Very low power amp 1-2 watts

[dr_iggi]

Hello forum! First post here.

I have a couple of mid power amps that give me all the volume I need: Zinky Blue Velvet 25 watts and samamp VAC 45.

I would love to build a very low power (1-2 watts) head or combo that I can crank all the way for basement/bedroom practice.

What I have in mind is a single-ended class A self-biasing design allowing interchangeable preamp, output, and rectifier tubes.

Maybe something like the Emery Microbaby... I think that such a build will be lots of fun.

However, I do not know where to go for resources regarding such a design. Your help and advice are appreciated.

Thanks!

[gregarious]

Doc, You'll probably find all you need at http://www.ax84.com/

[RJ Guitars]

Dr. Iggi,

Welcome to TAG forum... it's a great bunch of guys that are extremely helpful and generally can provide you real time saving wisdom on almost any amp build you try. There might even be some female contributors but if there are I don't know who they are...

Here is a post I put out a while back on a 5 watt push-Pull triode output amp. This is a really cool amp and I gig with it in the small venue setting...

You could build this with a smaller output triode like a 12AU7 if you want to knock the watts down a little more.

http://ampgarage.com/forum/viewtopic.ph ... highlight=

See what you think.

rj

[d95err]

I would love to build a very low power (1-2 watts) head or combo that I can crank all the way for basement/bedroom practice.

1-2 Watts is way too much for "bedroom" volume if you want to crank it. (Remember that 2W is just 10dB lower than 20W).

I have a powerscaled amp, and I've found that I need to turn the power down to about 50mW (yes milli Watts) to get bedroom volume when cranked. Oh, and that's through a fairly inefficient speaker...

So, it's almost impossible to build an amp that can do bedroom volume when cranked, unless you use powerscaling or some kind of attenuator.

[slajeune]

Well,

if 50mw is considered the optimal bedroom volume, you can try this amp:

http://www.sophtamps.ca/mambo/index.php ... &Itemid=38

This is roughly 35-40mW. Not a lot of distortion available but, with a simple boost in front, it can grind!

Sadly, the tubes are not current production tubes.

Cheers,
Stephane.

[krash]

well you can make 100mW or less from any 12A_7 tube if you bias it right and give it the right load/plate voltage. Just running 1/2 of a 12AT7 single-ended with about a 1K cathode resistor, about 200-250V B+ into about a 10K OT primary will make well under 1/2 of a watt. push pull with a 470 shared cathode resistor is under 1 watt this way.

Yes 1-2 watts is much more than "bedroom". Into a regular 100dB/w/m type guitar speaker it's about grand piano volume level when cranked. I build a lot of these amps.

I would suggest that the easiest and most effective way to get 50mW of power out of a design that makes 1-2 watts (like most of my low-watt amps) is to use an L-pad between the output and the speaker, just turn her down.

There are a lot of tricks to getting a low-watt to sound GOOD. What I mean is, not "pretty good for a small amp" or "a lot better than a MV amp turned down that low", but I mean, just absolutely as good as any other boutique amp without regard for power level, tubes, or dB's.

[66merc]

Hey,

Also new to forum.

I have had good luck lately building small practice amps using 12FX5 tubes. I just finished a 5F2A"ish" amp with one 12FX5 instead of the 6V6. Sounds good and overdrives nicely at true bedroom volume.

I also built a 12FX5 amp with two self split output tubes. I used this design and ultralinear OT but used the same 5F2A front end. Also sounds nice.

http://www.diyparadise.com/ultracompact.html

Pics on this page somewhere.

http://s155.photobucket.com/albums/s315/spammeup/

BTW, the 12FX5 tubes can generally be found NOS for about $1-2 each. I also cut costs by using back-to-back radioshack power transformers. I take the 12.6v heater current off the secondaries of the first PT and then feed those secondaries into a 2nd backwards PT. Rectify the output with bridge. A five buck OT from AES works nicely. I generally have more in chassis/tube cage cost than the rest of the parts!

66merc

[66merc]

oops,

This is the actual design my self split amp was inspired by

http://www.diyparadise.com/simpleel84.html

66merc

[gearhead]

well you can make 100mW or less from any 12A_7 tube if you bias it right and give it the right load/plate voltage. ...

There are a lot of tricks to getting a low-watt to sound GOOD. What I mean is, not "pretty good for a small amp" or "a lot better than a MV amp turned down that low", but I mean, just absolutely as good as any other boutique amp without regard for power level, tubes, or dB's.

I see that this is your biz, but any tricks you can share?

[krash]

Well, the main trick is this.

You have to understand that everything, EVERYTHING has to scale. If it doesn't scale, then you have to compensate somehow.

The main reason a 30W guitar amp with 2x 15W speakers sounds killer (like an AC30 for example) is because the whole amp is cooking. We're talking the power transformer, the power supply wiring, the output transformer, the rectifier, the speakers, etc.

