After the massive worldwide success of the Sortaflex preamp, I wanted to try something more like a ‘desktop’ unit that would function both as a tube bass preamp and a DI box. It uses an AC input and a single 12AU7 tube. This one has no EQ, but I like the way it sounds. On the other hand, since there isn’t any passive EQ stage to reduce the gain like in the Sortaflex, this one’s output is probably too hot to be used as a DI, and I had to use the input pad when I plugged the pre-transformer unbalanced output into a bass amp.
For this one, I burned my own circuit board. It’s just a simple two-stage amplifier with a 1 MegOhm pot controlling the input to the second stage.
Here is an action-packed video of this doodad:
This is the first time I’ve done an audio circuit with a ‘desktop’ layout instead of a pedalboard-type configuration. I was able to cannibalize a PCB, a power transformer, and a switch assembly from an older Sortaflex project, but there are a few things I didn’t think through too well, like the positioning of the LED. It’s running from a voltage divider ensconced on the rectifier board (square green one) way back by the power transformer. That’s fine for a pedal-type enclosure, but I wound up having to run the wires all the way up to the ‘front’. If I do any more of these, I might try working the rectifier section and the main section on to a single board.
Schematic, part layout, and image for etching the PCB:
If you look very, very closely at the photos, you might (or might not) notice that some of the resistor values aren’t the same as they’re marked on the schematic. I experimented with shorting R6 and R11, and finally settled on 470 Ohms. I got a stronger low-frequency response by increasing R13 from 220 kOhms to 1 MegOhm, which makes sense because R13 is in parallel with whatever the input impedance is of the output transformer (I still haven’t found that out). As I recall, everything else is the same, except that I forgot to wire up the unbalanced (1/4″) output after R14 before installing the board, so I wussed out and ran the unbalanced output from the junction of R13 and R14 because it was easier to reach.
From left, the controls are ‘pad’ (switch), 1/4″ input, LED, gain, XLR out, ground lift (switch, top), and unbalanced out (1/4″, bottom). The switch at the right looks like it has something to do with the 1/4″ output below it but it doesn’t – it’s connected to the XLR output. Something else I didn’t think through.
For more about the transformer, the ground wiring, and the components, see the Sortaflex project (scroll to the bottom). It’s mostly the same stuff except for the enclosure, which is a Hammond 1455B1601BK and costs a little more than the Hammond 1590D I used for the Sortaflex.
Last year (2019), I decided I wanted to build the preamp section of the Portaflex. I wasn’t super-strict about sticking to the original, but I did keep the EQ section more or less the same. Hence the name “Sortaflex”. What I came up with worked, but it had a lot of issues, so I set it aside for a while.
Usually I’m a hermit, but now that everyone else is too, I decided to take another shot at building a tube bass preamp that runs on AC power.
The Sortaflex has three controls: Volume, Bass, and Treble. Input is 1/4″. Outputs are unbalanced 1/4″ and balanced XLR with a ground-lift switch.
Layouts: The two PDFs below contain layouts for the two Sortaflex boards. The first one is for the main board and the second one is for the power (RC-filter) section. Each component on the layout diagrams corresponds to the components in the schematic diagrams above.
Layout diagram: I seem to have misplaced it but I’ll post it here as soon as I find it, I promise.
Update 8/30/2023: Never mind about the layout diagram. I don’t know what I did with it.
The main differences between this gizmo and the venerable Ampeg Portaflex are:
12AU7/12AX7 nine-pin tube instead of a 6SN7/6SL7 octal tube. Most of the fliptop schematics I’ve seen specify 6SL7 for the preamp tubes; one showed 6SL7s. I used a nine-pin tube here to make it easier to stand the PCB up and stuff everything into a Hammond 1590D enclosure.
Different plate resistor values (R3 and R14 in the schematic) and a different biasing voltage. (See the last paragraph in the ‘About the XLR Output’ section below for more of me complaining about my own failures). R3 and R14 could be tweaked for different gain and different impedance, but since the output signal is pretty hot to begin with, I’ve tried to keep the overall gain down.
