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:
Running the Numbers
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.
Variations on a Theme
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.
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All photos taken with a Lumix ZS-25. Click or tap on any picture to go to a higher resolution (larger) image.