A few observations after spending some time tweaking this thing:
- When people talk about Hiwatt circuits having “chime,” they aren’t kidding around. It’s kind of a stupid term, but it’s hard to think of a better one — it just means that the amp is really bright, a little like a Fender, but the highs are never shrill or harsh, they sound natural and almost acoustic. When the circuit is completely clean, it’s the ultimate jangle tone. The normal channel will get you there with single-coils, the bright channel will do it for humbuckers.
- For a while, I was using either the normal channel or the bright channel, and thought both were great and really usable in different ways. But when I started using both of them at the same time, my mind just got blown. I used the two outputs of a stereo delay pedal to send the same signal to both channel inputs, which allows you to crank one channel into distortion while you add in some clean of the other channel, combining the voicings in different ways. It’s like having a third, and fourth, and fifth channel in the amp… really really fun.
- A lot of the tone-shaping is “active” instead of passive. For example, the normal channel is unaltered until it hits the tone stack, but the bright channel has a bass roll-off of 6dB per octave below 350Hz (done by a coupling cap), followed by a 6dB boost above 4kHz on the next gain stage. Likewise with the presence circuit, which is most “neutral” at midpoint on the knob. Below that, it’s actually loading down the V3A input to the power amp, bleeding off high frequencies, and above, it’s bypassing the phase-inverter cathode resistor to actively boost high frequencies there. The V3B triode is even set up as a dedicated low-impedance voltage source (as a cathode follower) to stiffen the bias for the phase inverter. No signal flows through it, it just keeps a solid voltage on the phase inverter’s inputs. The PI breaks up later as a result, maintaining clean headroom.
- The tone stack doesn’t even begin to approach linear, with any combination of settings. It’s very much a part of the “Hiwatt sound.”
- I can hear a much more significant difference between different types of preamp tubes in this amp than in other amps. I’ve long held that when played clean, any small contribution by tubes to an amp’s sound is largely swamped by the amp’s circuit design, and inaudible. That’s still mostly the case here. But when driven into overdrive, there’s a consistently identifiable difference between different types of tubes. The old Chinese 12AX7s that I threw in the amp for testing purposes were pretty gnarly with the gain up. There are JJs in there now that are squishier and less crackly when driven hard. I’ll probably try a 5751 or ECC803s in a few positions, and a 12AT7 in the phase inverter to see what they’re like.
- The difference between power tubes is negligible, as expected, unless the amp is driven to the point where they break up (insanely ear-bleedingly loud). At house volumes, I can’t hear any difference between different power tubes. At breakup volume, the Chinese KT66s have a nice 60s-style sound. The JJ KT66s, which I’m fairly certain are not actual KT66s, sound great in this amp (and every other amp I’ve tried them in) and break up later and more gracefully, maintaining more bass response. They’re more like a 6L6GC but with higher transconductance (i.e. they’re louder at the same level of input drive) and lower max dissipation… a good compromise between 6L6 and EL34. EL34s in this amp sound just like you’d expect them to: squishy, squawky, and LOUD LOUD LOUD (true pentode high transconductance!). I have a feeling it will end up with JJ E34Ls in the long run if I use the dirty sounds more, and JJ KT66s if I use the clean sounds more.
- Using a tube rectifier makes a pretty significant difference when the amp is turned up. It requires a rebias to switch from diodes to a GZ34, and the tube rectifier adds enough sag at high volumes that the power tube breakup gets a lot more compressed and gooey. The bass response is attenuated. It’s a sweeter sound, definitely, and more Marshall than Hiwatt, which can be a cool thing.
- This amp is really stupidly loud for a 50-watter. The post-phase-inverter MV is great.
This prototype took about a month to build. It’s a 50W single-channel head based on the Hiwatt DR504, but with some added flexibility. I used Weber’s 6H50 chassis/case and some parts, to speed up build time, but not their schematic.
The preamp is a part-for-part clone of the late 60’s DR504, which I was really impressed with after using one on the most recent Rosetta record. I liked Ted Weber’s addition of the post phase-inverter master volume, so I included that.
The power amp and power supply are quite adjustable, depending on your taste — you can use KT66s (which I am now) or EL34s, just rebias. Grid stoppers are Hiwatt spec (22k), not Marshall. I’m also using a solid-state rectifier of exactly the same specs as the DR504 (two diodes on a center-tapped HT secondary), but it’s mounted in a tube socket, so you can swap in a GZ34 or equivalent simulator to give more squish. Using a GZ34 with KT66s and biasing hot, you can get a JTM-45 power amp attached to a Hiwatt preamp. Killer!
This amp has a better grounding scheme than Hiwatt used, and is considerably less noisy than the 1968 DR504 I modeled it on. It uses a rigorously-implemented multi-star grounding scheme, with the pre stages on a star, the phase inverter/NFB/presence on a star, and the power amp and supply on a main star connected to the chassis. It’s so quiet you can’t tell whether the amp is on standby or not unless you look at the switch or play through it. The bias circuit uses 1%-tolerance components so that the amp biases easily and precisely. Weber’s TRS jack bias monitoring system is excellent and this chassis was drilled for it, so I used it.
The layout obviously is nothing like the DR504, but I had a hunch from the beginning that that wasn’t really as important as many purists claim. I’ve learned over time that component values and the actual circuit are what matters most. Lead dress does matter from a noise/oscillation/crosstalk perspective, but gross tonal problems caused by wiring, without associated functional failures, aren’t very common. It seemed more important to try to replicate the discipline of the original Hiwatt layout than the layout itself. That turns out to have been the right philosophy… this is not exactly the same as the Hiwatt I used to record ADOM, but it’s got everything that amp’s got and a quite a bit of extra latitude for tweaking. It’s surprising that it has as much clean headroom as it does, given how high-gain the first preamp stage is. And balls! It will punch you in the gut… way more bass power than the Marshall circuits of the era.