Addition of capacitor on cathode of V1A. Can be omitted. See schematic.
No phase inverter or power stage (because it’s a preamp). More about this below.
68k grid-stop resistor at the grid of V1B. I’m not sure why I thought that should be there, but it’s part of the 2013 project as well. Maybe it’s a holdover from a previous guitar-amp project.
The volume control is after the Baxandall EQ and before the V1B input. In a typical fliptop amp, the volume control would be just before the phase inverter (from what I’ve been able to learn from various schematics).
Different values for the coupling capacitors. I used mostly big WIMAs and small WIMAs. Most of mine are 1uF, but “canonical” values (22nF) could be used instead.
So, does the Sortaflex sound like a Portaflex? Sort of. Maybe some… or not. Using a different tube probably makes it apples to oranges by default.
And it’s just the preamp section. Although I can’t prove it, I’ve always thought that the phase inverter stage of the Portaflex contributes to a quality in its sound that I have trouble describing: strech-y? Rubber-band-y? Slightly compressed? Something in the midrange that I can’t quantify.
Update 8/30/2023: I’ve concluded that it doesn’t sound like a Portaflex. The EQ is close but the unique sound of the B-15 just isn’t there. Maybe I should rename it the Notaflex.
Another update 8/30/2023: Tube Depot doesn’t sell the power transformer I used for this project and that’s quite unfortunate for me because the 1) that PT was perfect because it sourced only about 5mA from the primary and something like 300 or 400 mA from the heater taps, which is perfect for a single-tube project. If I ever build any more doodads like this, I’ll have to go on a hunting expedition for a suitable replacement.
dimensions of that PT were perfect for a Hammond 1590D.
What I like most about the Sortaflex:
With a 12AU7, it’s got plenty of clean headroom.
The low-end response is respectable and the EQ is wide and responsive.
I’m not hearing any AC hum – it’s quiet as a church mouse.
The balanced (XLR) output sounds pretty good from what I can tell.
It looks cool in the dark.
What I like least about the Sortaflex:
It’s big and heavy, and seems like overkill for a preamp.
It’s f***ing expensive to make.
About the Circuit Boards
The PCBs are one-sided so that they can be etching to copper clad board, but I had some made because I’m insane in the membrane. They are 0.062″ with a 1-oz. copper thickness, not the thicker kind (0.094 or 0.125 with 2-oz. copper thickness) that one often sees in conjunction with tube amps. I made the traces nice and wide though – most of them are 0.04″, and 0.07″ for the ground traces on the RC-filter board. If power tubes had been involved, I probably wouldn’t have used PCBs at all. But so far I haven’t had anything go wrong with either of them.
The small holes in the boards are for the screws that hold the standoff posts in place. On the back sides, you’ll see clearance around the groundplane so that the standoffs are isolated from any traces. On the main board, the big hole toward the bottom allows the pair of filament wires from the power transformer (the thick green twisted ones in the photos above) to pass through and solder to the heater connections from the back.
About the XLR Output
The balanced XLR output is achieved by a 12:1 Crimson Audio CT-121 di transformer (http://www.crimsonaudiotransformers.com), the cost of which (about $50 US) is somewhat out of proportion with a typical D.I.Y. project’s budget. I had an extra one because I bought two for another project. I’m quite pleased with the result.
The CT-121 di has an 11.5:1 (primary to secondary) ratio. I don’t know what its input impedance is but it seems to be similar to (but less expensive than) a Jensen DT-DB-E, which is 12:1 with a 140 kOhm input impedance. For a version with no balanced output, the PCB could be populated without the transformer section, by omitting C11, R18-R23, C12, C13, the transformer, the XLR jack, and the ground-lift switch (and C14 and R24).
Incidentally, I don’t remember why I thought the value of R18 should be so high (220 kOhms – see schematic).
