Billm Audio

or, what does this mod actually do?

Almost everyone who has looked into the heart of a Blues Junior has noticed that extra half of a 12AX7 that’s just sitting there, doing nothing. There are lots of things it could be doing… a second input, a line input, another gain stage…. Or take a page from the venerable Fender Bassman, and use it as a cathode follower. The cathode follower is a non-gain stage that separates the preamplifier from the tone stack.

Why would you want to do that? The tone stack, with multiple paths to ground through capacitors and tone control potentiometers, places a rather heavy load on the previous preamp stage. The preamp stage is high impedance and does not drive a load all that well. The cathode follower has very low input capacitance and very high input impedance, so it is a very light load for the preamp and a better match impedance-to-impedance than preamp-into-tone stack. On the other side, it has very low output impedance and is a powerful current source, so it can drive the tone stack effortlessly.

So why wouldn’t you want to do this?

The biggest reason is that it doesn’t sound all that different. The theoretical bark is worse than the actual bite, and despite all the impedance mismatches, lots of signal gets through to the tone stack. So you have to question whether it’s worth the effort. On the other hand, nobody ever accused a Bassman or a Marshall (which copied the circuit) of sounding bad.

Mark Huss did this mod on the green board Blues Junior years ago. He has instructions and some sound clips on his site:

http://mhuss.com/BluesJr/index.html

At a customer’s request, I recently re-engineered and adapted the mod to the cream board. It’s not all that difficult. Here’s how, below. I’ve also re-engineered Huss’s mod for the rev. C and D cream board. Photos are at the end of this page.

This is the circuit we’re trying to create. I cut-and-pasted another tube section into the Blues Junior schematic. It’s laid out just like the schematic for the Bassman, and it’s technically a DC-coupled cathode follower. The half-tube connected by the colored lines is the unused half of V2, designated V2A. The only additional component, other than the wires, is the cathode resistor. Although it’s unmarked on the schematic, it’s 100K, just like the plate resistor of V1A. This may seem strange if you’re used to cathode resistors in the 1K-2K range, but the high-potential balance between the plate and cathode is essential to the cathode follower’s operation.

The voltage drop across the cathode resistor raises the cathode to a high voltage that gets modulated by whatever signal appears on the grid. Unlike amplification stages, the cathode follower does not change the phase of the signal. What goes in is what comes out.

V2B, by the way, is the tone stack recovery amplifier.

(Note: If you’ve changed the plate resistor to some other value, as with the high-voltage preamp, the new cathode resistor needs to be the same value.)

This is the stock circuit board. The first step is to clip both ends of this jumper, close to the board.

This jumper is very convenient for our purposes; it’s the path from the output of V1A to the tone stack. We’ll feed the cathode follower from one end and feed the output of the cathode follower to the tone stack with the other.

It’s essential to clip components when desoldering them on printed circuits because the leads are typically bent onto the trace on the back of the board. Trying to pry or wiggle the entire part loose while keeping the solder molten is an excellent way to tear the solder pad from the circuit board.

I’ve removed one of the stubs from the jumper and am about to remove the other. A solder sucker is the best way to do this because you don’t have to linger on the joint and it cleans out the stub and the excess solder.

This trace brings filtered high voltage from the “X” point of the power supply to the plates of V2 and V1, but only half of V2 is supplied–no sense in sending power to an unused portion of the tube. I’ve marked the best place to tap into the supply.

I used a #60 drill in a Dremel tool fitted with an inexpensive accessory chuck. The chucks are readily available by mail order and in home centers and hardware stores.

If you drill at low speed (I use a variable-speed foot controller), you have excellent control over the drill. If you start at high speed, it could skitter off over the surface of the circuit board.

We also need a place for the new 100K cathode resistor. I created a pair of holes, one that connects to the new wire that will go to the V2A cathode, one that connects to the diamond-patterned ground area on the circuit board.

Scrape away the green coating so you can make a good solder connection. A 1/2 watt resistor is a perfect fit between these two points.

The brown wire will go to V2A’s cathode; the yellow wire (green on the schematic above) goes to the grid.

