"µicrovalve": a 12V power amp distortion idea

[Steben]

Using 12V 12U7 triodes as in the Firefly amp. should give around 0.012 W power. The first speaker could easily be swapped for a small inductive load (only 12 milliwatts you see).

[Steben]

or otherwise in SS: maybe one could make an inductive load straight to a minibooster/µ-amp?
Could that giove it a poweramp-twist?

[earthtonesaudio]

I don't know much about tubes, but is the grid of the bottom valve supposed to be connected to ground?

[morcey2]

I don't know much about tubes, but is the grid of the bottom valve supposed to be connected to ground?

Yes, it's supposed to be.  It's a form of a self-split output stage.  The top triode is a standard common-cathode power stage.  The bottom one is what's called a grounded-grid amplifier.  The input signal actually comes in through the cathode and is created by the varying cathode voltage created by the un-bypassed cathode resistor from the top triode.

Matt.

[bancika]

did you build it yet?

[ambulancevoice]

im not good with figuring out what does what in circuit
so, what is the purpose of the tubes, transformer, and speaker??? and stuff

[morcey2]

im not good with figuring out what does what in circuit
so, what is the purpose of the tubes, transformer, and speaker??? and stuff

The triodes/tranny/speaker are basically a scaled down version of a Tube Amp Push-Pull output stage, very similar to the AX84/Doug Hammond Firefly output stage: 

http://ax84.com/media/ax84_m276.gif

The J201 is just used as a gain stage and the LM386 is the real power amp. 

I think it will work fairly well and the self split output is very inefficient, so it will keep the power of that stage way down.  If the tubes are too hard to push into distortion, it would be fairly easy to rig up a jfet cathodyne PI and throw a bypass cap across the 12u7 cathode resistor.  That would give more gain from the 12U7, without adding any pre-amp type distortion. 

Matt

[ambulancevoice]

im not good with figuring out what does what in circuit
so, what is the purpose of the tubes, transformer, and speaker??? and stuff

The triodes/tranny/speaker are basically a scaled down version of a Tube Amp Push-Pull output stage, very similar to the AX84/Doug Hammond Firefly output stage: 

http://ax84.com/media/ax84_m276.gif

Matt

thanks but, i know where its from, i know what they are, i wanted to know what there purpose in the circuit
are they just for distortion??

[DougH]

I would dump the first "dummy" speaker and use a power resistor instead.

[Steben]

thanks but, i know where its from, i know what they are, i wanted to know what there purpose in the circuit
are they just for distortion??

yes, to emulate pushpull distortion in amps. On itself rather overkill (compared to minibooster-stuff), yet there is the loading of the transformer by an inductive load.

I would dump the first "dummy" speaker and use a power resistor instead.

yet that would eliminate the inductive loading. Like I said, it should be fairly easy to make an inductive dummy load, since it is such low power.

[DougH]

IME, I don't think you really need an inductive load for something that is such low power. I used a resistive load in the line-out for my Firefly and it sounded fine. I experimented with a resistive load attenuator with low wattage amps and it sounded more "midrangey" than an inductive load did. But given that it's in an early stage of your amp, you should be able to EQ that out if it's a problem (and it may sound good anyway). I would definitely experiment with a resistor there before I bothered with inductors and etc.

IMO the main advantage of an inductive load in a tube amp is when used in an attenuator, you will get more of an "accurate" sound when the signal is attenuated a lot because it presents a load that is similar to a speaker to the output stage. But a resistive load doesn't necessarily sound "bad"- it just depends on the sound you want. IMO it's an issue of sound reproduction vs. sound generation. With an attenuator you are concerned with accurately reproducing the sound of an external amp at lower volumes. With your amp, you are generating the sound from scratch so you can do whatever you want (i.e. whatever sounds "good").

[earthtonesaudio]

Wouldn't the output half of the transformer present an inductive load even without the speaker?  Or do they have to be separate to make it a "load"?

[Steben]

@doug

Interesting.
It makes me doubt a lot, though 

You see, if what you say makes sense - of which I am 95% sure it does - one shouldn't bother at all putting them transformers in preamps. Since their saturation is way above the rating of the milliwatt-pushpull and the slight difference in inductive loading,... it could be just plain overkill.

I guess the inductive loaded minibooster is worth a shot though...

[DougH]

Well I can't speak for your transformer, as you actually have a power amp built into your preamp so I suppose it makes sense. I've opinionated and debated on here too much in the past on the perceived value of putting an output transformer in a SS circuit to mimic a tube amp sound- I won't go into that in this thread.

I think your idea is good and worth experimenting with. I'm just suggesting trying a resistive load in there too, as you may find the difference between that and inductive is negligible. And if so, you could simplify the design and build.

[earthtonesaudio]

What about an inductive load with variable current through the "unused" side, to manipulate the core saturation point?

[morcey2]

Doug's probably right with the reactive load being good enough for this, but here's another idea for replacing the first speaker:

http://www.aikenamps.com/spkrload.html

Throw a reactive load in there.  With this being a 0.1 W amp (off the top of my head), the components won't need to be as beefy as Mr Aiken specifies. 

Matt.

[Steben]

Doug's probably right with the reactive load being good enough for this, but here's another idea for replacing the first speaker:

http://www.aikenamps.com/spkrload.html

Throw a reactive load in there.  With this being a 0.1 W amp (off the top of my head), the components won't need to be as beefy as Mr Aiken specifies. 

Matt.

As a matter of fact, that's exactly what I was talking about, matt... 

for 8ohms, You should have a 0.5mH choke and a 25mH one. forget the 3 Amps prescription. It isn't even 0.1W, yet 0.01W !!!

[DougH]

That's a good point, at low wattage you could get by with some pretty small inductors. Heck, I'd say give everything a try!