I should have thought about what would happen when the tube and the Mosfet buffer are tied to the same B+. If I ever build another version of this, I’ll try separating them with an RC stage. Without the Mosfet buffer, the B+ sits at around 200 Vdc and the voltage at the plates of the tube is around 125-130 Vdc, which is nice. But with the tube and the Mosfet buffer tied together, the B+ is only about 100 Vdc and the tube plate voltage is around 70 Vdc. In the future, I might try changing the resistor values in the buffer.
About the Ground Wiring
Since this is a high-voltage project, I tried to use star grounding even though it’s just a glorified pedal. That’s why you see so many thick black wires connected to the RC-filter PCB (see photos above).
For the RC filter board, the diode rectifier, the filtering capacitor that follows it (C1), the next RC filter stage (R1 and C2), and the voltage divider for the LED all have their own own ground wires (see schematic and photos above). They run to the terminal strip along with the ground wires for the power transformer’s filament supply and the tube circuit board. The two articles referenced above will explain why such a seemingly fussy approach can be desirable. It’s worked well for me when I build tube amps. Or what I should say is: I always use this approach for tube amp projects –> I usually get very little hum noise in said projects –> I’m superstitious.
By the way, referring to the two photos above, the purple and orange wires extending from the PCB are for the bright blue LED. A voltage divider prevents the LED from being fried (see schematic above).
About the Components
The tube in the videos is a JJ ECC82 / 12AU7. I’ve also tried an Electro-Harmonix 12AU7 and a Tung-Sol 12AU7, with similar results, and 12AX7s from Electro-Harmonix and Tung-Sol, which yielded much higher gain. That could be nice if you want tube bass fuzz.
The power transformer SKU is TR-PW-13. I get them from TubeDepot. Their website has a data sheet for it. It can drive a single tube, with ~= 200VDC/5mA at the secondaries (not center-tapped) and 6.3V/300mA for the filaments (center-tapped).
The RC-filter board uses Nichicon electrolytic capacitors rated at 450 Vdc for smoothing the oscillating AC into a useable DC voltage. They’re pricey but I like them. I get them from TubeDepot or Amplified Parts.
The AC switch assembly came from someone who sells on Amazon. There are many similar listings but not all are appropriate for this project. The fuse is worked into the unit. Separate components for the plug, the switch, and the fuse would also suffice. This sort of mousetrap should always be fused. And there should always be an Earth ground, distinct from any other ground points (see schematic and photos).
Most of the resistors are Vishay 2-Watt metal-film. I like them better than 1/2-Watt. I get them from Mouser.
Most of the nonpolar capacitors are WIMA, also from Mouser. The larger ones are in positions where a higher DC tolerance (> 100V) is required.
The tube socket is from TubeDepot, SKU SK-9PINPC2.
The enclosure is a Hammond 1590D.
For wiring, I prefer the stranded, tin plated, aerospace-grade tefzel wire I get from TubeDepot.com, just in case someone wants to take one of my homemade preamps to Mars.
Most of my PCB standoff posts come from Angela Instruments. The RC-filter PCB has 1/4-inch standoffs and the ones for the main board are 1-1/2 inches, to make room for the pots and the 1/4″ jacks.
I’ve always wanted to build a one-knob guitar amp, so I did. It’s another Champ/Princeton clone without the tone control. 1 x 12AX7, 1 x 6L6 (not 6V6!), ‘canonical’ 5Y3 tube rectifier. The main lesson I learned was that a one-knob guitar amp is cooler in theory than it is in practice, and that adding minimal tone-shaping (in other words, a ‘High’ or ‘Bright’ rolloff) is worth the (minimal) effort.
On the other hand, it was interesting to try to balance the output for a range of instruments (guitars). I used a Telecaster, an SG, a Strat, and a Les Paul with custom pickups when I was deciding what values to ‘hard-code’ for bypass capacitors and whatnot (see C2 and C5 in the schematic below). The video was done with just a Tele because I was too lazy to include anything else.
For questions and comments, email firstname.lastname@example.org
Here is an action-packed video of the ‘Uno Knobbo’:
And here is the schematic:
Uno Knobbo schematic
Note: the schematic shows the feedback resistors connected between the output jacks and the cathode of the second preamp stage. I thought the result was muddy so I disconnected them for the video above.