But first I had to remove the ground connections between pins 1, 2, and 3 of V2. If a tube section is unused, good design practice is to short all of the elements to ground so there is no chance of parasitic oscillation or other electronic weirdness.

Always test with an ohmmeter to make sure there is no hidden thread of copper connecting the pins.

I’ve desoldered pin 1 here. There’s a nice pocket right next to the socket pin. When I desolder the other two, I’ll be able to poke the ends of the wires down along the pins and solder them securely.

All finished! The new 100K cathode resistor is under my thumb; sorry for not having a clearer view. The red wire carries the plate voltage, the yellow wire connects the plate signal from V1A to V2A, and the brown wire carries the signal from the cathode to the tone stack.

You can see how the cathode follower essentially replaced the jumper wire, with the addition of plate power and the 100K bias resistor.

Rev C and D green board cathode follower:

On the rev C and D green board, the layout isn’t nearly as convenient as that of the cream board. The first step is to tap into the trace from the plate of V1A to the grid of V2A, our cathode follower. Huss did this on the tube board; so did I. The yellow wire runs from pin 1 of V1A to pin 2 of V2A. Note the cut traces between pins 1, 2, and 3 of V2A. including the ground.

The trace where the red wire goes into the circuit board is power to the preamp. We tap into it and send it to pin 1, the plate of V2A. The trace just above it is ground. One end of the cathode resistor goes into the ground trace. The other end is in an open area of the board. holes for the brown wire and the black wire shown in the next picture are drilled next to the lower end of the cathode resistor and all three are connected together.

The black wire sends the signal developed on the cathode into the Blues Junior tone stack. Note that the trace between the black wire and the plate side of R10 is cut. Thus V1A receives power from R10, but the signal from V1A never reaches the tone stack. Instead, the yellow wire (above) carries it to the grid of V2A to drive the cathode follower.

One final thought:

The 12AX7 in V2 may not be the best choice for a cathode follower, since the 12AX7 is designed to be a high-gain voltage amplifier. It’s not designed to supply significant amounts of current. It also has high internal impedance, which is part of the problem we’re trying to solve in the preamp/tone stack interface.

The cathode follower, however, is a current amplifier, not a voltage amplifier. Other tubes in the 12A*7 family are better suited to deliver current, most notably the 12AT7 and the current-driving champ, the 12AU7. They have lower internal impedance and are designed to deliver significant amounts of power. Putting a 12AU7 or even a 12AT7 into V2, however, would reduce the gain of the tone stack recovery stage, exactly like turning your master volume down halfway or more. That’s a bad idea.

There’s one tube, however, that is ideally suited for V2 in the Blues Junior: the 12DW7, also called an ECC832. It’s a hybrid tube; one half is that of a 12AX7 and the other is a 12AU7. It’s specifically designed for tasks that require an amplification stage and a driver stage. Fortunately, the triode on pins 6, 7 and 8 is the high-gain 12AX7 side, a perfect match for V2B. The section connecting to pins 1, 2 and 3 is the 12AU7 side, perfect for a cathode follower.

What’s the Difference?

So does the cathode follower actually sound any different? My customer was looking for more touch-responsiveness and feels that cathode follower tone stacks deliver this. I couldn’t say for sure, but I felt that the follower gives the amp an additional measure of clarity, especially in the bass. I haven’t done an analysis of the tone stack’s impedance, but it stands to reason that the modded tone stack, with the increased bass and mids, loads the preamp more on the low end. As a result, you get unequal amounts of distortion on the low strings vs. the high strings. Remove the load on the preamp, and everything evens out. This is not an easy mod to switch on and off, so I couldn’t do A/B comparisons. But I’ve heard enough Blues Juniors that the tones are pretty much engraved into my brain. This mod slightly increases clean headroom; the amp takes pedals better.

Swapping the 12AX7 for a 12DW7 (actually a JJ ECC83 and a JJ ECC832) did make a small additional improvement, but it may just have been the small, but audible difference between the two 12AX7 stages in V2B, the tone stack recovery stage. Then again, the internal capacitance of the 12AU7 stage and the 12AX7 stage is different enough that it could also affect tone.

My conclusion: If you make the effort to do this mod, you should try a 12DW7 or ECC832 to see if you get additional benefit.