RossD UltimaDork
4/23/13 7:36 a.m.

August 2014 PREFACE: Whenever I start a new tube amp project, I'll just add to the end of this old thread!

Someone in the hotlink thread asked for a build thread of my 1-2 watt per channel stereo tube amp. So here goes!

So I got most of the design from the forum on One of the guys on the forum designed a super simple single stage preamp ('4S preamp') meant for either a 12au7, a 12ay7, a 12at7, or a 12ax7. Another forum thread dealt with micro-powered amps (less than 2.5 watts per channel), and I had recently found two very small output transformers that would be around that wattage.

So after asking a couple of questions on what power tubes I should use, 6aq5 in a triode operation (since I have a box of old tubes from a garage sale), I came up with the final schematic. Actually this isn't the final schematic. I moved the on/off switch to directly behind the fuse, before the split to the transformers so that you could not turn on the high voltage transformer before the heater transformer. If I used a tube to rectify instead of diodes, I probably could have skipped the stand by switch all together.

From the same garage sale as the box of tubes, I got a couple of old short wave radio chassis. I stripped off the old grimy bits, painted it and bolted a bunch of parts to it:

The two transformers at the back are actually 120-208-240v multi-tap primary and 24 volt secondary controls transformers. The all silver transformer is the 12.6v heater transformer from radio shack. The two NOS Stancor output transformers are beneath the two aluminum covers on the right side. The covers are from the old chassis and just happened to line up with the mounting feet of the output transformers, so I decided that they looked cool and used them. Lots of switches and resistors are from radio shack, some of the other parts are from, and the tube sockets are from Triode Electronics.

I'll have to add a picture of the internals, since I don't have one at the moment.

Here's one of it in 'action':

Since it's around 1 watt per channel, I need to use some higher sensitivity speakers. The MTX speakers it's sitting on in the last picture are around the 91 dB/m/w so it gets pretty loud at that one watt, about 91 dB at one meter.

It's mostly hum free, but there's a little there. I'm not too worried at the moment. I've been mostly listening to Dave Brubek (Jazz). Some faster hard rock tends to lose some feeling. I'm guessing this little amp will be the jazz/classical/piano/dinner music type amp.

Questions and comments?

Zomby Woof
Zomby Woof UberDork
4/23/13 8:02 a.m.

Might be good for a high sensitivity full range/horn project?

RossD UberDork
4/23/13 8:06 a.m.

In reply to Zomby Woof:

Do you have a link to something like that?

I've been considering some Fostex 4" full range drivers and building a simple box that they detail in their documents.

crankwalk Reader
4/23/13 11:55 a.m.

The fostex 4 inch have very peaky mids for my taste. They need at least a ported box and a 10 in sub to go with them. I played with audiophile stuff for a little while with a friend and he had them. They are clean but need more bottom end.

Zomby Woof
Zomby Woof UberDork
4/23/13 12:20 p.m.

I'm partial to transmission lines.

e_pie HalfDork
4/23/13 12:42 p.m.

That's awesome.

RossD UltimaDork
4/23/13 7:02 p.m.

You can barely see the rectification diodes in the lower left -ish. The signal ground kind of looks like a 'J' mostly centered. The tiny little output transformers are on the right and you can see the aluminum covers through the holes in the chassis. The three white rectangular are the 470 ohm, 5 watt power supply resistors. The filter capacitors are across from them. The larger yellow capacitors are the coupling capacitors. They block the DC voltage and allow the AC signal to pass through. The filter capacitors work in much the same way as the coupling caps but dumps to ground the small amount of AC left from rectification while not letting the DC getting to ground.

The green and yellow twisted pairs of wires are the 12.6 volts for the heater/filaments.

RossD UltimaDork
8/18/14 8:32 p.m.

So, I'm back at! With our first baby arriving in late November, I'll need to finish this so I can lull the baby to sleep with some sweet musical gold! I was at the local antique mall and come across another old table top radio for cheap. It wasn't in working order and would have been more work to fix than I'd want to deal with, but since it was so cheap, it's now my 2A3 platform.

Here is one of my attempts to layout everything. My other option would be to place the output xfmrs in the middle and the large choke to the back right, while shoving the two smaller chokes into the inside of the box. I might go with the latter. I have a 120v neon lamp (from radio shack) that I've managed to fit into the original bulb location but can I wire in series between the on/off switch and the power xfmr; will the current be to much?

I also am going to put some new veneer on the top and try to repair some of the other spots.

