"Micro Mace"

[1blueheron]

The purpose of this project is to build a smallish amp that can be used for low level practice in a small but lively bedroom, or in a classroom after school without disturbing others and still have great tonal quality and range from very clean to full on distortion.  I anticipate an output of 1/4 Watt or so will be more than adequate. My objective is not a loud 1/4 watt either.  It seems like all the micro amps out there are trying to squeeze max volume out of less than 1Watt.  That is not my objective.

Using the solid state preamp, tube output stage concept of the Peavey Mace/Duece amps seems appropriate.

Conceptually, I am thinking an LM386 preamp Like the Ruby amp, driving a pair of 6SN7's as output tubes just like the Firefly R does, using a small reverb transformer as the output transformer Like Rob Robinette did with the "Micro Deluxe".

Or, use the Ruby Amp as a optional second preamp channel. Going this way, I think I would change out the 12AX7 in the Firefly and replace that circuit with a 6SN7 preamp section and leave it in place as a low gain, clean, all tube option.

How crazy is this idea?

I originally posted this in misc. schematics section but got no response and wasn't sure if it should be there or here or maybe in the solid state section?  Can move it again if necessary.

[VMS]

I hope others will chime in here too but i have a feeling that pair of 6SN7 on full distortion is going to sound loud. So my approach would be to build great sounding (maybe two channel) tube preamp and then maybe use that ruby as a power amp.

[1blueheron]

I hope others will chime in here too but i have a feeling that pair of 6SN7 on full distortion is going to sound loud. So my approach would be to build great sounding (maybe two channel) tube preamp and then maybe use that ruby as a power amp.

I guess I missspoke (typed).  The LM386 preamp would be driven into distortion but the 6SN7's would run as clean power.    Does that sound more reasonable?

[sluckey]

The LM386 is a power amp, not a pre amp. It has a very low output impedance meant to drive a low impedance device such as a speaker. I think you have the cart before the horse.

[shooter]

The nice thing about Ic's, if you have a good breadboard, you'll have it working in an hour, Rev 9 before bed 

[VMS]

I guess I missspoke (typed).  The LM386 preamp would be driven into distortion but the 6SN7's would run as clean power.    Does that sound more reasonable?

oh, i see. I was thinking that you were aiming for tube distortion at low volume.

[PRR]

> LM386 is a power amp, not a pre amp.

It can work as a preamp. Over-kill, but less than a buck.

[1blueheron]

I admit I am out of my element here but in the past I have built SS amps running an LM386 as a preamp into a TDAXXXX series amps.  I was able to use a multiposition selector switch with a series of Caps between pins 1-8 to get a wide range of distortion levels and got some amazing sounds.

I have never paired one up with a tube amp but with the vast number of distortion pedals using the LM386 as their basis, it seemed like it must be possible somehow.

I have been listening to clips of the Purple Plexi which is an LM386 based pedal and pretty much covers the spectrum of Marshall sounds.  It seemed like building something similar to this,  into a really clean, low gain tube power section would be a versatile addition to the collection. 

If its a terrible idea, I will scratch it as an onboard preamp.

Regardless, I still want to build an all octal, low gain, all 6SN7 version of the FireFly R or the Micro Deluxe.   Once I have that nailed down and working, perhaps I could build a purple plexi,  or distortus maximus type LM386 based pedal.

As PRR says though, its only a buck and if its possible, what do I have to lose trying it?

[sluckey]

It seems that chip is more versatile than I first thought. Why not just build that pedal and plug it into that big amp you just built? And turn the amp volume down to a whisper. Or up to a roar when needed.

[1blueheron]

It seems that chip is more versatile than I first thought. Why not just build that pedal and plug it into that big amp you just built? And turn the amp volume down to a whisper. Or up to a roar when needed.

The pedal approach certainly has its attractions, however, carrying that big amp I just built plus a cabinet to school for practice is neither fun nor practical, and... I still have gobs of free parts and piles of 6SN7 tubes left over from the organ and the eico 377 I took apart.  What better to do with them?


So far I have gathered:
Power cord
Fuse holder/fuse
On/off switch
indicator lights
all the 6SN7's I can use
all the octal sockets I can use
Power transformer (275-0-275)
Choke Coil
Headphone jack
1/4" input jack (self shorting)
1 1M pots
LM386n IC
IC socket


Honestly, I think all I need is a small Output transformer, Caps, resistors and possibly a voltage regulator.  Once I have the list of resistors nailed down, I may have quite a few of those as well.

[PRR]

> It seems that chip is more versatile than I first thought.

It's a special-purpose chip which can be used for a lot of audio.

Of course it is a 9V 1/4W 16r audio power amp for pocket radios. You can even squeeze 0.4W in 8r.

It has low idle power (for pocket battery life), so no big sin to use it for low/no "power output" jobs.

It "is an opamp" but with dedicated NFB resistors. For DC you let both inputs fall to V-, the output will set at half-supply, done. For AC/Audio the naked gain is about 20, extra pins let you set it to 200, a nice range.

Here the Gain pot trims from say 40 to 200.