Also, the schematic specifies a 6V6 as the power tube but I used a 6L6 instead, because I thought it was a little more articulate at high gain levels. As I recall, I used the 5k tap of the output transformer for the 6L6 (brown wire), instead of the 8k tap (red wire).
What I came out with this time was fiddly and awkward because I didn’t think the layout through as well as I could have. A better layout for this sort of 5-Watt clone would be the the previous project referenced above. If I do anything like this again and I use one of those Hammond Enclosures again, I’ll probably use that layout, with separate, smaller turret boards. Here is the layout for good measure:
Uno Knobbo layout: not my best effort
I tried the Uno Knobbo with several speakers and I like the Eminence Lil’ Texas the best, maybe because they rolled off the piercing high end and the two Jensen speaker I tried didn’t. All the speakers I tried were 12″ because I don’t have any 10’s.
By the way, if the enclosure looks weird, it’s because I tried doing my own powder coating on it, with limited success. That was fun though.
Also, I stuck a TH Audio badge to it because I had a few lying around – I’m not planning to sell these (yet) under my side business.
(Public service announcement: comment are disabled because I got tired of the endless spam. For questions and comments, email email@example.com.)
Here are two videos of the Spirit of 86. The first one is done with a Fender Telecaster and the other is with a Gibson SG. Scroll down for photos and an erudite philosophical discussion.
This is the Spirit of 86… or as I like to call it, the Buttrock Express. It’s like a little shrine built around an EF86 tube (https://en.wikipedia.org/wiki/EF86). A couple of years ago, I started wondering why I knew of a number of amps that use an EF86 as a first preamp stage, but I’d never heard of any amps that use a preceding tube to drive an EF86 (of course, that doesn’t mean there aren’t any). I thought it might have been because it would just sound horrible or because resonance and microphonics would get the best of the EF86, but I really wanted to find out. So after dawdling around for several months, I put one together last year, with parts and components I happened to have handy, and I tweaked it every few weeks until a few days ago when I decided to immortalize it in a blog post and stop fiddling around with it.
To the right are a schematic (above) and a diagram for a possible layout which is close to the one I used for the amp. Of course it’s not the only possible layout, and the positioning of the elevated heater supply (upper right) is arbitrary – I wound up kludging it onto the end of the turret board.
In the layout, the ground connections (thick blue lines) are an approximation, but most of the points that connect to a specific location such as the from the Gain control to the turret board are drawn that way for a reason – to reduce noise – because this project was probably the most noise-prone one project I’ve done to date. The biggest problem was AC hum. Randall Aiken has a good article about grounding in tube amps and why it’s not always good to perceive the ground as a sort of uniform field:
… and I try to do whatever the voices in Randall Aiken’s head tell me to because it almost always pays off. Side note: I tried grounding the EF86 tube to a couple of different points on the turret board instead of running the ground back to the star. I really thought it would make a difference in noise level but didn’t, to my ears anyway.
This project has been interesting and educational for me, but I still can’t decide if I like the way it sounds or not. The overdrive is aggressive and “gnarly” for lack of a better term (which makes sense because the EF86 is a pentode), but it doesn’t have a lot of sustain or the “squishiness” of cascaded triode stages. I suppose that might be a good thing for articulation and clarity if this amp were being played by someone with better guitar skills than I possess.
The Buttrock Express might serve as a template for an overdrive channel at a future date. Getting a nice loud clear clean tone out of it is an exercise in futility.
The Thomas the Tank Engine sticker on the front face is there to cover a drill hole that I would up not using. The TH Audio badge is there because I had some extras – this is just an experiment, not something I’ll be offering through TH Audio (my side business for effect pedals and, eventually, amplifiers).
Controls are, from left: Bass, Mid, Treble, Gain, Bright, and Master Volume. The Bass/Mid/Treble controls are a fairly standard Marshall-style 3-band EQ positioned after a cathode follower. The EQ stage uses a couple of unusual values: 68k for R7 (see schematic) and 250k log for the Bass pot (instead of 500k or 1M, to counteract the mounds of low frequencies that pile up). The Bright and Master controls are connected near the input to the power tubes, like one might see in a Vox AC15.