The old baffle is an ancient piece of corrugated cardboard and is ugly beneath the old fabric. I've got some speaker fabric from the Fender Champ scratch build that haven't finished ( I'll get there, I swear ), so I'll be using that with new a 'baffle'. Baffle in quotes because there will be no speaker inside the cabinet but there will still be a hole so that any tube light that may get thought the fabric might be seen. Since the new fabric is a bit easier to see through, I'm replacing the cardboard with a thin piece of plywood that will get a dark coat of stain.

I'd really like a nice piece of furniture when I'm done and if someone thinks we are using this old radio for decoration, even better when it lights up and makes some beautiful music. My last couple of amps have been less than visually appealing. The 6k6 stereo amp wasn't bad but wasn't as concise of a design as I'd like.

I was hoping to use the original rotary on/off switch that was the volume pot (without using the volume pot) but I was reading a solid 30-70 ohms after being turned on. The shafts are good 2" long, and haven't been able to find a SPST rotary to match... I might just use a toggle hidden on the back of it to keep the original pots/knobs in place and looking pretty.

If you're interested in the schematic or how I got this far:

sobe_death HalfDork
8/18/14 10:22 p.m.

That Fender fabric is awesome! I have a bit of that silver/turquoise stuff that I was going to redecorate a pair of Polk Audio towers with...

RossD UltimaDork
4/11/16 8:33 a.m.

A couple of months ago I finished the amp side of my tombstone 2A3 amp.

I plugged in a 80's AM/FM turner from a component system into my '4S' 12A*7 Universal Preamp ( ) and finally into my 2A3. I used some '70s thrift store speakers (pictured in the bottom right of the photo) and found the sound coming out to be quite lack luster. I was a bit disappointed but chalked it up to the amp needing some hours through it to get things loosened up a bit. Tube guys talk about that kind of thing but I've never really experienced it, but I'm a "DIY'er in an Audiophiles World".

I've used the 4S Preamp before and it sounded good. I've used the thrift shop speakers for a while now and had not found them particularly disagreeable. I was bummed out but I didn't even consider the FM/AM Tuner! I plugged my phone into the preamp to listen to some MP3s, it was transformed from something kind of dark and muddy to something quite bright and beautify, well as bright as beautiful as Antrax can be.

RossD UltimaDork
4/11/16 9:26 a.m.

At this point I switched over to one of my first 'tube amp projects', high efficiency full range speakers. Thanks to DrBoost on his theater room thread to jump start my stalled project. I have some Fostex FE126En 8 ohm drivers and used the bass reflex cabinet design from their website.

I cut the holes in the birch plywood I had previously cut up. I used that drill bit with the outrigger bit. It was a little scary at first but I survived.

From it suggested to use some Duct Seal to keep the speaker frames from ringing. It's an oil based clay that stays malleable.

I also used it to friction fit the ports into the cabinets because I'm an oaf and cut the holes to large.

I used my card scraper to remove the excess.

When I used my table saw to cut the birch plywood, I was not very used to using it and I had some issues. Let's just say the cuts were not perfect and I had a couple of kick backs and it left some marks.

As such I used some Duct Seal to fill in the gaps. It's not my most shining moments but, hey, we're all friends here.

I pre-soldered the internal conductors and disassembled the rear jacks so I didn't have to solder with the cabinet on the bench in front of me.

After soldering the drivers up and carefully assembly, as to not puncture the speaker cone with the screw driver, I stuffed them with acoustic fill. I have no idea what I'm doing at this point so I used half of this bag between the two speakers and just tossed it in there.

To finish up, I reassembled the rear jacks and buttoned everything up.

I connected them up to my first stereo tube amp build, a 6AQ5 power tube with an integrated 4S preamp. It's low on power but also low on price since it was built from some old tv repairman's stash and radio shack parts. This will probably be my work office setup as the Fender Greta isn't quite working the way I'd like it to with the old school Raymer Model 822 AM/FM Tuner. (The tuner's output is too high for Greta and with the volume all the way down, it's still almost too loud and can get a bit overdriven/distorted.)

All of the speaker components, sans wood, came from and I'd like to thank them! They don't have a store front per se, but more of an office and a warehouse in Madison, WI. One afternoon a couple years ago, I just stopped in and disrupted their lunch break to buy these parts. They were very helpful and were just as excited as I was at buying these parts. I highly recommend them if you need any sound equipment!

petegossett UltimaDork
4/11/16 10:35 a.m.

FYI I have a pretty good sized sheet of CBS-era Fender silver-face cloth. If anyone here needs it I'll make you a good deal.

RossD UltimaDork
4/11/16 10:47 a.m.

I still have quite a bit of the tweed stuff, so I'm good. Don't forget the Guitar thread, post it up there. Jumper K. Balls built a couple of nice looking amps, he might want it.

DrBoost UltimaDork
4/11/16 11:08 a.m.

