5W Head to Preamp

I've been wondering what to do with this Kustom Defender 5H that was my introduction to tube amp building, as I don't really have much use for it lately. Somehow the idea popped into my head to use it as a bass preamp.

At first I tried the Garnet Herzog method - use the existing OT, hook up a dummy resistor, and tap the voltage off of it for the line out. It works okay, but the bandwidth is restricted/shaped on the OT. Not terribly useful for bass, as the stock Kustom OT is weak sauce. This worked fairly well for guitar though, and was simple.

Then I considered wiring up the EL84 as a cathode follower, but the required heater elevation and loss of a gain stage steered me away from that. If I had a couple more tubes, it might be worth it.

Eventually I settled on this:

The real key here is that Triad line matching transformer. It boasts 20Hz-20kHz response and only costs $6. I used a 5k pot to keep the output impedance low, and so I could balance the dropping resistor. The EL84, like any pentode, has an insane ra so the Zo is dominated by the Ra, in this case 2k5. The 220k resistor ensures that the pentode isn't excessively loaded and lowers the output voltage to something more reasonable for whatever power amp this gets connected to. After a pair of 12AX7 gain stages and an EL84, the output voltage could feasibly have been in the 200VAC range.

Oh yeah, and the load and cathode biasing resistor were picked off the load line. My Kustom is running around 350V B+ so I just roughly plotted the steepest line I could without nudging the max dissipation curve and guesstimated the plate and screen currents to pick a cathode resistor. It idles around 9W but clipping is symmetrical so there's no particular benefit to biasing it any warmer. Wattages could've been picked more carefully, but components were grabbed out of the parts bins.

Rk is bypassed; wish I hadn't missed that.

Capacitors are obviously non-ideal too, but this is just to publish the general idea.

As a bass preamp it sounds good, but it still needs a lot of tweaking and voicing before I'll post the schematic. The Triad transformer is good to its word and passes even a 5-string bass without attenuation. Plenty of bass and none of the shrill treble typical post-power amp line outs suffer from.

Also the Triad transformer can be wired for balanced operation, of course. The only other note I have is that this transformer is incredibly sensitive to radiated fields, so keep it away from the PT.

Clips and full schematic once I've got it really dialed in. I've got a Matamp-style preamp in there now and it's nice and versatile but not fantastic with heavy overdrive. Probably understandable considering there are only three gain stages.

I'll probably wind up dropping the screen voltage, but the headroom is so low on the EL84 already. Hmm.