It's summer, time for a road trip. See you in a week or two.
A warm evening in summer, the heady scent of the Star Gazer Lily wafting upon the soft breeze, a quiet secluded corner of the garden... it's inevitable that thoughts and hearts turn towards love.
Crickets will sing their mating songs, nesting sparrows will snuggle a little closer, a young couple will hold hands and talk in hushed whispers about the future. And yes, even slugs would hold hands, if they had any to hold.
In Part 2, we learned that it may be possible for someone with only a basic Neolithic skill level and a shaky grasp on electronic theory and reality (me), armed with the tools on hand in a typical cave dwelling home workshop, to assemble a guitar amp out of mostly scrap and junk components. But - does it even work? And will it sound better than it looks? This time, let's do some in-circuit voltage measurements, and do a short analysis of that data, as well as finally taking this beat up jalopy out for a spin.
Editor's note: the events depicted in this post are not in actual time sequence; in reality, we made sure the amp worked first, before reading voltages.
Once again, here's the schematic, as built, of this particular amp, with voltage measurements annotated. I should apologize for my sloppy schematic drawing skills, but 5 minutes with a pen beats an hour+ at a CAD program:
Okay, let's see what's going on in the output section. With an old Sylvania 6V6GTA plugged into the octal tube socket and a Sovtek 12AX7WA in the 9-pin socket, an 8 ohm speaker hooked up and the input shorted (normal condition with nothing plugged in), volume control zeroed, and almost exactly 120VAC at the wall outlet, we see 17.2VDC at the 6V6 cathode (pin 8), and 317VDC at the anode (plate, pin 3).
Let's do some simple math, using inversions of Ohm's Law: 17.2V divided by the measured actual resistance of the nominal 470 ohm cathode resistor, in this case 452 ohms, results in a calculated current draw across that resistor of .038A, or 38 milliamps. Now let's take the calculated voltage differential across this (tube) device, which is 299.8V (317V plate voltage, minus 17.2V at the cathode), and multiply that by .038A, and we come up with 11.39W, or about 11 and a half watts of plate dissipation at idle, which is roughly 80% of design rated maximum for a 6V6GTA (14W).
Plugging in a Fender Sylvania STR 6L6GC instead of a 6V6GTA results in 21.2V at pin 8, which gives us 13.9W plate dissipation, which is a lot less than that tube's 30W design max. In an ideal world, we should be able to up the output with a smaller value cathode bias resistor, but that presupposes a power transformer that would actually supply that extra juice. Although I have no spec sheet to support this, I suspect that the small-ish power transformer we have wouldn't support much higher of a current draw, no matter how much we fiddle with the cathode resistor value.
Finally, looking at the 12AX7 preamp tube, 1.4V and 1.5V at the cathodes of each triode section is pretty much right on target, well within this tube type's design operating parameters. And anything close to 200V at the plates of the 12AX7 double triode, in this case 191V, gets us to a good balance between gain, vintage tone, and headroom.
Well, simple or not, that's about enough math - time for another picture:
Sometimes, after a repair or mod, I'll flip the switch, and... nothing. After tracing through the circuit, it might turn out that the Heisenberg Compensator had been soldered to the Relative Dimensional Stabilizer - what a boneheaded move! This time though, I got lucky - it worked straight away.
When the assembly was done, I plugged the 99 Cent Champ to AC through the handy home-made light bulb current limiter. The bulb's glowing dimly, good, and there's that nice low vibration coming from the power transformer, also good, since it means that the transformer is working. Shut it down, pop a couple tubes in, plug in a cheap test bench speaker, breathe deeply, and flip the switch again. After about 5 seconds, here comes a really faint hiss from the speaker - alright!
After letting it sit powered on for awhile to see if anything pops, explodes, or oozes, it's time to turn it off and then see how it sounds as an actual amplifier. I hooked up a '70s Oxford/Fender 12" in an open back cab, plugged in a '91 MIM Strat equipped with Classic Vibe pickups, flipped up the power switch, let the tubes warm up again, and cranked the volume pot. Not only did it work, but it worked well, and sounded... a lot like a Champ Amp. Well actually, a lot like a Champ through a 12" speaker.