So when we typically scale things down we are thinking only about tubes. We tend to forget about things like supply wiring, filter capacitors, power supply impedance, and that kind of thing. No I'm not saying build the whole thing with 28ga wire but you may need to compensate. For example, you are not going to get ANY power supply sag unless you design it in. Likewise, you will not get ANY saturation of the OT if you use a normal commercially-available p-p OT (mine are rated for 12 watts). So then what do you need to do about that? Something, I bet! Same for a speaker. You are not going to get any speaker breakup with less than a watt of power. You should know what that is going to do to the tone and make sure you are not depending on that for good tone.

Also remember that when building an amp with triode push-pull output design that the tube itself has far different characteristics vs. big tubes. For example, a 12AU7 has plate and grid capacitances of just under 2 pF, while an EL84 is more like 10pF for grid capacitance and an EL34 more like 15pF. So you may need to compensate with actual capacitors added in to add capacitance so it responds more like a big tube.

One other tip is not to cut any corners. Build a whole entire guitar amp the same way as you would a whole entire big amp. Just use a lower power output tube setup. But the rest of the amp needs to remain the same. Now since I am building commercially-available amps I want ready replacement tubes so I use 12AU7s or 12AT7s rather than using some oddball NOS $1 ebay tube. If you go my route then you will be limited to about 300V B+ so plan your design accordingly. So in other words, don't pick a basic design (Fender) that is going to require 400V on the PI in order to work right. Remember the plate voltage is the voltage behind the load resistor, so you may have B++ at 400V and the plates on the PI under 100K resistors look more like 220V or something, but when you connect that 400V B+ through the OT to the power tubes, it looks a whole lot closer to 400V and you'll fry a 12AU7.

And lastly, we are talking about 50mW amps but remember nobody builds a 1 watt EL34 push-pull amp. So a 50mW amp with a 12AU7 would be just as bad of an idea. We tend to run big tubes biased at 50-70% of their dissipation limit at a given plate voltage, and depending on the tubes, sometimes at or above their rated plate voltage. And we carefully match OTs to the plate load characteristics to get the kind of tone we want. You know everyone goes on and on about how using the right OT Z for a wreck type amp makes a big difference. The same thing holds for little amps. Bias the tube at 70% or more of its dissipation limit (fortunately there's only one grid so it's easier to calculate, but good luck getting dissipation specs for tubes like 12AX7s), run it at or very near the limit for the plate voltage and match the plate load like you would for a big amp. For a 12AU7 that means running it into about an 8K primary for a push-pull amp, and about a 400 ohm or so shared Rk. For less power you could use a 12AX7 or 12AY7 but you will have to experiment and likely blow up a bunch of tubes to figure out which OT works. You notice nobody ever publishes any info about using a 12AX7 into a load smaller than about 47K.

Oh one more thing. IMHO, the PI makes a huge difference in tone of a guitar amp. Unless you are really looking for a single-ended amp kind of tone, then don't make the mistake of thinking a Plexi preamp mated to 1/2 of a 12AU7 running single-ended is going to sound anything like a Plexi. It's going to sound like a poorly-modified Champ.

There are a number of other tricks floating around on the web. If you look around you will find them, but just wait until you get an amp built and it has problems, then search around for info on fixing the problem.

It's really cool to have amps that you can literally run on "10" all the time without ever worrying about blowing a speaker or an eardrum. So what if you eat up tubes? My 12AU7s last me about 1 year running in my personal Serena and I play it probably 10-20 hours a week, all of that time with it on max. It costs $9 to retube the power stage.

hey check out this latest recording I just did with my Serena (no it's not a Revelation demo, just a real song): http://joshkarnesmusic.com/media/like_heaven.mp3

[dr_iggi]

Thanks for all the replies... lots of great insight Scaling everything makes so much sense, rather than just concentrating on the output stage.

[El_Martin]

Josh, tnx for your great input!

Small amps are great...

Did you see my Lil'Wreck? Two tubes, back-to-back 12 V transformers for heating and HV, 100 V output transformer...
no PI, no PP ...

OK, it just cleans up w. guitars on volume "1"

Going to bias the output triode a bit hotter and reduce plate load (tnx to your hints)

Ciao
Martin

[RJ Guitars]

Josh, tnx for your great input!

Did you see my Lil'Wreck? Two tubes, back-to-back 12 V transformers for heating and HV, 100 V output transformer...
no PI, no PP ...

Ciao
Martin

I agree, Josh it's great to have your input... one of my favorite builds was inspired by your Serena amp...

Martin, can you lead me to that schematic? I love to build a scaled wreck.

rj

[El_Martin]

@Rich: I am working on the schematic...
Edit: just added the schemo, see TW section
Ciao
Martin