The purpose of the article about changing the anode resistor value was to improve the clean channel and smooth the dirty channels. That is exactly what it will do, but I wanted to talk about conceptual ideas regarding this change. I’m going to use pedals for illustration and then translate that over to tube amplifiers.

Analogy

I used to build guitar pedals and I’ve spent a lot of time cascading circuits into each other. While transistors and op amps react differently than tubes, the overall result on dynamics is similar from one component to another.

A basic idea with multistage distortion devices is determining the amount of gain and loss to use for each stage. A Big Muff has a lot of distortion throughout three transistor stages and uses symmetrical diodes in a feedback loop at each stage to further distort the sound. The Sustain control is the same as a Distortion/Gain control for another device and is after the first stage. As the Sustain is increased, the sound gets “fatter” and is quite bold, but looses definition as it rips the signal up.

Many distortion/overdrive devices which use diodes to clip the signal will usually place them near the end of the amplification, or as part of a single op amp stage (Tube Screamer and its derivatives). This approach produces a smoother, less bold distortion by comparison to overloading from the front. The sound gets compressed much later. In tandem with controlling the lows with filters, these types of distortion devices have less thump to them and remain a little bit clearer.

An Experiment

In interesting experiment, which produced my favorite homemade pedal, involves running two op amps into each other. The first is followed by hard clipping LEDs and requires a decent signal gain to create distortion. The second has diodes in the feedback loop. Use pots on each stage and experiment with varying the parts and see how changing the levels changes the sound. It provides an interesting chance to judge the merits of differing configurations.

Back To Amps

If a person reduces the anode value of stage 1a in a Dual Rectifier, the sound will be less bold. Part of the way the stock amp reacts is to immediately produce a very powerful signal and send it down the line. Reducing this signal will make the following stages react less to the same positioning of the Gain control. This is great for Clean and also works well for the dirt modes.

Codependent Stages and Cold Clipping

When I look at the Dual Rectifier I see 4 main groupings for making changes to distortion: 1) Stage 1; 2) Stage 2 and 3; 3) Stage 4 and 5; and 4) the power amp. Stage 1 is configured like a clean boost. Stages 2 and 3 work together as an overdrive run into a distortion generator (which eviscerates half of the signal) . Stages 4 and 5 are another powerful, bold, boost and distortion which is driving a cathode follower (which clips and compresses the previous evisceration in a smoother way). Each section pushes the next and each grouping works together in this chain.

Many amplifiers, and the Dual Rectifier in particular, create a lot of level gain in the front to push the distortion being generated later in the chain. The cold clipping stage (V2b) depends on this early gain to be driven hard enough to create the grind for which the Recto is famous. If a person reduces the resistance value for V1a, V2b will not be driven as hard and will be less compressed. This is one of many solutions for creating conditions to have more room on the Gain control before massive clipping occurs at the V2b.

Another solution is to change V2b’s cathode to a lower resistance value. This mod would be for the person who doesn’t just want to have more variety on the gain dial. It fundamentally changes the distortion characteristics of the entire preamp by affecting harmonic content being generated during clipping. If this route is taken, it might be necessary to experiment with changing the amount of gain being dumped to ground between stages to find a sweet spot where a high Gain control setting will drive the cold clipping into massive distortion.

Potential Dividers

As just pointed out, the potential dividers upstream from V2b are perfect places to make adjustments to modifications. If the a person wants to keep their Clean sound unchanged, the Channel 2/3 voicing circuit, the Channel 2/3 Gain control values, and the grid resistor into V2b are the places to consider.

An Experiment

For the person who wants to ditch their boost pedal to tighten an amp: decrease lows from the voicing circuit, increase the Gain control to 300k or 500k, slightly increase the V2b grid resistor (or decrease the V2a load resistor it connects to), and change the cathode value of V2b to a value between 22k and no less than 10k. By shaving some low frequencies, creating less dumping in two places, and decreasing the harmonic content generated by V2b, the sound will be smoother and can still be driven into massive distortion if care is taken to experiment with subtle changes.

Decreasing the low frequency content to a roll off around 150 Hz – 180 Hz in the voicing circuit will make the Gain control brighter and firmer. Increasing the Gain control value also makes the sound brighter and decreases the amount of signal thrown to ground by the potential divider created by the voicing circuit and the Gain pot, giving a great increase in signal. These changes are much like using a boost. One Gain pot could be kept stock to retain an unboosted sound on that channel.

Changing the potential divider after V2a is meant to increase the drive into V2b. Reducing the V2b cathode resistor value will change the clipping characteristics, but also gives a slight gain increase to the stage. The combined result would be a distortion generator with more sustain, less grind, and more balance between the harmonics. V2b will now drive V3 harder.

If a person finds V3 to be driven too hard, increase the V2b load resistor from 330k to a greater value (470k or 680k would be my recommendation). An effect of a relatively low value load resistor is that it loads down V2b and diminishes the gain by a very small amount. Increasing it slightly will make the distortion more prominent, but reduces the overall level being passed to V3a. The previous increases to level in two places and the small gain increase at V2b will offset losses at this point.

Advice

I personally would incorporate these final ideas into a new design. I think a person should have the right amp for their needs. If the Recto isn’t working for a person, it seems to me that another amp could get them closer to their goal.

If a person has been through several amps and gets really close to where they want to be with a Recto, a subtle change could be a good idea, but I don’t advocate radical changes to any amplifier of such cost and quality.