In Part 1, I'd written that a Champ has a tone that is "clean, sweet and chimey at lower volumes, and progressively becomes more raw and edgy as you turn up the knob, finally topping out at a roaring overdrive, all at living room, bedroom, or studio control room levels", and that's exactly how this junkyard beast sounds. Playing a Strat with vintage style lower output pickups, the 3-way selector jiggled into a "between" switch position, and the amp volume pot cranked, gets a pretty close approximation of a Tweed Champ "Layla" kind of tone, especially if you get your ear close to the speaker cone and pretend you're one of Tom Dowd's Neumann mics.
With a guitar equipped with humbuckers or P90s, like my '77 Les Paul TV Special, the tone is every bit as clean and chimey at lower volume, but the higher pickup output results in a nice smooth singing distortion when the volume is dimed. Depending upon the speaker(s) used, it can sound Neil Young-ish or very like a Marshall - plug up to a Celestion or a "C"-clone in a closed back cab, and it's close. After all, Marshalls have the same basic preamp first gain stage, with only slight differences in component values.
I experimented with having the feedback loop disconnected (22K resistor from the speaker output to the cathode of the 2nd triode), and although the amp was only a very slight bit louder, it was also more distorted and harsher sounding at all volume levels, so I hooked the feedback loop back up. I may eventually put in a switch to have the option of that other sound.
Some Champ-type circuits have a cathode bypass capacitor on the 2nd stage; with one in place, the 99 Cent Champ had an almost entirely different character. The range of clean tones at the lower end of the volume control's sweep was very limited; there seemed to be a lot more gain, and it went into overdrive a lot quicker, along with a noticeably higher overall output volume. The OD tone tended to be harsh and not very pleasing, although that sound may appeal to some players. Putting a sizable grid stopper resistor just before the 2nd triode would most likely sweeten things up, but I just took the cap out instead and went back to a more pleasing tone.
As expected, when a 6L6 was substituted, the maximum volume level was higher than it was with a 6V6, but not by very much. Tone wise, it was a bit cleaner sounding, with noticeably less overdrive when dimed. There also seemed to be more of a warmer tonality, but that may be due to the sound of a Fender Sylvania STR; I'll try out other 6L6s in the near future, and see how they work out. (note: a Russian Mil-Spec 6P3S-E, known in this country as a Sovtek 5881, sounds f***ing fantastic):
In any case, a vintage 6L6GC in this circuit is running at less than half of its design maximum current, and more than 200 volts less than max plate voltage, so it's just loafing along, taking it easy, and that's the way it sounds - smooth, non-edgy, and overall more Hi-Fi than a 6V6. And, being so far from its stress point, a 6L6 should last darn near forever here.
Overall, I'm pretty happy with how this junkyard Champ turned out; some circuit mods worked, others didn't. In the end, it really is a sweet sounding amp - quiet and almost noiseless at lower volumes (thanks to short wiring runs and a choke in the power section), with a rich full bodied tonality, as well as having the ability to go through various stages of overdrive with a twist of the volume knob. Alternatively, you can just turn it up all the way, and go from clean to mean right at your guitar's volume pot - nice! Of course, it's not really a loud amp, probably no more than 4 watts output, but it's nice to get some real, actual, and honest tube OD sounds at lower than face-melting volumes.
Like I said earlier, I just got lucky; the components which had unknown operating conditions (like the transformers - did they even work?) all turned out to be okay. I will take no credit for anything here: basically, all the hard thinking and heavy lifting was done a half century or more ago by tube amp circuit designers who really knew what they were doing. And I am, like everyone else these days, only following in their genius footsteps.
In the near future, hopefully within a couple of weeks, we'll get around to adapting an old Kodak 8mm film projector speaker box into a cab for the 99 Cent Champ. Also, by that time, we'll have been able to see how this amp sounds with different preamp and output tubes, as well as a variety of speakers, old and new(ish). And that's not all: in response to some requests, I've promised an exclusive interview with the little rubber lobster.
And now let's do a safety disclaimer: As tube amp guru Gerald Weber is fond of saying - and here I'm just paraphrasing, it's not an actual quote: "The voltages and current levels inside even the smallest tube amplifiers can be LETHAL; take all necessary safety precautions, and if you aren't comfortable poking around inside a live circuit, then just don't do it."
