Ask Amp Man: Expanding the Tone-Shaping Power of a 20-Watt Marshall Clone
Can you add separate bass and treble controls to a Marshall-style amp channel with a single tone pot?
I recently purchased a handwired clone of a Marshall 2061 JMP and I love it. But is there any way to add a bass control or maybe a bass-boost switch? I want the bottom end without losing clarity. The amp currently has high and low inputs for both the lead and rhythm channels. The head was a kit designed by Soultone Amps.
Okay, Ask Amp Man readers, it turns out Bobby is local to me. Because I thought this might be a cool project, I had him drop off the amp. It has two channels, lead and bass, each with a single volume and tone control. We discussed what Bobby wanted, and it turned out that the single tone control on each channel weren’t cutting it for him—he needed more control over both the treble and bass. He already bridges the channels with a jumper (we used to call this “double jacking”).
I decided the best action would be to retain an individual volume control for each channel, and then install global bass and treble controls for overall tone shaping. Here’s a description of that mod, along with a couple of additional changes to enhance the amp’s performance.
First, I removed the original control set from the amp (Photo 1), leaving it connected as an assembly (Photo 2). This makes it easy for any future owner to return the amp to its stock configuration.
Next, I installed new volume pots in the front panel, placing them between the two sets of input jacks, each next to its respective channel. The other two locations can now become individual treble and bass controls.
Wiring up the volume controls is simple: The CW leg connects to the output capacitor for the individual channel outputs of the first preamp tube. The CCW leg goes to ground, and the wiper connects to the respective 470k mixing resistor.
Now for the tone controls: I didn’t want to add to the existing circuit board, so I simply built the tone stack by attaching the components directly to the controls (Photo 3). This, by the way, is the true definition of “point-to-point” wiring. I chose the tone stack values traditionally found in older Marshall amps: a 470 (or 500) pF treble capacitor, 0.022 ?F midrange and bass capacitors, and a 56k dividing (slope) resistor. The input to the tone stack comes from the junction of the two 470k channel mixing resistors. (The one for the lead channel has a bright cap in parallel). I disconnected the original 220k grounding resistor from this point, since it’s no longer needed, thanks to the new tone stack.
The tone stack’s output (the treble pot’s wiper) is connected to the input (grid) of the phase inverter. Since there’s no midrange control, I chose a value of 10k to approximate a mid-position control setting. This value could be anywhere between zero ohms (jumper) and 25k—or even higher, depending on your personal midrange preference.
That completed the new control set, but I wanted to make a couple of other changes as well. This particular amp design uses only two preamp tubes. One is dedicated to the phase inverter, so there really isn’t much gain in the preamp stage. Most of the amp’s drive characteristics are generated by pushing the output stage into distortion—which is great, since it’s a 20-watt amp with two EL84 output tubes. Since we added a traditional tone stack to the circuit, putting additional load on the signal path, I compensated by increasing the gain in the phase inverter.
I did this by decreasing the value of the cathode resistor of the phase inverter tube. The original value was 8.2k, but I lowered this to a 2.7k (Photo 4), noticeably increasing the gain. The other change I incorporated was to parallel the bass channel’s 220k mixing resistor with another 220k resistor to better match the level of the channels. (Photo 5). (I simply repurposed the disconnected grounding resistor.)
There you have it: a cool Marshall-style 2061 amp with full tone control. Now it’s a more versatile low-power gem. Enjoy!
Warning: All tube amplifiers contain lethal voltages. The most dangerous voltages are stored in electrolytic capacitors, even after the amp has been unplugged from the wall. Before you touch anything inside the amp chassis, it’s imperative that these capacitors are discharged. If you are unsure of this procedure, consult your local amp tech.