Interesting experience with Hiwatt phase inverters

[cdemike]

I built an SAP clone a few months ago, and I grew pretty frustrated with it after the honeymoon phase ended. It felt very stiff, and I couldn't find an appropriate volume setting where single note lines would sustain while the chords weren't too distorted. In other words, either I'd have a semi-clean rhythm sound that I liked but anemic sounding leads, or over-the-top rhythm and leads that sustained. I also found there was an unpleasant quality to the overdrive with a fatiguing fuzzy quality when in the upper ends of overdrive. To me the amp sounded best with the master volume somewhere between 12:00 and 3:00: lower settings with similar overdrive levels sounded gutless, and higher settings worsened the fuzzy quality. So I figured the issue lie downstream from the master volume.

So I decided to see if I could smooth the amp out by changing the phase inverter circuit from the later style where the PI cathode follower passes no signal to the earlier style where it does pass signal. I originally used a PI circuit similar to the "Jimmy Page" model with the 1.8M and 470K resistors and with it drawing power from its own B+ node rather than sharing it with V1. The NFB network was the same as other later style circuits (not sure if the Page schematic has the NFB circuit partly missing or if it's just an oddball). In any event, you could describe it as being the same as the 80s SAP schematic with different power supply values or like the Page schematic with the more common-to-Hiwatt presence control circuit. The amp was built by adapting a Mojotone kit, so I used their supplied schematic for the new version, which is the same as the early 70s linked input version on Mark Huss' site.

I'm pleasantly surprised how much of a difference this made. The amp is much more approachable with more of the compression I was looking for to match leads to rhythm, and the overdrive lost the fuzziness. Wanted to share the experience since I found it surprisingly difficult to find more information about how the different PI circuits affect the sound. One of the only sources I could find describing the comparative impact on sound was from a Jimmy Page clone amp that was documented on the Marshall forum (post #71 on https://www.marshallforum.com/threads/% ... 713/page-4), and interestingly the builder, Shakti, preferred the later style circuit.

[beasleybodyshop]

I built a clone of the Jimmy Page SAP amp a few years ago, and had the same experience you did.

I ended up rebuilding that amp to early 70s spec preamp with the same PI you ended up with and was much happier.

[cdemike]

That's interesting! I actually haven't played an amp with the regular-spec Hiwatt preamp without the cathode follower first stage. What did you read as the difference? After running LTSpice sims ad nauseum and messing around with boosts into the front end of my amp, I don't think the CF is contributing much if any clipping (maybe and only maybe some light compression, but I'm missing the reference point without knowing how a non-CF preamp feels).

The unused triode at V2 drives me crazy, so I've been considering adding a regular-spec preamp as a mod. Due to the severe output impedance mismatch, I think the best approach would be to add tube with both sides of the the regular-spec preamp and run that into the unused V2 triode. In other words, just use the unused side of V2 as a plate mixer. What was it that led you to prefer the regular-spec preamp?