Hybrid designs with all-tube preamp?

[Hardtailed]

Anyone has some cool hybrid designs they could refer me to?

I'm interested in trying to build one, where all the juicy tone would be created in the preamp and then I would feed it a ginormous SS power amp that would work at almost any volume. It's more of a curiosity thing and seeing if I can pull it off.

The point is not necessarily to get like a cranked Plexi tone at any volume, but rather have an amp with a useful tone that will scale well to various needs.

But if I wanted to go full blown, I guess I could build a complete tube amp with a 12AU7 power section with a dummy load then feed that to the power amp. I already have the transformers for it. But I'm trying to get the part count low.

So yeah, anything to share fellow solder fumes junkies?

[defaced]

People have done it, but it's pretty few and far between. Usually it's a tube preamp into a Class D solid state power amp. I'd like to try it because all of my sound is from the preamp but haven't gotten that far with the project. Here's a Class D power amp that has been mentioned on another forum I frequent. Hypex Electronics B.V. Click on products, and then on UcD180.

[Hardtailed]

Well, the SS part I'll probably just bridge a pair of LM3886 (or two pairs and make it a stereo affair) or use an off-the-shelf unit.

It's really the preamp side I'm looking for ideas.

As for you, I find that I'm getting most of my sound from the preamp already since I can pretty much never push my power amp. Even on stage, my volume is under 9 o'clock.
Sure I could crank it, but I prefer hearing my voice than my guitar on stage!

[Steve Conner]

I like hybrids and have built several. My latest (and favourite ) is basically just a Champ or Valve Jr.-like amp with one 12AX7 and one EL84. The output from the OT speaker winding goes to a three-position switch that selects three destinations:

Speaker via 10dB attenuator: Output power 0.3w, can be cranked in living room

Speaker direct: Output power 3w, can be played clean in living room or cranked in coffee shop

Speaker via 40w solid-state booster amp: Can be played clean in a coffee shop or cranked at a gig

As you can see the output varies in factors of 10: I wanted to test the old saying that it takes 10x the power to be twice as loud. I also added power scaling by varying the EL84 screen voltage to give some flexibility.

I invented a new output stage topology where the OT speaker winding drives the output transistors directly, and they provide current drive to the speaker like pentode or beam tetrode tubes do. I'm not publishing this yet, I'm going to try and sell it first.

But the important thing is to arrange for current drive to the speaker. My output stage is just a simple, low part count, protected way of doing that. Note, I don't claim the concept of current drive as my invention: it's the low parts count, Class-AB biasing and protection.

Regular SS power amps give low-impedance voltage drive. Teemu Kyttala's book (free, google it) has examples of current-feedback power amps, also see Rod Elliot's site and schematics for the Marshall Valvestate amps.

YMMV, but I think the general topology of a Champ-style amp driving a dummy load with a feed taken to a current-feedback SS power amp is a good bet.

[defaced]

It's really the preamp side I'm looking for ideas.

That's a pretty vague statement. Could you be more specific? Do you want suggestions for cool preamps? Or are you looking for exactly how to do this or what?

[BackwardsBoB]

Carvin has a circuit that drops the supply to the PI to reduce wattage and get some power-stage-like breakup. It does sound a bit like push-pull power stage distortion, probably more than a Champ circuit driving a dummy load.

Point is, maybe you can get by with just adding a PI you can overdrive to your preamp to improve toobiness. Not sure what to do with the outputs, but there are lots of possibilities.

[Steve Conner]

It does sound a bit like push-pull power stage distortion, probably more than a Champ circuit driving a dummy load.

That's not really surprising given that a Champ doesn't have a push-pull power stage. An overdriven single-ended power amp is a different flavour of distortion, but it still definitely sounds "tubey".

My topology would also work with a little push-pull power amp. Well, I say "my" topology, but it's about what you'd get if you plugged a Hughes & Kettner Cream Machine into a SS power amp.

Bob: How low does the Carvin drop the voltage?

[Hardtailed]

That's a pretty vague statement. Could you be more specific? Do you want suggestions for cool preamps? Or are you looking for exactly how to do this or what?

Yeah, I mostly mean examples of tube preamps circuits that are known to work well with an SS power stage.

I'm much more into the Marshall type of sound, so I do want it to sound closer to a push-pull Class A/B type of amp. Keeping the PI stage but dropping the tube power amp is an interesting idea. Saves an OT too!
As I said, it's not like I'm getting much power tube saturation right now anyway.