Finally, I'd like to say that if anything I've written seems wrong, or if my math or even my assumptions feel like they're based on a dubious grasp on reality, let me know. I welcome your comments; as I've said, I'm only a hobbyist having fun, and I'm always willing to learn something new.
When you walk up to the edge of the gorge at Peter Skene Ogden Park, you can see just how hazardous it is. It's several hundred feet, straight down a sheer vertical cliff face, with just a low stone wall between you and eternity.
In Part 1, we looked at the classic Champ Amp circuit, and set up a project goal of putting together a guitar amp similar to a Champ, using nothing but dead junkyard parts - a hare brained scheme worthy of young Dr. Frederick Frankenstein himself. Maybe it should be called the Abby Normal Amp?
But questions remain: how does a pile of trash heap parts get turned into an amp, how will this stitched together beast sound, and what's the purpose of the little rubber lobster anyway? To find out, please read on.
An Old Piece Of Scrap
The copper plated chassis came with holes pre-punched for three 9-pin tube sockets, as well as a bunch of other holes of various sizes, and a huge gaping rectangular cut out that had to be filled; I used a scrap piece of sheet aluminum and screwed it into place. Some new holes had to be drilled for the wires to and from the output transformer and choke, and one of the socket holes was enlarged to fit the octal output tube socket. I used as many of the existing holes as possible when bolting up the transformers and terminal strips, and to a certain extent that determined the component layout.
You can see that there are a couple of empty holes on the front panel; I had planned on including a tone control and make the amp closer to a 5F2A Princeton, but just a volume knob is enough, really. Eventually, there might be a pilot lamp, but so far I haven't scored one for the right price - nothing. Besides, until I can put the amp into a cab, the tubes themselves indicate when the amp is powered on; not to mention the power switch flipped up - duh.
Now, a few words about the old-school wiring method I used. Here's a photo of the inside of a mid '50s Conn organ amp:
Most all electronic equipment from the classic period was hooked up using what I call "true" point to point wiring. The resistors, capacitors and transformer leads themselves are used whenever possible as the direct signal paths between tube sockets, pots, and jacks, with as little additional wiring as posssible.
All Fender amps made until the early '90s used a different wiring scheme, using turret or eyelet component boards to hold the smaller parts, plus interconnecting wires running to and from the board and other components: jacks, pots, transformers, tube sockets, etc. Today, that layout method is referred to as being "point to point", although at the time those amps were being produced, it wasn't called that at all - it was known as "circuit board construction". That technique was a brilliant manufacturing expediency; amps could now be made in record time, and quality control was much easier, but it can and usually does add a lot of additional wiring length. This requires careful attention to "lead dress", which usually means keeping those wires as short as possible, and squashing it down close to the chassis, in order to avoid noise buildup in general, and sometimes weird audio artifacts and oscillations.
Interestingly, modern printed circuit (PC) board construction, which most tube amp hobbyists dislike, can have much shorter signal paths than the older Fender style circuits, and so can be a lot more quiet and hum-free, as well as keep manufacturing costs down. That's the reason why all mass produced amps manufactured today are made using PC boards; most sound just fine, and some of them sound great. However, PC board construction is a repairman's nightmare, compared to earlier point to point wiring, which is very straightforward to troubleshoot.
The art of planning out a 1940s style "true" point to point wiring scheme in advance is something that is beyond my abilities, but I do something close by using a Zen-like technique: not thinking too much, putting a few terminal strips in likely places, and just start lacing in parts and soldering it all up, while following the schematic. Sometimes, beer and loco weed help out in the process.
This results in what looks like a chaotic mess, similar to the inside of my brain. But look inside any old radio, organ amp, or military communications set and be amazed it even works (!), compared to the well planned out, logical and easy to follow layout of the typical 1950s to '80s Fender amp with their eyelet board construction. The upside of true point to point, though, is a noticeably reduced noise floor and less AC induced hum, especially when using isolated jacks and shielded wire at the inputs, neither of which Fender bothered with.
