Bluesmaker tone stack

The original 1965 Bandmaster AB763 employed 4 preamp tubes and 2 6L6GC output tubes.  The total filament current draw was 3.0 Amps.  The 125P7D power transformer was rated for 3.15 Amps.  This is typical Fender engineering, where the transformers were sized no larger than they absolutely needed to be.  With the Bandmaster converted to a Bluesmaker, we now have 5 preamp tubes and 2 EL34 output tubes (which draw 1.6X more filament current than 6L6s).  The total Bluesmaker filament current is 4.7 Amps.  This is 50% higher than the rating of the power transformer.  The good news is that the Fender (Schumacher) transformers are known for being very permissive of this kind of abuse, and should be able to run like this for many years, albeit probably much warmer than usual.

Signal path of the #1 High Gain channel.

McIntyre Bluesmaker Bandmaster

John McIntyre deviated from the Bluesmaker circuit in several important ways with this amp.  While it still incorporates the trademark parallel gain stage with "Tone Blend" pot, it also includes EL34 power tubes, reverb, a master volume with bypass switch, a Pentode/Triode switch, and perhaps most interesting, two Very Low Gain inputs taking the place of the Vibrato channel inputs, which on the original Bluesmaker were simply disconnected.

This is interesting because this was about the time that John McIntyre was developing his Bluesmaker II stereo guitar amps for studio work.  These VLG (Very Low Gain) amps would eventually become the Lexicon.

I suspect that this amp may be a very early application of John McIntyre's low-power amp design.

I have endeavored to reverse engineer this amp in order to document its unique features and provide an accurate schematic.  (for posterity, if not just out of curiosity).

Don Hayward

1995 McIntyre Bluesmaker
Bluesmaker McIntyre Tone & Gain

Signal path of the #2 Very Low Gain channel.

This McIntyre Bluesmaker (Bandmaster/Cameron) uses 10 triodes (five 12AX7 preamp tubes), compared to the original Fender Bandmaster, which used 4.  The Marshall JTM-45 has 4 preamp triodes, as does the Fender Bassman 5F6-A that it emulates.  The Fender Deluxe Reverb employs 8 triodes, of which 3 are dedicated to the reverb circuit.  In this Bluesmaker, McIntyre uses only 2 triodes (Driver & Recovery) for the entire reverb circuit, leaving 8 triodes dedicated to gain (two triodes form the phase inverter, as with the other examples, but are still gain stages). 

(The cathode follower driving the tone stack of the Bluesmaster is not a true gain stage – in fact it attenuates the signal somewhat, but that is also true of the JTM-45 and the 5F6-A.  The Bandmaster and the Deluxe Reverb do not use a cathode follower to drive the tone stack, instead the first triode signal of those amps leads to the tone stack).

The first gain stage, V1, is John McIntyre’s signature parallel-triode arrangement, where each triode of V1 is voiced very differently and can blended by the “Preamp Balance” pot.  The tonal variation is dramatic:  the first triode of V1 is very Fender-like – clean and bright, with lots of headroom.  The second triode is high gain and reminiscent of the Marshall high-gain amp circuit – crunchy and thick, with pleasing full distortion.  You can readily see the difference in the following plots of the two circuits from the calculator:

Here is the control layout of this Bluesmaker amp.  The high gain channel incorporates a Deep switch, Preamp Balance, a Mid control with push/pull Boost, and a Presence control with push/pull Response Shift.  These were the features of the original Bluesmaker as described in the Guitar Player article of 1993.

The unique features of this particular Bluesmaker are:  EL34 power tubes,  Reverb control, Master Volume with push/pull Bypass, Pentode/Triode switch (on the back of the chassis), and the Very Low Gain channels, each of which is voiced differently.  (These are in addition to the "normal" Bluesmaster modifications).

Only the Channel 1 Volume and the Channel 2 Treble and Bass controls and Bright switch  retain their original function as on the donor 1965 Fender Bandmaster.

