What's the mojo to the Hiwatt design?

[Bear]

Not questioning the mojo, just trying to figure out where it lives. Circuit-wise, that is, not so much interested in discussing the transformers at the moment.

The TMB stack is apparently essential. Anything else to replicate or at least to factor in as a contributing element to approximate in some way or another?

[Jana]

The phase inverter.

[selloutrr]

The hiwatt has a monsterious sound. Extreme head room and if given a place you can crank it to distortion the break up is very musical and rich. The biggest draw to the amp is the quality of the build mil spec point to point it truly is art. Tonally the original British tubes, the purity of material and size of the partridge transformers, and the mustard caps. All play a large part in the hiwatt sound. Much like a tw ever part plays a key roll but no clone will come close to a 103 with out those ingredients.

[txbluesboy]

Like Jana said, the mojo in a Hiwatt comes from the fixed bias phase inverter. No other amp I have ever seen uses this setup. It gives it its distinctive clean sound.

[M Fowler]

Exactly as Jana noted,

the amp has 1/2 of the third 12AX7 is used as a voltage regulator to provide fixed bias to the PI to keep the signal optimally balanced, minizing distortion, and giving a clean sound. The 4th 12AX7 is the PI.

[selloutrr]

So you could reconfigure the phase inverter to a traditional configuration and get a distortion monster? Ie modern vintage tone monster?

[Jana]

More like, you could reconfigure the PI and kill the magic.

[Andy Le Blanc]

What hiwatt used is based on a Schmitt inverter, modified to incorporate
global feedback between ground and what the inverter see's as ground.

There's a pretty famous paper, "Cathode Phase Inversion" Otto H Scmitt
1941.

In the the paper there is a statement that this inverter provides useful gain
and because of the coupled cathode a "current" linearity independent of frequency.

Its the same type of inverter used in McIntosh Hi Fi amps.
Their also usually directly coupled to the preceding stage which reduces any phase anomalies
introduced by the coupling cap, many manufactures drove this style inverter
with a direct coupling to a high gain pentode stage.
Hiwatt tried a direct coupling to a low impedance source, so you also get the
benefits of a good bandwidth and tone and good frequency response
of the inverter.

I did a bass build around it, the hiwatt form, the feed back was critical,
but you can drive the thing with any form of follower and have good results.

[Andy Le Blanc]

Here's one development, Hiwatt continued to modify how the inverter was sourced.
You can see the evolution in hiwatt schematics, check out Mhuss hiwatt pages.
The bargain bin section at schematic heaven has several other examples
of how this type of inverter was implemented.

[neon333]

What a great thread It's got me wondering if this is one of Dan Bouls tricks at 65amps. Lil Elvis(el84), the Tupelo(6v6) and the SoHo(el84 w/ ef86 frontend) are all wonderfully clean sounding amps with a lot of headroom. Lift the tone stack (the "bump" circuit) and the amps get nice and crunchy.

[Merlinb]

One of the great HiWatt mysteries will always be, what is the voltage follower actually for?! (in the DR-201 for example)
It doesn't regulate anything! it just provides a voltage reference, no better than a potential divider+cap feeding the LTP directly. There appears to be no useful purpose to that follower except to use up an extra triode!

Maybe HiWatt realised it was redundant and went on to change the design to the more conventional cathode follower DC-coupled arrangement...

[Andy Le Blanc]

He probably saw or read of something some where and went bench noodling,
If it sounds good it is good.....

[jaysg]

The CP103 shows you that the classic tone stack isn't critical, however, Mark Huss's work -- Marwatt, Hiwatt footstomp box, shows that you can use that tone stack in front of a normal PI and still get the goods. Mystery is right..

[imaradiostar]

depending on the version of Hiwatt you're referring to that triode carries signal. On the earlier ones it's a level shifted cathode follower that establishes DC bias for the PI and couples the signal to the PI.

I don't know if I've ever posted here before...odd. Sorry if I'm jumping in out of turn!

jamie

[skyboltone]

depending on the version of Hiwatt you're referring to that triode carries signal. On the earlier ones it's a level shifted cathode follower that establishes DC bias for the PI and couples the signal to the PI.

I don't know if I've ever posted here before...odd. Sorry if I'm jumping in out of turn!

jamie

No such thing Jamie as out of turn. Welcome and please hang about.