Enough talk; here's some photos of the build in progress (no snickering in the back row about the wiring job - I'm just a hobbyist. "Dammit", Jim said, "I'm a hobbyist, not a doctor!" Apologies to the late great DeForest Kelley):
That piece of 12 gauge solid copper wire (one of the conductors from a piece of 12-3 Romex cable), with the two right angle bends in it is the ground bus bar flying above and through the inside of the amp. Using a ground bus is an old grounding technique from the days when almost all electronics was true point to point wired. Attached at only one point on the chassis, in a place away from the audio input, and with as many of the circuit ground connections as possible made directly to it, the bus helps to ensure the reduction of hum caused by multiple paths to ground, and the resultant ground loops.
A New Home
Here's the funky old speaker cabinet, a Kodak film projector cab, that the 99 Cent Champ will eventually go into:
Next time in Part 3, we'll measure some in-circuit voltages and run the plate dissipation numbers, and do a subjective tone analysis, ie: how does it sound, anyway? Stay tuned.
At a recent party, I mentioned I was starting a blog, and someone said, "I wish you success with it!" Success? I hadn't thought about that. I thought I was here to have fun; in fact, where I live, fun is a minimum daily requirement (see above).
Makes me think about those folks who can't have a hobby without turning it into work: "Between my vocation and my avocation, I don't have time to take a vacation." Or all the poor souls bombing past me on the river path who have to make every bike ride a workout.
What is the measure of success, in relation to a blog? Page views? Number of followers? Total ad revenue from selling sidebar space? Consider some of the most popular blogs out there: the masterfully subtle political snark of Josh Marshall's Talking Points Memo; the laughably out of context conspiracy mongering aimed at a 10-toothed rube audience at Drudge's sludge fest; the cheerfully sunny, always-summer world of Elsie Larson's Beautiful Mess - they all have one thing in common: each blog only does more or less one thing, and they each do that very well.
Wait - forgot to mention The Blog Of Dr. John H. Watson... what? That's not a real blog? Oh. Does it matter?
I can't do just one thing; I can't sit still. It's hard for me to get on the interstate and turn on the cruise control. I know a little about a lot of things, and a lot about some, and somewhere in there are a couple of opinions, too. As Bob Dylan once said, "I got a head full of ideas that are drivin' me insane!" I'm one week into this crazy little thing called Blogger, and it's been a blast so far. I think I'll just keep having fun.
One night, after a couple glasses of Joe's Black Toad, I had a sudden thought: just how inexpensively could a usable and great sounding tube guitar amp be built?
A good place to start might be the classic 1950s Fender Champ Amp circuit. It's simple, which equals cheap, since the component count is low; and depending upon which speaker you run through, it can sound astoundingly good.
So, the goal for a project (as opposed to merely a beer fueled vision) would be to scratch build an amp, which hopefully works, has a tone that is somewhat close to the legendary Champ sound, and - here's the most important criteria - as cheaply as possible. I guess the title of this post kind of gives away the total end cost of this project, and the photo above shows some idea of the amp's beauty factor, which is as low as its price.
Doing The Circuit
Let's talk tone. I've had the good luck to have played through most of the various Champ models that Fender made over the years, as well as a few kit clones and similar amps from other makers. They each have their own flavor, but the baseline tone comes from this basic preamp design:
Actually, the vast majority of all guitar amps ever made, no matter what make or model, have had minor variations of this same basic preamp, and through countless recordings and live performances, it has come to define the sound of the electric guitar. A notable exception was the Vox AC-4, which had an EF86 pentode front end; as well as some lower power US-made amps which used 6AU6 pentode first gain stages. But by and large, the sound we associate with electric guitar is a vacuum tube triode 1st gain stage, followed by a volume control, and then into a 2nd gain stage.
One reason Champs in particular sound so darn good is because their tone is just simply that almost universal guitar tone, but without refinements, effects, tone controls, or cascaded gain stages. A Champ circuit is essentially the basic dual triode tube preamp shown above, with a one-tube single-ended power output section added - no more, no less. It is clean, sweet and chimey at lower volumes, and progressively becomes more raw and edgy as you turn up the knob, finally topping out at a roaring overdrive with a guitar equipped with low output single coils, or a flat-out slamming distortion with humbuckers or higher output singles, all at living room, bedroom, or studio control room levels.