Drawback in g-world: the input is only 50K, awful low for G-pickups. The "Ruby Amp" adds that JFET for a high input, and a volume knob.

Detail critiques: the 220uFd output cap is for a pocket loudspeaker, and MUCH bigger than needed to slam tube grids and mix-resistors. But hey 220u@16V costs pennies, and they are not my pennies, and using it means it could be re-purposed to drive a speaker without finding a big cap. The <1r output of the '386 will totally SWAMP the output of the tube preamp, we need switching or mixing here, but we can fret that later. The two-12AX7 preamp has far more gain than the '386 preamp; I wonder if the tube-side even needs that much gain (even for insensitive self-split output). So it may be "dangerously high gain", too easy to hit FULL POWER SPLATTT, but I do not think Jericho will fall, and gain-tweaks are easy to do after smoke-test. 

[1blueheron]

PRR, Thanks for the feedback and ideas.  Working on revisions.  Hope to have schematic together soon.

[1blueheron]

Drawback in g-world: the input is only 50K, awful low for G-pickups. The "Ruby Amp" adds that JFET for a high input, and a volume knob.

Detail critiques: the 220uFd output cap is for a pocket loudspeaker, and MUCH bigger than needed to slam tube grids and mix-resistors. But hey 220u@16V costs pennies, and they are not my pennies, and using it means it could be re-purposed to drive a speaker without finding a big cap. The <1r output of the '386 will totally SWAMP the output of the tube preamp, we need switching or mixing here, but we can fret that later. The two-12AX7 preamp has far more gain than the '386 preamp; I wonder if the tube-side even needs that much gain (even for insensitive self-split output). So it may be "dangerously high gain", too easy to hit FULL POWER SPLATTT, but I do not think Jericho will fall, and gain-tweaks are easy to do after smoke-test.

With a couple notes in mind on the above, I have set out working on some drawings. I believe there is only 1 pre-amp tube in the firefly and micro deluxe, though both use 2 output tubes.  The way it is shown drawn in first post is a bit obtuse.  Will swapping the one 12AX7 out for a LM386 be about an even swap as far as gain capability to drive the output tubes?  We will see.


Attacking power supply section first.  My experience with LM386 is that even though it will run on as little as 5VDC, it runs out of steam and gets farty sounding at that voltage.  It sounds much better on a solid 12VDC supply.  Tightens up and gives a good chug chug rather than blat blat.  I would like to derive the 12VDC from on board PT if at all possible.  Draw of the LM386 is a mere 4- 8mA so I think it is realistic.  The chosen PT previously heated 4 tubes so I don't think it will be too much load. 

The power supply in the Eico 377 appears to be 275-0-275  with a 6.3V filament circuit.  This will be verified before I put it in service.  It previously used a 6X5 rectifier tube.  I am opting to change this to SS rectification as I have read horror stories about the 6X5's sparking out.  If I recall correctly, that is the reason the 6X5 is missing from this one and the unit ended up in the discard pile.

Using the same initial schematic as Rob Robinette drew for the Micro Deluxe for rectification,  I have added a second rectifier and voltage doubler (ICL7660) to the filament output section to run the LM386 section with 12VDC.  (I noticed Hoffman sells some packaged rectifiers, if they would do the job better than this then I could go that route.)

Open to suggestions... since I am rectifying to DC for one, wouldn't be too much trouble to make all heaters DC... 

Working towards better impedance matches for the audio portion.

[1blueheron]

Schematic from the Eico 377 I stole the PT from.

[1blueheron]

So I have started putting pieces and parts together.  This will be my first "scratch" build starting from nothing.

PT is sourced from an Eico 377

Chassis is an 8"X 8"X 6" electrical junction box with bottom portion cut off.

6SN7 tubes and octal tube bases salvaged from Allen organ.

Project concept is merging a small, low watt tube amp with a small, LM386 solid state amp.

For those familiar with the Peavey Mace/Duece amps, a solid state front end was used to drive six 6L6 power tubes.  In this case, a Solid state LM386 based micro amp will be used as a pre-amp to drive 6SN7's as power tubes and create a baby version. 

I originally proposed this as just a solid state pre-amp into tube power amp stage but after further thought, I think I am going to leave the option open to use this as a clean, all tube amp, with switch selectable ss pre-amp stage for the LM386 distortion channel.  I admit this is largely an experiment and probably makes no sense to many but to me I think it makes an interesting and unique project which will be very versatile.

Still fine tuning the schematic.  A TL071 has been utilized to give a better input impedance match to the LM386 it is not there for increase in gain.  It will be socket-ed so a whole range of op amps could be swapped in and out easily for a range of different sounds.  The NE5532 is likely one I would try in it as well as the OPA2604 but starting with the TL071.

The Rob Robinette "Deluxe Micro" layout has been adopted for the tube portion but with the modification of making it (3) 6SN7's as I am not looking for very much gain.  I assume I do not yet have all of the values correct and they will need to be tweaked.

Thanks to those who provided input on previous post.

[1blueheron]

Pic2

[1blueheron]

Schematic