Hi-Fi tube guitar amp?

[dave251]

I'm looking for a hifi tube design, schematic in particular. Must be suitable for matching with the typical impedance needed for electric guitar...1 megohm input.

A simple tone stack is fine, in fact...the simpler the better. A single tone control with the "correct" turnover point is really all that's necessary. 15 to 30 watts of "clean" power is what I'm looking for.

This amp is for use in conjunction with my line of "hybrid" acoustic/electric guitars. I've been using an Eminence Beta 12LTa speaker and it sounds great with my guitar designs.

I'm assuming I'll need to find an old movie theater/PA amp design....

Dave
Guitar Repair Kansas City | Luthier Acoustic Electric Guitar

[Dave H]

I like this one Mullard 5-20 from my old Mullard 'Circuits for Audio Amplifiers' book

[loudthud]

The terms "HiFi" and "Guitar Amp" conflict. The transients created by a guitar, even a acoustic guitar, require enormous power to reproduce at a decent level without some clipping, and most "HiFi" amps don't clip gracefully. Combine that with a full range two or three way speaker system, the results won't be as expected. It's ok for recorded music because the recording process limits and compresses the signal. This is done to make it sound good.

HiFi amps use lots of negative feedback to reduce distortion and raise the damping factor. In a guitar amp, these things contribute to a cold lifeless sound. It would sound much better to have moderate feedback and distortion to warm up the sound, combined with a speaker system that only extends to the 5kHz to 10kHz octave.

Just my .02, YMMV.

[Chuck H]

I'm going to back Mr. thud. Guitar amps aren't reference amps and therefor shouldn't be limited to clean reproduction of already produced sounds, but rather with built in failings that offer softening and compression to spikes and resonant peaks. Without that quality musical instrument amplifiers would be as useless for the task as a Marshall head would be as a PA! If you get the reference and meaning. That isn't to say that a guitar amp can't be designed with more fidelity in mind, but it needs to remain graceful WRT the anomalies that are common in in instruments and this requires design that would be unacceptable for strictly reproduction amps. As is implied with the term "hi fi".

[jazbo8]

Why not just get a acoustic guitar amp if you are looking for a clean, "hi-fi" sound?

[Chuck H]

And there you go. In fact, having worked "hand in hand" with an acoustic amp designer, I can tell you that acoustic amps are basically mini PA systems. But in reality I'm thinking the OP will be wanting something special just for his guitar!

[nashvillebill]

Would some of the early Ampeg designs be more suitable for the OP, since Everett Hull didn't care for distortion in the Ampeg amps?

[teemuk]

Would some of the early Ampeg designs be more suitable for the OP, since Everett Hull didn't care for distortion in the Ampeg amps?

They might have been considered "HiFi" when they were made.

Several. Decades. Ago.

Today, not even close.

HiFi with tubes is something akin to likes of McIntosh or Audio Research amps. You won't find that sort of stuff from guitar amps, let alone from ages old Ampegs, which were borderlining high fidelity already even when they were conceived.

[eschertron]

How High the "Fi"?

@Dave

Are you looking for an amp that has a 'sweet spot' with just enough compression and distortion to sound tube-y, or are you looking for transparent cleans at any volume? The latter won't be gotten with a tube power stage, IMHO.

A tube preamp with a SS power section would allow you to dial in the amount of tube that you want (the idea behind all those A.R.T. boxes). Oh! Hybrid again!

[bob p]

Back in the 1950s all MI gear was derived from the best available HiFi amps of the time. Yes, things have changed a lot since then.

I agree with all of the theory that's been discussed, but I'm still reluctant to say that HiFi amps will always sound bad in MI applications, or that MI amps would always sound bad in HiFi applications. There's always going to be a unique situation that requires a solution that violates all of the rules that are derived from conventional widsom. Sometimes that means you'll find a good solution in a place where everyone tells you not to go.

Yes, highly damped amps can sound dull and lifeless for guitar, but there are a lot of bass players who rely on racked preamp + power amp setups, using preamps to shape their tone, and using clean power amps that provide tremendous amounts of highly damped clean power. HiFi/ProSound amps work for them.

I once heard an acoustic guitar player who had the best live amplified tone I have ever heard. After the show I asked him what he was using. Much to my surprise, he used an Audio Research VT-100 Mk II (HiFi reference amp) for amplification. It was an interesting choice -- that amp has MOSFET current sources driving a quad of 6550 on each channel. Even though it only produces 100W per side (clean), it is incredibly loud and it clips gracefully without blocking distortion. It sounded fantastic amplifying an acoustic guitar. Without a doubt that guy had the best amplified acoustic guitar tone that I have ever heard. A HiFi Reference Amp worked for him. (I wish I could remember his name.)

It's not that uncommon for people to use HiFi gear in MI applications. Now here's something that nobody would ever recommend: using a modern MI amp work for HiFi:

I have a set of Infinity Reference Standard HiFi speakers that have a nasty LF impedance dip to 0.8 ohms. They're famous for being amp killers, because if you drive them with any sort of program material that has demanding bass content, they will smoke any amp that can't act as a constant current source. To keep the LF impedance problems from effecting the rest of the signal I drive them bi-amped. I've tried lots of amplifiers for LF drive. The bass array has driven many of the best SS HiFi and Prosound amps to their knees, either smoking them outright or driving them into protection. The only amps that have successfully driven them hard with a demanding signal without burning up have been Krells, ARCs and SVTs.

Yeah, I actually said that -- SVTs. I've got a pair of racked SVT 2 amps in my bass rig, and just as an experiment I decided to give them a try. They did the job surprisingly well.