Here's the schematic for the justly famed Fender 5F1 Champ Amp:
Simple! What we have here are two gain stages, each 1/2 of a 12AX7 dual-triode tube, with a volume pot between them; the 2nd triode stage is also the driver into the 6V6GT power output tube, which connects to the speaker through the output transformer, which is a device that matches the relatively high circuit impedance to a low impedance speaker. Along with the power supply circuitry that provides the voltage and current necessary to operate the amplifier, that's all there is.
And now here's a schematic of that same basic circuit, but with a few changes that were made along the way while putting this project together - it's kind of an "as built" circuit diagram, to use an obscure engineering term. I used to be an obscure engineer:
I may have been an engineer, but as you can see, I wasn't an artist - hey, it's freehand. The 5Z1 designation is just some bad humor. Moving on, here are the differences between this circuit and the Fender Tweed 5F1 Champ that it's based on:
• Only one input jack, and one 33K grid stopper resistor going into the first gain stage, which is very close to the 5F1's two 68K resistors in parallel = 34K.
• A 6.8uF cathode bypass capacitor on the first triode section of the 12AX7 reduces the "woof" (unneeded or unwanted low end that can make your guitar tone sound flabby, and sucks power while trying to reproduce a mostly inaudible spectrum), relative to the response of the 25uF cathode bypass cap used in virtually all Fender 1st gain stage preamp tubes.
You may have noticed that most Tweed Champ schematics show no bypass capacitor here at all, but if you look closely at those diagrams, you'll see two dots on either side of the 1.5K resistor. Someone in the past has erased that cap and left the tie points in, and everyone since then has file-shared that schematic, and so it goes. No cathode bypass capacitor at the first gain stage results in less gain, less overall volume, and way less great gritty overdrive when you dime the volume knob - in other words, a Tame Champ - so, let's make sure that cap is in there.
• The 5F1's .02uF coupling capacitor between the plate of the 1st gain stage triode and the volume potentiometer has been reduced to a .01uF, once again to reduce the woofiness a bit, helpful when running any speaker larger than an 8-incher.
• In a nod to the Blackface/Silverface era, I've added a 1.5K grid stopper going into the 6V6 power tube, a 470K on its screen grid, and subbed a choke at the top of the main power supply "Pi" filter, instead of a 10K resistor. The 1.5K and 470K may help out when you run a 6L6 instead of a 6V6 (good to have the choice), and the choke simply reduces hum - always a good thing.
• A 10K resistor instead of a 22K at the second Pi filter ups the voltage at the plates of the 12AX7, which may add headroom. Or it might not, but I like a little more clean before the onset of dirt, and this seems to do the trick.
• The filter cap values are just what I happened to have on hand in the
• There are two IN4007 diodes for rectification (converting AC wall voltage to the DC voltage necessary for the operation of a tube amp), instead of a 5Y3 tube rectifier. The power transformer I have doesn't have the 5V heater filament tap needed to run a rectifier tube such as a 5Y3 or 5AR4. We could have used a rectifier tube with a 6V filament, such as a 6X4, but the few extra volts gained by the use of diodes instead of a tube rectifier is good in this case, since our power transformer has fairly low voltages at its secondary legs (255VAC - 0V - 255VAC).
• The 6.3VAC heater circuit of the .99 Cent Champ is balanced, with both legs referenced to ground via 100 ohm resistors, instead of the single ended filament supply circuit in the original Champ. Per conventional wisdom, this also reduces hum; I happened to have a couple 100 ohmers on hand, so why not?
• Finally, the AC power cable is a modern 3-conducter with ground, and there is no .05uF "death cap" (see 5F1 schematic above) referencing one leg of the AC to the chassis, along the cable to your guitar and through the strings to your hands, and heart. When I was just starting to play electric guitar, I almost died as a result of the deadly combination of a stock '50s Fender amp and a club's improperly wired sound system. A shocking experience, which I survived; Les Harvey and others weren't so lucky.
