Moonbeam: Therapeutic vacuum tube reconstruction for a guitar
amplifier and a man
I constructed this tube guitar amplifier using the chassis and speaker cabinet of a solid-state combo amp with several objectives in mind:
I enjoy all aspects of tube DIY design and construction: the circuit, the layout, the cabinet, etc. However, I don't consider myself a natural craftsman, so it takes me a lot of time and effort to perform the electrical and mechanical work to the standard I set for myself. I didn't feel up to taking on the whole load for my first project back, so I decided to improvise a useful amp out of stuff I just had laying around.
The main ingredients for this project were:
Peavey provided a chassis suitable for tube-amp reincarnation - a
simple 16 gauge steel U-channel with a minimal number of holes in the
bottom. I made 4 holes on 2" centers for the octal sockets with a
Greenlee 1" conduit punch, and a 3/4" hole for the miniature 9-pin
socket with a Unibit. Once again, my friend and co-worker Arpad
made quick work of the power transformer cutout with the Bridgeport
mill. I could have made the cutout with a drill, hacksaw, and
file, but it's nice to have a friend with the tools, skills, and
willingness to help. One note about drilling/cutting steel: it's
a lot harder than aluminum. I use the minimum speed on my drill press,
lubricate the drills/Unibits with oil, minimize the pressure, and pause
between step-drill steps. Using minimum pressure also reduces the
size of the burr that has to be cleaned up on the exit-side of the
|The front panel of the Peavey
had a few hundred holes for all the pots and switches typical of the
knob-happy solid-state amp genre. I used the Unibit to enlarge
the ones I needed for the full-sized volume and tone pots, toggle
switch, and pilot lamp I would use.
The back panel wasn't quite as perforated, but I did need to make a hole for the external fuse holder. The fuse for the original amp was inside on the PCB.
|The populated turret board and
the tube sockets were lifted straight from the prototyping chassis
where they had lain dormant most of the past 5 years. When I
originally made this turret board, I accidentally flipped the pattern
left for right from what I designed for my Velleman chassis
layout. It turns out to be the correct orientation for this
The circuit is essentially the same as my first Moonlight, with a power and output transformer appropriate for a push-pull 6V6 output stage. I hope you will forgive the lack of an as-built schematic. Here is a text description of the power supply and other important circuit elements:
Here are the guts all wired up with transformers and switches
Debugging issuesBecause I already knew that the turret board and sockets were wired correctly, I didn't spend as much time as I usually do checking my wiring before applying power. I checked the power supply up to the standby switch, ramping up the AC gradually on a variac and watching the voltmeter - no problems. The whole amp worked on the first try, but with fizzy distortion at low volume. I tracked it down to a loose resistor I hadn't soldered on the turret board. That solved, I cranked the volume and was quickly rewarded with the dreaded sound of high-pitched oscillation with the tone control anywhere near the treble side. I spent a while sifting for cold-soldered joints, bad grounds, and shorting grids and cathodes to ground with a cap, swapping tubes, and being perplexed. In desperation, I connected one side of the output transformer secondary to ground, and the oscillation disappeared. On the bench, I had connected the speaker to the OT's flying leads with test clips and no ground. I had planned to connect the speaker directly to the OT flying leads in the cabinet, but decided to obtain the necessary ground connection by running the leads back inside the chassis to a 1/4" phone jack mounted on the back panel. That seemed slightly less of a hack than running an extra lead back from the speaker to a terminal screwed to the chassis. Note the speaker jack is right next to the preamp tube socket, with the OT secondary leads close by. These are the kinds of things I normally go to great pains to avoid in my "careful" layouts, and I am happy to get off scott-free this time.
Tubes, from left to right: 5Y3GT rectifier, 2 x 6V6-GT output, 6SL7GT phase splitter, and EF184 preamp. The power transformer is the previous edition of the Weber W025130, with 120V primary, 540/680 VAC@150mA HV windings, 45V bias, and 5VAC@3A and 6.3VAC@5A filament windings. The output transformer is the Weber W041318 6600 ohm primary / 8 ohm secondary. The choke is the Weber W022699 9H@120mA.
|Here is the rear view of the
complete package. The cabinet is nothing special, cheesy MDF
covered with a decent tolex job. It arrived for re-animation in
pretty grubby condition, with rusty hardware and covered with a quarter
inch of dust and grime. The shop-vac and Formula 409 took care of
the grime, but it shall carry its rusty screws onward into its new
life. It's only rock and roll. The speaker is a Weber
Signature 10f that I bought for just such an occasion all those years
What does it sound like?
|I'm relatively pleased with how
it sounds. I like pentode preamps because they respond well to
changes in the guitar volume control, and are pedal-friendly.
This amp has a good amount of clean headroom, which the original
Moonlight sorely lacked. I thought the problem was caused by the
output tubes being overdriven almost all the time, and that the cure to
was to use a lower gain, higher headroom output stage. It turned
out that it's much easier to get clean headroom by using a lower gain
phase splitter tube, e.g. a 6SN7 instead of a 6SL7.
In this amp, both strategies are effective. The 6V6's have plenty of headroom even with the high gain phase splitter, and can really accept a lot more useful (non-flabby) bass than a pair of 3.5W triodes. With the low-gain phase splitter, there's a couple more numbers of clean on the volume control for jazz-style playing.
It's not as bright as a blackface Princeton or Deluxe, which I wouldn't expect with cathode bias and the 320V B+ I'm running. It might sound 5E3-ish. When cranked, the distorted tone has the bluesy growl and singing sustain that makes it easy to get well-controlled fundamental and one octave higher feedback all over the guitar, something that I found it much harder to do with blackface-style amps.
It is conventional wisdom that pentode preamp tubes, e.g. EF86, EF184, can have microphonic issues in combo amps, because of the vibrations in the cabinet. It doesn't seem to be a problem for this circuit, though, probably because the pentode gain is set for the lower end of the possible range.
|I'm almost afraid to get caught
up tweaking with this thing, because I may end up wanting to play it
instead of giving it to my daughter. But she needs an amp to take
to college, and it sure as heck is not going to be
the '67 Princeton Reverb I lent her to play at home. I'll
give myself a month to play with the higher voltage 6V6 (and maybe 6L6)
options. The present B+ is probably low enough I could get
away trying 6K6's, though I might need to adjust the cathode resistor
or screen supply voltage. I have a 16 ohm 10" alnico in my parts
stash that would give the correct impedance ratio with the Weber output
Cosmetically, a new control faceplate, speaker baffle, and grillcloth would de-Peavey-fy it to the point that someone might even think about stealing it. I'm not sure I can call it complete without attending to the cosmetics, but it's far enough along that I can start working seriously on the next build. And that was the main objective.
While doing the research for this project, I found a USA-made multi-tap output transformer to run 6K6's at the proper impedance ratio at Triode Electronics. They also sell a complete line of Fender-style power and output transformers from the same company (Magnetic Components of Chicago) at really good prices. While I decided to stick with the Weber transformer set for this amp, I couldn't resist picking up the similarly-sized set from Triode, along the a pair of JJ 6V6-S's. The Magnetic Components transformers arrived and look really beefy. I think they will be mounted on my next prototyping rig in the near future. Of course, buying another transformer set nullified the objective of reducing my amp part inventory, but you can't have everything. Where would you put it?