with a slightly altered circuit. The Range Master essentially does two things. It high-pass filters the input with a cutoff frequency of about 1 to 1.5 kHz, and it boosts the gain by up to 30dB. But the Range Master also has one other important aspect, which is often overlooked. It has a very low input impedance, which loads your guitar pickup and makes it less trebly.
Even though Eleven Rack does not have a treble booster effect, you can get pretty close using the built in Parametric EQ and manually setting True-Z. Set the Low Band of the Parametric EQ to the HP6 type, which is a 6dB per octave high-pass filter. Set the frequency to around 1kHz, and the output gain to +24dB. Then go to the input block and set True-Z to “32k Ohm + Cap”. Now max out the Normal channel on the AC Hi Boost model, and you’re done. The Range Master has a little bit more gain at the max setting, so if you want you can add another Parametric EQ or the Dyn 3 Compressor before the amp to add a little bit more gain, or you can bump up the gain by mixing in a little bit of the Brilliant channel.
I’m also familiar with the Matchless DC-30 circuit, and it’s quite interesting how similar the DC30’s Channel 2 is to a treble booster into the normal channel of an AC30. I’m sure this is not just a coincidence. The rotary tone switch on the DC-30 is basically a variable 6dB per octave high-pass filter. The cutoff frequency of the high-pass filter can varied from approximately 20 Hz to 700Hz. At the highest setting the result is quite similar to a typical treble booster.
Even though we did not model Channel 2 of the DC-30, it’s possible to get pretty close using Eleven Rack. First set True-Z to “1 Meg + Cap” to match the impedance of the tube input circuit. Set the Parametric EQ to the HP6 type. Set the frequency to between 20Hz and 700Hz and the gain to +18dB. Then run that into the AC Hi Boost Normal channel. On the DC-30 there is also a gain bypass capacitor, which has the effect of increasing the relative amount of high-frequencies as the gain is lowered. You can approximate that with the HiShelf filter in the Parametric EQ, by setting frequency to around 2kHz and the Q to 0.5. Adjust the gain to taste, depending on how bright you like it.
Here are some presets you can download, so you can try it out for yourself.