The amp dishes about 20 Watts, with a 6.6 kOhm output transformer and a 6L6 push-pull pair. At 20 Watts, the 6L6’s aren’t doing much, but I keep them there because a) the OT I’m using is a nice match for them (better than the 4k OT I was using previously, b) a 6V6 pair sounded glassy and midrange-y, and c) I wanted to hear the EF86 overdrive without an extra layer of power-tube breakup.
I used a tube rectifier instead of some diodes because… well, because I could, I guess… although the tube rectifier might be introducing a little ‘sag’ that mellows the general gnarliness of the pentode overdrive. To be honest, I don’t know if it does or not.
For this project, I used Tung-Sol tubes because… well, because I already had some. I’ve swapped out the Tung-Sol set with an Electro-Harmonix set, and I don’t hear much difference at all. In either case, it wasn’t difficult to get something close to 90V at the plate of the EF86, once I figured out the voltage drops. Normally, I like JJ tubes and have used them in a number of projects, but to my ears, the JJ EF86 sounds weak in this particular project (I tried two different ones).
One last thing: this amp is *not* a clone of the Dr. Z Route 66. The Route 66 uses a single EF86 into a passive EQ section, to a phase inverter and a fixed-biased KT66 push-pull pair. The Spirit of 86 uses 12AX7 -> passive EQ section -> EF86, into a phase inverter and a cathode-biased 6L6 pair (which is hardly doing any work).
This is more or less a clone of the head section of the Ampeg B15n, known as the Portaflex or “fliptop.” I’ve been wanting to try building a tube bass amp for a while and I’ve been curious to find out why fliptops sound the way they do.
Below is an exciting video documenting the tail end of this build. Please keep in mind that I’m not exactly Tony Levin.
My fliptop interpretation preserves most of the “canonical” aspects of the original(s), like the extremely high input impedance (5.6M Ohms), the Baxandall tonestack, -50V fixed-bias (*not* cathode-bias!) and the unusual phase-inverter configuration. But this one sports a few departures from the original:
1. The instrument input uses a decoupling capacitor (22nF) before the biasing resistor of the first preamp grid input.
2. The resistor and capacitor values for the DC filtering stages aren’t anywhere close to the original(s) – I used 50uF can caps because I had some available. The difference in the filtering stages probably changes the whole sound but I don’t have a way to find out because I don’t have access to a real fliptop. I’ve seen some B15n schematics specifying diodes for rectification and others specifying tube rectification. Mine uses diodes. I wound up with about 465V at the plates of the power tubes, which might be a little too high, but I haven’t done a round of tweaking lately.
3. My version has one channel instead of two like most others do, because I wanted to keep the interior as uncluttered as possible for the inevitable 5 million little adjustments I made.
4. I used a great big filter choke rated for 500 Volts, because I could. I found that a choke helped reduce noise and hum.
5. I always try to use elevated heater supplies (see schematic), even for push-pull amps. In this case, the filament divider resistors are tied to the -50V bias (see schematic). That’s one way to deal with 60Hz/120Hz hum from 6SL7/6SN7 tubes. I guess this doesn’t qualify as a “departure from the original” but it’s how I got rid of most of the AC hum, along with isolating the ground lines for the filter capacitors from the other grounds and busses, like I should have in the first place.
6. I found that I could coax more clean headroom out of the unit by using 6SN7 tubes for the preamp stages instead of 6SL7 (think: 12AU7 vs. 12AX7, sort of). For bass, I like clean headroom. I looked at the signal from input to output with an oscilloscope and found that clipping was most likely to happen first in the preamp stage. That can sound nice, but for this unit, it sounded awkward and nasty, to me anyway. Swapping 6SL7′s (higher gain) for 6SN7′s (lower gain) reduced the strength of the preamp signal but gave more headroom before the nastiness kicked in. The drawback for 6SN7′s is that they demand twice as much filament current as 6SL’s (~=600mA instead of ~=300mA) but the power transformer is quite capable of handling that increase (see schematic).