McIntyre Bluesmaker Schematic
Bluesmaker Tone Stack
1965 Fender Bandmaster

The output transformer is a Stancor A-3801, something of a legend in itself - I have seen these NOS on Reverb selling for $500, although I've seen used ones (rarely) on eBay go for $50.  The A-3801 was used in the Train Wreck Express amp, and is a very highly regarded 35W output transformer, no longer made.

Very Low Gain Channel Input #2
The VLG Input #2 is significantly louder than Input #1, since it does not have the reverb circuit bogging it down.  The guitar input signal has a 100K grid stopper, and then lands on the grid of V4a.  The V4a triode has a 100K plate resistor and a 1.2k cathode resistor bypassed by a 22 µF capacitor.  It is a high gain stage (~61 gain) with wide response (see the V4a frequency plot, below).  The signal passes to the Master Volume and then to the Phase Inverter and the output tubes and output transformer.  There is no reverb, no tone control,  just the Master Volume.  The Presence control still functions as expected, along with the Presence Response switch.

Bluesmaker High Gain V4a
McIntyre Bluesmaker Schematic/Cameron
Bandmaster McIntyre Bluesmaker controls
1995 McIntyre Bluesmaker

The schematic of this unique John McIntyre Bluesmaker amplifier.  There are 5 preamp tubes, all 12AX7, and the high gain channel uses all of them.  Component deviations from the original Bluesmaker are shown in red.  Measured voltages are shown in blue.

This is Don Cameron's 1965 Fender Bandmaster.

The chassis serial number has been ground off, (hmmm...) but I figure it is a 1965 Bandmaster, probably late 1965, because it is an FMIC amp, after Leo sold the company in January of 1965.

This amp has been modified to the "Bluesmaker" circuit designed by John McIntyre, who has been modifying and building guitar amplifiers for years. His articles in Guitar Player magazine on amplification mods have set standards in tone excellence.   One such article in GP Feb. 1993, was titled "Turn a Humble Fender into the Ultimate Blues Machine" ;  this is the origin of the Bluesmaker.

Don Cameron sent his Bandmaster to John McIntyre in Calgary to be modified in 1995.

The inscription, signed by John McIntyre,  reads:

 For Don



1995 McIntyre Bluesmaker

1965 Fender Bandmaster / McIntyre Bluesmaker

Signal path of the #1 Very Low Gain channel (with reverb).

Very Low Gain Channel Input #1
The VLG Input #1 leads directly through the 220K grid stopper and a 470 pF cap to the grid of V3b, the reverb driver.  From there the signal path is identical to the high-gain channel, above.  This input has only one true gain stage, V4a.  The Volume control has no effect on the signal, and the signal does not go through the tone stack, so tone controls do nothing, and neither does the High Mid Boost switch.  The Master Volume gets very noisy above about 3 (hiss).  The MV bypass works as intended.  The Presence control still functions as expected, along with the Presence Response switch.

Bluesmaker McIntyre Tone & Gain

The tone stack is tweaked a bit here with the slope resistor of 47K (Fender used 100K), the load resistance of 2.47M (Fender typically 1M), and the source load of 100K. The net result of these changes is to boost the Bass between 10Hz and 100Hz.  The upper plot is with the Treble, Bass, and Mid controls all set midway, at 5.  Typical of this AB763-derived tone stack, turning the Bass and Mid pots all the way down deletes all except the high  frequencies (see lower plot above).  Turning all the pots "off" removes the guitar signal, essentially turning the volume off.

The unique McIntyre High Mid switch acts as a low-pass filter connected to a high-pass filter.  

The signal leaves the tone stack via the Treble pot wiper, goes through the 470K load resistor and the 470pF coupling cap, and lands on the grid of V3b, the reverb driver.  The V3b plate signal ties to the NTE- 4L miniature transformer, which connects to the Reverb pan, which is an Accutronics 8EB2C1B.  The output from the reverb pan goes to the grid of V4b, reverb recovery, and from there leaves the V4b plate to connect to the Reverb control pot through a 500 pF coupling cap.  The signal from the Reverb pot connects to the grid of V4a, a gain stage, and leaves the V4a plate through a .022 µF coupling cap to the Master Volume pot.  The Master Volume pot can be by-passed with the DPDT switch.