Looks great! That pic of the glowing tubes might have pushed me over the edge. I REALLY want to build a tube amp now. I have my 5.25" Celestion drivers that are simply stellar and are 93 dB efficient. I need to measure the T/S parameters though since i have NO idea what they are.

DrBoost UltimaDork
4/11/16 11:12 a.m.

Oh yeah, I wanted to add that I'm glad my thread gave you the kick in the butt needed to keep going.

T.J. UltimaDork
4/11/16 11:28 a.m.

Good use of monkey E36 M3 (duct seal).

Looks like a fun project.

RossD UltimaDork
4/11/16 12:10 p.m.

Thanks for reading along fellas! I've got more tube amps to build and even less time coming! So don't hold your breathe for a follow up any time soon, but if you have tube amp building question or what not feel free to toss them up here.

Once my speakers loosen up hopefully I'll revisit this thread with a bit of a review.

stuart in mn
stuart in mn UltimaDork
4/11/16 1:07 p.m.

There is at least one place out there on the interwebs that sells speaker cloth for vintage applications...I used to have a link but it was on my old computer, I'll look around tonight to see if I can find it again.

RexSeven UberDork
4/11/16 1:50 p.m.

I love this kind of stuff. I've been getting into headphone hi-fi lately and I have a couple of amps (one tube amp, one hybrid tube/transistor). I don't have the skills to build one, but I will be watching this thread for inspiration in case I get the time to do it.

@RossD, I have a grip of NOS vacuum tubes. No 2A3s, but plenty of others (esp. 12AU7 and 12AT7). If you need some, let me know via PM!

VWguyBruce Dork
4/11/16 3:23 p.m.

Cool build/builds. Stuff that dreams are made of.

You, gentlemen, are making me lose focus on important things to lookup tube amp plans.

RossD UltimaDork
4/11/16 3:29 p.m.

Really, the only skill you need is a calm hand and some patience. I like to pick designs with low part count to keep it easier to make and it's cheaper! As for the soldering, I use a giant solder gun and hamfist most things. I have a solder pen for if I need to do something intricate or with a PCB, but I haven't needed that yet.

Here's my wiring of the 4S preamp:

And here's how the guy who designed it wired it:

I call it 3-D wiring.

It's a lot different than the way Fender's amp guys did their thing: They use a board with turrets or eyelets and will sometimes have components/wiring on both sides.

This is how I wired up the same amp:

Patience, calm-ish hand, and the ability to follow a schematic.

As for designs, I like the designs by Suncalc on He designed my 2A3 amp and the 4S preamp above.

Doc Brown
Doc Brown Dork
4/11/16 9:27 p.m.

This stuff has always like sorcery to me. The art of building tub amps is a magic I'll never fully understand.

Anyway, I spotted a solid state rectifier in the schematic and its got me wondering. Would a tube rectifier do the same thing? ... just curious

RossD UltimaDork
4/12/16 7:54 a.m.

The thing with silicon parts are they have very specific limits, and tubes have recommendations. So you can get away with excursions into the red for short periods of time with tubes and if you pass the limit with a component made from sand they get all smokey!

Doc Brown, if you're interested in tube rectifiers:

I can kind of do some load lines for the different stages of amplification, but I usually do something wrong and I really don't fully comprehend whats going on. I'm a Mechanical Engineer pretending what I remember from my college electrical engineering classes makes sense with tubes.

The forum is quite good at times, but the tube world seems to have a lot of black magic used in terms of 'golden ears' and 'this is the way they did it in 1922 so it's gotta be better than anything else'. That's why the gentleman that goes by Suncalc on the forum seems so relevant to me. He uses actual engineering and has an excellent grasp on whats happening and has the ability to show the math. I love math. He's the one that wrote the tube power supply article linked above. The other stand out, in my mind, is gofar99, who professionally designs amps and is on the forum too. He's designs use more silicon components and are usually more complicated.

Here's a repop Tube Manual:

Or a bunch of free pdfs:

paranoid_android74 SuperDork
4/12/16 9:30 p.m.

Wow, this is great stuff! Building a tube amp is still high on my electronics to build list. I have designs for both a guitar and a bass amp (Fender and Marshall clones respectively) but never did anything with them. I do still have a bass guitar project somewhere... Hmmm...

I'd also like to have one for the house, but put that off because of my (then smaller) kiddos.

Way cool stuff. If any of the Michiganders here need support staff for a build, I'm your huckleberry!

RossD UltimaDork
4/13/16 1:58 p.m.

My next build might be a Fender 5E3 Deluxe out of this little mono hifi amp.

But instead of having four inputs, I would use just one and mini toggle switches to determine which of the inputs are being used. That way I can us all of them which is something you couldn't do with just the original fender amp. And I don't even know if that's a good idea!

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