My version puts out somewhere around 30 Watts at the most, from what I can tell. I used a Classictone 40-Watt output transformer, so I don’t want to try to push the output higher than it already is. To be honest, I’m guessing at the Wattage, assuming about 20% or 25% power loss for the push-pull output transformer.
The tubes are all Tung-Sol. I tried a pair of Tung-Sol 7581′s for the power tubes, and a pair of JJ 6L6GC’s, but I liked the Tung-Sol 6L6GC-STR’s for their relatively wide frequency response.
This is my first single-ended amp build. It puts out about 5 Watts, which is plenty loud, but not bonecrushing. It’s based on the Fender 5f2 “Princeton” circuit, which is quite similar to the 5f1 “Champ,” the main difference being that the 5f2 has a tone control. This is not a kit – the chassis, cage, and filter choke are from Hammond and I kind of winged it with the drilling. The chassis is aluminum and the cage is steel. The transformers are from Classictone and the tubes are JJ.
Now that I’ve done my own Princeton/Champ clone, I can see and hear why so many people like having at least one of these around.
If you see anything I’ve said here that might be inaccurate or wrong, please let me know so I can correct it.
Proof that this doodad actually works:
Original schematic… or at least, the one I see all over the place.
Update 8/30/2023: It still works, after ten years. I extracted it from a closet yesterday and hooked it up to my Dr. Z 1-12 cabinet, and it sounds fine.
Another update 8/20/2023: I’m really sad that Classictone went out of business because I used to get most of my transformers from them.
Below are the schematic and a possible layout for this build. The layout diagram is close to what I actually built but not exact, and it’s not to scale – it just shows where things can go for minimal wire-crossing and decent ground distribution.
Take the layout diagram with a grain of salt – the resistors that connect directly to the grid inputs of the tubes (R2, R5, and R10) really should be soldered directly to the socket pins, with as little exposed lead as possible. Also, the cathode resistor for the power tube (R11) probably should be rated > 3W.
Since I’ve never gotten my grubby hands on an original 5f2 or 5e2, everything I’ve learned has come from schematics, sound samples, video clips, other people’s blog posts, and discussion boards. But from what I do know, the main differences between this build and a ‘canonical’ 5f2/5e2 are:
1. Different configuration and location for the ‘tone’ control. I put ‘tone’ in quotes because in my version, it’s just a low-pass filter pretty much like the one in a Rat pedal. For this build, ‘tone’ is situated after the second triode just before the power stage, as a 10n capacitor to ground and a 250k pot.
2. Larger values for the B+ filter capacitors. The 5f2 used ~= 8uF whereas this build uses a 10uF after the rectifier, and 47uF for B+2 and B+3.
3. 6L6 power tube instead of a 6V6. I connected pin 3 of the octal socket to the 5k tap (brown lead) to make it more 6L6-friendly (closer to 4-5k impedance). Before that, I used the red lead of the OT for a 6V6, for about 8k impedance. Personally, I like the sound of the 6L6 a little bit better – it was more… smooth, maybe? But I realize that using a 6L6 for a power tube in a 5-Watt amp is severe overkill, since it draws a lot more heater current than a 6V6.
4. The 5f2 had a feedback resistor between the secondary side of the output transformer and the cathode of the second triode stage. This build doesn’t. Originally I had one but I thought it made single-coil pickups sound too harsh so I got rid of it.
5. This build has a (relatively) large bypass capacitor in parallel with the plate resistor of the first triode stage. It rolls off high frequencies and helps to alleviate the ‘icepick’ effect at high volumes.
6. This build has a 220k grid-stop resistor at the input to the second triode stage of the preamp. Like the cap mentioned in #5, it helps to roll off high frequencies.
7. This build uses an elevated heater supply (see schematic) with a DC offset of about 45 Volts for the filaments, to reduce AC hum.
