Micro Amp Fever: What Have You Done?

cometazzi

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I don't know that we've got a concrete definition of a "micro" amp (like say "under 5W" or such). However, you guys know what I'm talking about: low powered amps with 12AU7 or EF80 output tube(s).

RobRob's Major Valve Micro Series gives us iterations of the Champ, Deluxe, Bassman/JTM45, Blackvibe and JCM800. Last summer I built a ThunderTweak Heavy Watter, and in days of yore there was the AX84 Firefly.

I found the Heavy Watter to be a fun and engaging project, and the power level is perfect for apartment living. Excellent high gain tones, but it's a bit of a one-trick pony. Nothing wrong with an amplifier that just does one thing and does it well.

Since then, I've had the itch to build another Micro Amp, but before I decide on one of the great offerings above, I figured I'd ask what you folks have built and what you think of your creations.

So what have you done? Have you built anything other than what's listed above, and/or have you Micro-ized something else?

Perspiring minds want to know!
 

JSMac

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Maybe these don't qualify as micro amps but these little guys are micro in size and a few watts. 12ax7 and a 6bq5. Made from classroom record players. Something like a Kalamazoo model one/Lancer circuit.
F27A5B44-0EC0-47DA-BA64-4C23927689F4.jpeg
 
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SerpentRuss

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I called this one tight squeeze, 6N2P and 6CL6, maybe two watts, downhill with a tail wind. Gain, Tilt-Tone, Master Volume. The tubes are laying down horizontally and the works slide into an old industrial power supply enclosure.

Here's the build thread:

 

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Peegoo

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Here's a one-watt amp that drives a 6N1P power tube with a 12AX7. It has solid-state rectification, a volume and a tone, and off/on and standby. I stuck it in a homebrew cabinet with a '66 muscle car badge from the junk yard. Sounds Plexi-like through a 4x12. Rattles the windows, actually.

Nova-SS-Amp-Small.jpg
 

SerpentRuss

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Here's a one-watt amp that drives a 6N1P power tube with a 12AX7. It has solid-state rectification, a volume and a tone, and off/on and standby. I stuck it in a homebrew cabinet with a '66 muscle car badge from the junk yard. Sounds Plexi-like through a 4x12. Rattles the windows, actually.

Nova-SS-Amp-Small.jpg
Fun project. I really like small amps. Could you share a parts list for this. Are you running the 6N1P in parallel or push-pull?
 

ThePanda

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I have just built the RobRob's OG Champ Micro. Only special thing I did was plug it into my new 2x12 bassman cab with G12H Redback and Eminence SwampThang...
 

woodandglue

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I built one of Rob's micro deluxe's as my first build at the advice of a few smart folks. I went back and repurposed the bright switch for a midboost/ midscoop switch ala Merlin. It is nice to have a different voice at the flick of a switch.

IMG_5979.jpg

After a few conversions, I built a PRR 1/3 watt that uses two 12au6. I haven't boxed up this newest one, but it is fun to play and was relatively cheap to get all the parts. The weird thing about it is the knob on the right really gets the distortion grinding. I'm still figuring most things out, usually the hard way, but I think it changes the bias of the output tube.



IMG_6717.jpg
IMG_6713.jpg
 

owlexifry

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I also have a SLO Micro.
i’m looking at doing this one someday in the future.
got some EF80 tubes on the way.

my immediate concerns are:
- how to get the preamp plate voltages high enough, when the EF80 power tubes can’t take more than 300V…

options:
1)
- x2 separate HT windings, into separate x2 SS rectifiers. B+ from x1 goes to (2x)EF80 plate + screens. the other (higher B+) goes to PI+preamp
2)
- i don’t even know if it’s possible, and it’s probably a terrible idea: but is it possible to: route B+ first to PI, then preamp, and then final B+ for the EF80 plate+screens at the end..
(unusual scenario where power tubes require lowest plate voltage)
 

woodandglue

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Also interested. Also curious if "PRR" means the "PRR" forumite at el34world forums and other places. (I know him from diystompboxes.com)

@woodandglue, those are some nice looking builds. I used the same knobs on my first build, btw.
Yeah, PRR means the same character you're mentioning. He mentioned that he made the schematic as a joke. Well, I'm loving that joke!
 

draggindakota

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I'm glad this popped up! I've been looking through Rob's Micro amp pages, contemplating building one. What would be considered a big enough chassis for say a Deluxe Micro? I have a couple of retired firewalls I filched from work that might make nice chassis, but I'm concerned on the depth as they are both under 2" deep. One is 9.375" x 6.5" x 1.75" and the other is 8.75" x 5.875" x 1.75".
 

cometazzi

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Yeah, PRR means the same character you're mentioning. He mentioned that he made the schematic as a joke. Well, I'm loving that joke!

I figured as much. I'm not a member of el34world but their forums come up a lot in searches. As I mentioned I knew him from diystompboxes, and I've seen him in a handful of other places (though never here). Always 'PRR' and always the same avatar.

He's a bit terse at times with his replies, but he really knows his stuff. I've learned a lot from him either directly or indirectly.
 

wangdaning

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Two channel bassman micro, built it as tweed but in blackface chassis. It needs a cabinet home, but to get the table saw I need to wait. Had it ordered and then we went through some lockdown issues here. I think I am going to revive the thread on it when I get the kit and wood for the cabinet build.
 

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Paul-T

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I made the Deluxe Micro EF80; but haven't had time to fully trouble-shoot as the distortion was too fizzy for me. In the meantime I seemed to have accidentally acquired parts for a GA5 that will fulfl the same function so I think I'll rebuild as a JTM02 in a new chassis.

(So if anyone's in the UK and needs a cool cabinet for cost of wood, PM me)
 

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cometazzi

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Two channel bassman micro, built it as tweed but in blackface chassis. It needs a cabinet home, but to get the table saw I need to wait. Had it ordered and then we went through some lockdown issues here. I think I am going to revive the thread on it when I get the kit and wood for the cabinet build.

How do the channels differ? I see different numbers of knobs on each side, but the Bassman (at least the 5F6-A) has mostly 'symmetrical' channels.

I'll be interested in the build thread revival!
 




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