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Blackface tone is in the reverb circuit!
I have a 1970 Deluxe Reverb that's been my favorite amp for years. I also have a blackface Bandmaster head that I've been trying to like for years, but it's never sounded all that great. I've rewired this Bandmaster repeatedly, trying to get it to sound like my Deluxe, but with more power. I changed every component to Deluxe AB763 values, recapped it, tried different transformers, even tried Mojo Deluxe transformers with 6V6's in it so it was virtually identical to the Deluxe, but nothing made it sound better. I eventually put a set of transformers from a blackface Bassman in it and used it occasionally for louder gigs, but always wished it sounded more like my Deluxe.
And then it occurred to me that the major circuit difference between the two was that the Bandmaster didn't have reverb, and that the reverb mixer stage in the Deluxe is in the circuit path even when the reverb is off. I never use the vibrato in either amp, so I decided to rewire the Bandmaster's vibrato circuit as a reverb circuit like the Deluxe's.
I removed all the components of the vibrato circuit, including all the wires leading to the Speed and Intensity pots. Then I wired up the reverb mixer stage into the now empty eyelets in the circuit board, including a 50k fixed resistor in place of the tremolo intensity pot.
To make a long story short, it worked! All of a sudden this tired old Bandmaster had all the sparkle and grind that I love my Deluxe for, but with the power of 6L6's. OK, so what's going on here?
Refer to this AB763 Deluxe schematic for the following explanation:
http://www.doolinguitars.com/article...man/deluxe.jpg
I'm no engineer, so this is all speculation, but here's my theory... The reverb mixer stage is inserted between the "Vibrato" channel preamp and the phase inverter. The signal from the 2nd preamp stage goes through a 3.3M resistor (bypassed with a 10pf cap) to the grid of the mixer stage. The grid is also tied to the Reverb pot through a 470k resistor, and to a 220k resistor to ground. That 470k and 220k in parallel read about 150k. So, the 3.3M in series with that effective 150k resistance form a voltage divider, dropping the output of the preamp by about 22:1. But the mixer stage is half of a 12AX7, which has a gain of about 100. That means the signal has a net gain of about 5 in the mixer stage. (In the schematic, I've colored this "input voltage divider" RED).
The output of the mixer stage goes through a 220k resistor to the phase inverter, but it's also tied to ground through the 50k tremolo intensity pot. The 100k plate resistor of the mixer stage forms another voltage divider with the intensity pot, dropping the output about 3:1, bringing the net gain through the mixer stage down to 1.66, almost unity gain. (In the schematic, I've colored this "output voltage divider" GREEN).
What this amounts to is a little master volume overdrive circuit! The reverb mixer stage is running at 5 times the gain of the preamp stage, but the voltage divider at its output acts like a master volume pot to bring the level back down. It's not a lot of overdrive, not enough to really hear as distortion per se, but it's enough to add harmonic complexity, sparkle, and slight compression.
The 10pf cap bypassing the 3.3M resistor adds to this effect. It bypasses high frequencies above 3.1kHz, supplying a slight treble boost going into the mixer stage, so those extra highs also get distorted a bit.
As an added bonus, I realized I had a half a 12AX7 left over, so I wired it up like the reverb recovery stage in the Deluxe and hooked up an Alesis Nanoverb where the spring reverb would have been. (The reverb driver tube and transformer aren't needed to drive the Nanoverb). So now I even have nice digital reverb built in.
Here is the original Bandmaster schematic:
http://www.doolinguitars.com/article...bandmaster.jpg
And here is the hybrid "Bandeluxman" circuit I wired into the Bandmaster:
http://www.doolinguitars.com/article...andeluxman.jpg
And then it occurred to me that the major circuit difference between the two was that the Bandmaster didn't have reverb, and that the reverb mixer stage in the Deluxe is in the circuit path even when the reverb is off. I never use the vibrato in either amp, so I decided to rewire the Bandmaster's vibrato circuit as a reverb circuit like the Deluxe's.
I removed all the components of the vibrato circuit, including all the wires leading to the Speed and Intensity pots. Then I wired up the reverb mixer stage into the now empty eyelets in the circuit board, including a 50k fixed resistor in place of the tremolo intensity pot.
To make a long story short, it worked! All of a sudden this tired old Bandmaster had all the sparkle and grind that I love my Deluxe for, but with the power of 6L6's. OK, so what's going on here?
Refer to this AB763 Deluxe schematic for the following explanation:
http://www.doolinguitars.com/article...man/deluxe.jpg
I'm no engineer, so this is all speculation, but here's my theory... The reverb mixer stage is inserted between the "Vibrato" channel preamp and the phase inverter. The signal from the 2nd preamp stage goes through a 3.3M resistor (bypassed with a 10pf cap) to the grid of the mixer stage. The grid is also tied to the Reverb pot through a 470k resistor, and to a 220k resistor to ground. That 470k and 220k in parallel read about 150k. So, the 3.3M in series with that effective 150k resistance form a voltage divider, dropping the output of the preamp by about 22:1. But the mixer stage is half of a 12AX7, which has a gain of about 100. That means the signal has a net gain of about 5 in the mixer stage. (In the schematic, I've colored this "input voltage divider" RED).
The output of the mixer stage goes through a 220k resistor to the phase inverter, but it's also tied to ground through the 50k tremolo intensity pot. The 100k plate resistor of the mixer stage forms another voltage divider with the intensity pot, dropping the output about 3:1, bringing the net gain through the mixer stage down to 1.66, almost unity gain. (In the schematic, I've colored this "output voltage divider" GREEN).
What this amounts to is a little master volume overdrive circuit! The reverb mixer stage is running at 5 times the gain of the preamp stage, but the voltage divider at its output acts like a master volume pot to bring the level back down. It's not a lot of overdrive, not enough to really hear as distortion per se, but it's enough to add harmonic complexity, sparkle, and slight compression.
The 10pf cap bypassing the 3.3M resistor adds to this effect. It bypasses high frequencies above 3.1kHz, supplying a slight treble boost going into the mixer stage, so those extra highs also get distorted a bit.
As an added bonus, I realized I had a half a 12AX7 left over, so I wired it up like the reverb recovery stage in the Deluxe and hooked up an Alesis Nanoverb where the spring reverb would have been. (The reverb driver tube and transformer aren't needed to drive the Nanoverb). So now I even have nice digital reverb built in.
Here is the original Bandmaster schematic:
http://www.doolinguitars.com/article...bandmaster.jpg
And here is the hybrid "Bandeluxman" circuit I wired into the Bandmaster:
http://www.doolinguitars.com/article...andeluxman.jpg