MESA BOOGIE S.O.B. Weezer Blue Album Mark 1 Mod

[xenrelic]

Hey guys, just thought I'd share this fun project with anyone who is interested in modding their Mesa S.O.B. 60 into the Boogie Mark 1 to get those Weezer Blue Album tones. A buddy of mine brought this Son of the Original Boogie head in and we got it sounding awesome.

The Sprague 715P caps need to be swapped out with Sprague/CDE 6PS series capacitors or the Mallory PVC6 type for the tone to be accurate.

The phase inverter needs to be rewired to Mark 1 spec and the Boost and Bright Push Pull Pots need to be added for Volume 1 and Volume 2 for the mod to sound correct. The Limit Control is converted into a 10K Presence Control. We just opened the pots up and replaced the carbon track boards on them, so the original knobs fit correctly. The Push Pull Pots are a little bit trickier to open up and replace the carbon track boards on. I ended up using long shaft CTS push pulls. You can still order these on Mesa's parts website.

Other than that we just cleaned up the heater wiring and added a DC lift to it, along with adding a few extra shielded signal wires for a lower noise floor.

Then I dialed in the node voltages to get it just right while comparing to the album tones. I listed the voltages we ended up with on the schematic. Note that the S.O.B 2 has a higher B+ voltage than the previous model. There's about 472VDC vs. the previous model with a B+ voltage of about 450VDC. To compensate for this you need to add two 10K 2W resistors before the phase inverter power node in place of the stock 5.6K resistor, so that the voltage = 345VDC at the phase inverter node. Then you'll need to split the V1 power node from the V2 power node via a 22K 2W resistor and add a 47uF - 60uF 350v filter cap on the V1 power node. This will drop the V1 node voltage, so that you see about 268VDC.

Cheers!

[Reeltarded]

You killed the only one I like!

[Mark]

When you changed the P.I. stage did you notice a huge change in gain going from the S.O.B. phase inverter to the Mark 1 (faux Tweed Bassman/Marshall) phase inverter?

I tried putting the Mark 1 (TB/M) P.I. in with a 12AX7 and I had to pad down the signal as the amp was distorting very early. I had to add a 10K resistor to the last plate resistor (before the MV) and the power supply taking audio from the junction of the plate resistor and 10K resistor to get a workable output.
This isn’t a surprise as both Trainwreck and Dumble use this circuit to coax gain out of Fender black/silver face amps. I found the blackface style P.I. using a 12AT7 had about the right level and worked best with the amp.

What were your findings?

Thanks for your assistance.

IMG_1298.jpeg

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[xenrelic]

I think there would be too much gain hitting the stock S.O.B. phase inverter for it to even work without adding those extra 220K resistors in parallel with the master volume pot to ground.

Adding the stock Mark 1 phase inverter to the S.O.B. shouldn't be an issue at all as long as you add the negative feedback resistor to it and the 250pF cap between the plates. Though you might want to remove the extra 220k resistors from the master volume.

The Fender style phase inverter in some of the Mark 1's has a different feel that I think doesn't work well for the tone I was after, but some people might dig it for surf music or blues rock. If I were going to go the 12AT7 Fender P.I. route, I definitely would make the negative feedback adjustable with a 10k linear pot in series with the 820 ohm resistor.

When you changed the P.I. stage did you notice a huge change in gain going from the S.O.B. phase inverter to the Mark 1 (faux Tweed Bassman/Marshall) phase inverter?

I tried putting the Mark 1 (TB/M) P.I. in with a 12AX7 and I had to pad down the signal as the amp was distorting very early. I had to add a 10K resistor to the last plate resistor (before the MV) and the power supply taking audio from the junction of the plate resistor and 10K resistor to get a workable output.
This isn’t a surprise as both Trainwreck and Dumble use this circuit to coax gain out of Fender black/silver face amps. I found the blackface style P.I. using a 12AT7 had about the right level and worked best with the amp.

What were your findings?

Thanks for your assistance.

IMG_1298.jpegIMG_1302.png