8. This build has separate outputs for 16, 8, and 4 Ohms. The 5f2 usually had a single output.
I’m sure there are other differences I’ve forgotten to list. There is a little AC hum but it’s quickly overpowered as the volume knob goes up. Overall, the build is a little messy – I didn’t really know how it would go together when I started it, and I made a zillion little tweaks to it. I left the transformer leads a little bit long in case I reuse the transformers for other projects. Next time, I’ll use a chassis that’s higher than 2 inches, and a little wider for better component spacing – the choke just barely fits. Near as I can tell, I’m getting about 5 Watts at the output (assuming 50% loss for a single-eneded output transformer). That’s plenty of cowbell for just sitting around playing.
Like the original Champ, this build does not use a master volume control. That means the power stage is always running near full boil. As the Gain knob goes clockwise, the volume increases, but so does the clipping.
The Classictone output transformer I used (40-18031) has two different leads for 5k and 8k primary impedances. I’m using 5k here for a 6L6. 5k would also work for an EL84. 8k would be good for a 6V6.
Above, from left: 12AX7 preamp tube, 6L6 power tube, 5Y3 rectifier. All the tubes are JJ. I’ve been getting good results with JJ tubes lately, especially the JJ 6L6. The ‘sag’ from the 5Y3 rectifier is really obvious in the sound of the amp: to me, it’s like the signal is hitting a rubber wall when I lay into the strings. The knobs, from left, are volume and tone. There is no ‘master’ level control. The volume pot controls the strength of the signal from the plate of the first tridoe stage in the preamp to the grid of the second triode stage.
Below are photos of the interior. It’s pretty obvious that I had some issues with the layout.
For this build, I tried ‘floating’ the filament wiring (green twisted wires) instead of having them hug the chassis.
As of April 18 2012, my first D.I.Y. tube guitar amp appears to be functioning normally. I made a huge mess and a lot of mistakes with it but it came out fairly well for a first try, and was definitely worth the time and expense because I learned quite a bit.
Here is a schematic that’s more or less like what I made, which is based on the AX84 designs (see ax84.com, a website for tube-amp enthusiasts). This schematic is incomplete and has some mistakes in it – the tonestack is not represented and the filament supply of the Hammond 270FX power transformer should marked 5 Amps, not 3 Amps. The transformer, a Hammond 1650F in this case, is matched better to a pair of 6L6’s than to EL34’s, and the poweramp section isn’t right for a pair of 6V6’s, which start red-plating right away.
See below for why the tonestack is left out. If you want to see tonestack schematics you can probably bring up 500 of them in Google in about 20 seconds.
The vertically-mounted perfboard is for the the power tubes’ negative bias voltage, which I wound up not using because I opted to cathode-bias them instead.
One of the dumbest things I did was to cram most of the DC filter capacitors onto the turret board. I should have used dual can caps for all the filter-cap stages instead of just the first two, so all the ground lines could be run straight back to the star ground.
Since the poweramp is cathode-biased, I don’t have to adjust a negative supply voltage every time I change tubes. I’ve been able to load EL34’s, 6L6’s, and 6V6’s, all with very distinctive results, although I can’t keep using 6V6’s because they overheat in this circuit.
The EQ is not a standard 3-band stack. The bass control is just a high-pass filter, the mid is a twin-T notch, the treble is a low-pass filter. And they’re wired in series, respectively. I’m not thrilled with the EQ but a standard 3-band Fender-style EQ sounded awful with the overdrive stage, probably because of impedance mismatches and certain sections I didn’t wire up all that well.
The tubes in the photos are 6V6’s but I stopped using them because they red-plate (overheat). I used EL34’s for the videos but I’ve been using a 6L6 pair lately because they give me a better low-end response than the EL34’s. That isn’t surprising since the impedance of the output transormer (about 7.7kOhms) is way out of whack with an EL34 pair would expect (3-4kOhms would be better). I’ve got the amp wired for a single 16-Ohm output because at this point all I’ve got is a single 16-Ohm speaker.