[J M Fahey]

You might simply build a Mesa Rectifier channel and drive your SS power amp with it.
The way those preamps are designed, they create 99% of their distortion and characteristic sound straight from the preamp.
Their characteristic muffled sound matches well the perceived brittle SS sound, the end result is quite acceptable.
Personally, I have built tons of SS power amps to power JMP1's, Peavey Rockmaster, Rack V-Twin, Mesa Studio Preamps , etc. or small 5 to 18W tube combos, with good results.
So far as the SS power amp does not clip itself, the Tube sound gets through; otherwise it loses its character.
Plan on having at least 2x the SS power available than you will actually need, as in using a 100W SS amp to emulate a 30 to 50W tube amp and so on.

[defaced]

The way those preamps are designed, they create 99% of their distortion and characteristic sound straight from the preamp.

I was going to say about the same thing as Fahey. Pretty much every high gain cascaded preamp design is made to work this way. So pick your flavor, there are many many out there and when distilled down to just the preamp, they're really kinda easy to layout/build (assuming you only do one channel). Here's my personal list of "have built" or "would like to build":
VHT Deliverance (built)
SLO 100/Avenger/Dual Rectifier (they're all basically the same) (built)
Bogner Uberschall (on the list)
Peavey JSX (on the list)
JCM 800 (on the list)

[guitician]

SS Distortion

Another thing to consider is the non-linear distortion of the power section. SS can be made to produce even order(non-clipped) distortion that can add a warm tube like sound. Many simple discrete power sections can do this.

[BackwardsBoB]

That's not really surprising given that a Champ doesn't have a push-pull power stage. An overdriven single-ended power amp is a different flavour of distortion, but it still definitely sounds "tubey".

My topology would also work with a little push-pull power amp. Well, I say "my" topology, but it's about what you'd get if you plugged a Hughes & Kettner Cream Machine into a SS power amp.

Bob: How low does the Carvin drop the voltage?

The metalheads are everywhere, they crave power stage distortion, and at 100W, it's impractical. The distortion they want is what you'd get out of a ubiquitous 100W EL34-based push-pull output, not a Champ. Users of the Carvin circuit find that they can get close with the switch on the back of an X100B that voltage-starves the phase inverter. There are two basic versions of this circuit. The first is present on most of the original X100Beasts from the '80s and '90s, and it's "improved" in the new X100B series IV.

In the original, there's a 22uF cap on the PI supply, and a 470K pull-down to ground. The screen voltage is used through a 10K resistor at 100% power, they put another 470K in series for 50% power (cutting the voltage way more than half), then add an additional 1M in series at 25% power. The 50% sounds pretty good, but the 25% setting takes things too far, as if they neglected the fact that the PI would draw power:

http://www.carvinmuseum.com/pdf/amps..._schematic.pdf

In the re-issue, they've dropped the 470K and 1M resistors to 220K each, still driving a 470K pulldown. This circuit's performance is getting an even better reception, with some users simply leaving the switch set to 50%:

http://www.carvinservice.com/crg/sch...01C29apr08.pdf

[Steve Conner]

The way those preamps are designed, they create 99% of their distortion and characteristic sound straight from the preamp

The metalheads are everywhere, they crave power stage distortion

One of you must be wrong... Which one?

My first ever hybrid was a high gain preamp with two 12AX7s, driving a microphone matching transformer backwards to add some "transformer mojo", then into a graphic EQ removed from an old cheap and nasty stereo system, and finally a large Philips PA amp obtained from a flea market, driving a 4x12" with four different assorted speakers in it.

I have no idea what power the amp was, all I know is that it had eight TO-3 transistors in it and impressive black heatsink fins on both sides. It was pretty loud, and great for the metal I was playing at the time. Before that I used one of the old SR&D Rockman rackmount preamps, but this sounded loads better.

[JacksonRR]

Randall makes a few like that. Tube pre into an SS power section. They're all more or less metal amps, but that's Randall. They have some sort of "tubelike" response built in the circuit. Don't know about that.... Haven't tried one in person.

[defaced]

One of you must be wrong... Which one?

That is a bit of a contradiction isn't it? Most metal guys (me and most everyone I talk to) want a very tight sound at levels where you can keep up with a heavy handed drummer (ear shattering). The only way you get there is a tight power amp. And power amp distortion with high gain preamp distortion sounds like ass.