Parts Is Parts
After some years of messing around with tube amps, I've ended up with a couple small bins of old but still useful parts - resistors and capacitors in new or good used shape, some tubes and sockets, jacks, switches, left over wire, etc. I've also been lucky in knowing a couple of like-minded tube amp nuts, and I was able to rummage through their scrap piles and slip a few things into my pocket when they weren't looking, or actively distracted: "Whoa - is that an unopened beer over there?" "Where?!?"
After a period of scrounging around, I ended up with a small pile of cool old junk. The copper coated chassis came from some kind of ancient tube powered test equipment, as did the power transformer. The paper interleaved output transformer is from a 1950s organ amp that had four 6K6 output tubes in push pull parallel. I stared at the RCA Tube Manual until I got a headache, and finally decided that, with its center tap unconnected, it could handle one single ended 6V6, or even a 6L6, just fine.
I had some old 1/4" jacks, a fuse holder, some terminal strips and a switch laying around, and a good clean used octal socket, as well as (very important) a little red rubber lobster. So far, everything's been free. Here's where the 99 cents in the title of this post comes from: Gene, Glenn, and Linda at Thompson's Electronics, the greatest used and vintage Hi-Fi store anywhere around here and maybe anywhere, had a brand new but really old phenolic 9-pin socket for the 12AX7, and it cost... $.99.
In Part 2, we'll continue on, and try to make some kind of order out of this mess of old rusty parts, look at component placement and layout of the wiring, and discuss how a schematic circuit diagram gets turned into a wired up amp that, hopefully, works. Stay tuned.
My friend David acquired a 1991 Fender Strat with a strange history - someone had covered the entire guitar in white latex house paint. Literally every part of it: body, pickguard, pickups, neck, fingerboard, everything - it was like a "ghost" guitar or something. The guitar's next owner had done a good job removing most of the paint, and when I first saw it there were only a few traces left, deep within the grain of the rosewood fingerboard and in various spots among the bridge saddles. David, however, is a very thorough kind of guy, and when he's done with any project, it's as perfect as possible; when he acquired the Strat, he tore it completely apart and really gave it a good cleaning. Now, there are no traces of its former Sherwin Williams paint job.
While this MIM (made in Mexico) Stratocaster was apart, David also made a lot of improvements to it: he completely shielded the pickup cavities in copper foil, installed a toneful brass tremolo block, replaced the pickguard with a mint-colored Allparts loaded with Squier Classic Vibe alnico pickups, and shaped and fitted a new nut made of some modern super slippery material. On top of that, he did a complete fingerboard setup - leveled and recrowned the frets, beveled and smoothed each fret end, and even rolled the edges of the rosewood fingerboard. The result was nothing short of the finest playing Strat I'd ever had my hands on, and I've played more than a few. David also shined up the original "Beach Boys White" body paint and buffed up the finish on the neck, so the guitar now looked as nice as it played.
I guess it's just in my nature to mess things up. Soon after buying it a few months ago (wait, I forgot to mention that part; well, I did), I took it apart again and started changing things, such as installing a gold anodized pickguard with a couple Duo Sonic pickups, and nickle neck plate and jack cup. That's one of the cool things about Fenders and Fender clones - there's literally thousands of different parts and ways you can mod them to suit your own taste. The first thing I did was to remove the original cast zinc tuning machines (most likely made by Ping and actually quite good quality), and replace them with a set of vintage type gears. Gotoh in Japan makes some nickle plated Kluson style tuners that are probably hands down the finest ever made - smooth, stable, and with immaculate fit and finish.
First step is installing the replacement bushings. The factory drilled holes in this headstock are 10.5mm in diameter, and need adapter / conversion bushings, made to retrofit vintage type tuners with 1/4" shafts into headstocks that have been drilled to the larger size that newer style tuners require. They're the press-fit type, and pieces of 1" width, thin pine lath stock on each side of the headstock (to protect the finish on both the tuners and the headstock) and a C-clamp did the trick to press each bushing in individually, slowly and carefully.
What's not shown here is the next step, which was to drop the new tuners down each hole, and with their posts snuggled into the new bushings, the machines were positioned in a straight line - see the photo below to see what that looks like. While making sure all of the tuners' posts were perpendicular to the back of the headstock and free to turn smoothly without binding in the bushings, I used a small sharp scribing tool as a punch to mark the position of each of the seven mounting screws.