The signal goes to another .022 µF cap and then to the grid of V5b.  V5 is the Phase Inverter, more or less a copy of the Fender Bassman 5F6-A long-tailed pair phase inverter.  In the standard Fender (and others) circuit, the plate resistors are 100K (common grid) and 82K (common cathode), which act to balance the two halves of the phase inverter.  In this Bluesmaker, both plate resistors are 100K, probably to introduce additional distortion into the circuit.

The signal from the plates of the V5 phase inverter go to the grids of the two EL34 power tubes. The common cathode side has a 4.7K resistor (marked with a star) in series with the grid, presumably to un-balance the output stage and add even harmonics. The El34s have three 1N4007 diodes in series between the plates and the cathodes for fault protection (flyback).  This acts to prevent damage if the amp is operated without speakers connected.  And there are also 1N4007 diodes in parallel with the 1Ω cathode bias resistors, presumably as fault protection, so that the diode pulls the fault current and trips the fuse, saving the resistor.

The El34s are wired to a DPDT switch so they can run in either Pentode or Triode mode.  The amplifier could use 6L6GC tubes in place of the EL34s, although a re-bias might be required.

The EL34 outputs connect to the Stancor A-3801 output transformer, the secondary of which connects to the speaker output jack.

The circuit utilizes Negative Feedback (not highlighted in the signal path drawing), and the Presence control works as intended, as does the Presence Response (shift) switch.

A simplified way of describing the performance of the amp's High-Gain channel is this:  with the blend pot all the way CCW, you have a very Fender - like clean tone;  with the pot all the way CW, you have a very Marshall - like crunchy distortion.  In between these setting, you get a blend of both tones.   The Bluesmaster endeavors to be the penultimate guitar amplifier - A Fender and a Marshall in one amp.

The VLG channels (Bandmaster Tremolo inputs) are very low gain and well-behaved.   The #1 input has reverb, and no tone stack.  The #2 input has no reverb, no tone stack, and has higher gain than input #1, because it does not lose gain to  the reverb circuit.

The Pentode/Triode switch works well as a Half-Power switch. 

The original John McIntyre Bluesmaker schematic.

Bluesmaker Signal Path
The guitar signal enters the #1 (high) input, through 34K of the parallel grid stoppers, and connects to grids of both V1a and V1b, the McIntyre parallel gain stages.  V1a cathode resistor is 2.1K (2.7K on the Bluesmaker original schematic) and the cathode capacitor is 0.1 µF. The plate voltage is 340 V, the plate resistor is 47K.  This is the Fender-tone preamp.

V1b cathode resistor is 1.2K, and the cathode capacitor is 2.2 µF.  The capacitor is removed from the circuit by the “Deep” switch, which bypasses a 15K resistor in series with the cathode capacitor.  This 15K resistor stops the switch from “popping” by allowing the capacitor to charge when the switch is open.  Plate voltage is 340 V, and the plate resistor is 220K.  This is the Marshall-tone preamp.

The V1a signal from the plate uses a .033 µF coupling capacitor to connect to the left side of the 500K Preamp Balance pot.  The V1b signal has a .005 µF coupling cap and goes to the right side of the pot.

The blended signal leaves the wiper of the Balance pot and goes to the Volume pot, which has the standard 500pF Bright switch. 

From the Volume pot wiper the signal connects to the grid of V2a, a voltage amplifier, and the signal from the plate goes through a large 2M (1M on original Bluesmaker) stopper to the grid of V2b;  this stage also has a 220K grid leak resistor.  The big grid stopper is probably to prevent blocking distortion on this high-gain stage.  The V2b signal from the plate ties directly to the grid of V3a, which is a cathode follower driving the tone stack.  The tone stack is a copy of the Fender tone stack, with 250 pF, 0.1 µF, and .047 µF tone caps for Treble, Bass, and Mid.  This provides the classic Fender “Mid-Scoop” seen in this plot from the Duncan Tone Stack Calculator: