This is a follow-on to an old post on my personal blog at Anchor States.  It’s not so much that I’ve “improved” the mods I did before, as much as that my needs have changed and I’ve asked the Valve Juniors to do different things.  These changes reflect that.

I found that I had very few opportunities to run the amps wide open and actually use the really liquid distortion I got out of them. (The exceptions were on some Rosetta recordings, i.e. Temet Nosce and Blue Day for Croatoa, where they were fantastic.) I wanted a cleaner, more linear sound from the preamp, allowing me to get louder and bring out more of the power tube sound on its own. In the old configuration, the amp didn’t play well with pedals once it started to break up. Delay, tremolo, even OD and boost pedals would just get muddy and eventually disappear in the amp’s own raw drive sound. The amp had one good sound — a great sound, really, but still just one sound.

The update center-biases both preamp triodes for the most clean headroom in the preamp. I originally tried just center-biasing the first triode and keeping the second hot, but this just made the amp fuzzy at the transition into preamp overdrive. Keeping both triodes centered brings more drive out of the power tube, at a higher volume. Moreover, since the preamp is staying cleaner, I was able to fully bypass the first stage with a 47uF cap, making it full range. More bass frequencies coming through makes the clean sound on my baritone guitars much more usable. The second triode has the option of 1.) no bypassing, which cuts the gain in half and maintains full-range output, useful for clean, 2.) a Hiwatt-style “brilliance boost” with a 47nF bypass cap, making the old VR1 treble bleed unnecessary, and 3.) a Marshall style mid-high boost with a 1uF cap, useful for brash overdrive tones. R1 has also been moved to the input side of R2 so that it doesn’t form a voltage divider on the input.

In the power amp, I’ve done some loadline calculations and moved the cathode resistor up to 300 ohms, which with my voltages gets me to about exactly 90% idle dissipation. (My plate voltage is 322V, even with a 1k resistor at R10, which could be related to the much more efficient Hammond OT and its lower-resistance primary.  My screens sit at 300V, so with a 300-ohm cathode resistor, I get a -10.4V bias voltage and 34.7mA of idle current, for 10.8W dissipation. I have had no problem with tube life at these voltages.) The bias was too hot before, even moving up to 250-ohm from 220-ohm. Now the power amp has slightly more clean headroom, but still breaks up beautifully. I’ve also added a pentode/triode switch to get a spongey, darker, lower-volume overdrive from the power amp at 3 watts. The switch has a 100-ohm resistor in series just to protect the screen grid. I’ve also removed the grid-stopper switch on the black amp, since I never used it. It’s now the V1B bypass switch on that amp.

The end result is that the amp has more usable sounds. It has a really good clean now, which can get fairly loud if needed, or with singing power tube breakup. It does single-coils or humbuckers equally well. With the bypassing options I can also get a Marshall-y crunch sound with the volume cranked. Boosting the amp with a Tubescreamer actually sounds good now. Analog delay and chorus sounds are particularly improved, even as the amp starts to break up.

In the schematic below, red are my previous changes, blue are the new changes.