Presenting the "Harmless" stereo amplifier project, the great-sounding stereo amplifier harmless to tube guitar amp builders.

Guitar players know where to go for the best sound - tubes.   Sixty years after transistors supposedly killed tubes off, tubes are still with us, and still producing the best guitar tone.   This is not likely to change.   Ever.   But when it comes to listening to music in the tube amp workshop, it's not tubes, but transistorized amplifiers pushing tunes to amp builders' ears.   Why?   Simply put, tube-type hi-fi audiophiles and tube guitar amplifier aficionados are in camps with mutually exclusive goals and they rarely interact.   One has extreme fidelity as the goal, and one intentionally fosters different kinds of distortion for the pleasing musical sounds it produces.

Harmless' features:

• Very good tube sound.  Better, warmer, and "tubier" than any solid state amp.   Guitar amp builders will like the sound they get from this amp!
• The only solid state devices in Harmless are the power supply rectifiers; the signal path is 100% tube.
• Simple controls.   Volume and Tone.   The tone is a Passive Quad34 Tilt - which means it is a balance between complimentary bass and treble boost/cut circuits, a very effective tone control.
• Construction materials common to guitar amplifiers.   Guitar amplifier builders may find they already have nearly everything they need to build their own Harmless.
• Readily available tubes and other components.   Harmless' tube compliment consists of a 6SL7 and a 6V6 for each channel.   The output transformers are modern (re: upgraded) Champ transformers.   Harmless also sounds great with the Edcor GXSE15-8-5K (overkill) and should sound good with the smaller, open-frame Edcor GXSE10-8-5k.   The circuit board contains connection points for fixed and bypassed screen voltage should the builder opt for non-Ultralinear transformers like the Hammond 125xSE series or Heyboer XFRM-DOB20WSE available from Doberman Music Products .
• Harmless was designed as if it were a tube guitar amplifier, but without any tone shaping or overdrive - it's a clean channel from end to end.   Cathode bias.   Familiar topology: Input stage, cathode follower, tone stack, Class-A (single-ended) power output stage, 4 and 8 ohm 1/4" phone jack output jacks for each channel.   Guitar amplifier builders will be quite comfortable building Harmless.
• Connections are made to only one long-edge of the circuit board.   In other words, the circuit board is not pinned down by connections to front and rear chassis-mounted components.   This provides easy access to tube sockets, etc. located under the board (for instance, to reduce chassis size) without unsoldering the board.
• To accelerate construction, some of the components in Harmless are available for sale, including the board (see photo of board below), the chassis, and chassis-mounted small components.
  
    The price for the drilled board is $25 US.   Boards are 1/8" G10 Garolite.   We normally drill for Keystone 1514-4 turrets, but we can also drill for Keystone 1503-4 if you prefer the larger size.   The original Harmless was made with 1503-4, but subsequent boards use the 1514-4.   Please note that the "drilled board" price is for the board only (no turrets, no ground bus, etc.)!
  
    For the board with 1514-4 turrets and ground bus, it's $75 US.   The ground bus is 12 gauge copper with brass supports on the ends.   Optionally, we can supply the board with turrets instead of the ground bus for an additional $10.   Sorry, but we don't sell the turrets and ground bus hardware without being mounted on a board.
  
    Boards can also be supplied completely loaded with premium components, soldered to MIL-SPEC standards, pre-tested, and ready for installation.   The price for the loaded, tested board is $565 US.
  
    The chassis is available direct from Dirty Dawg Amps.   CopperAmps can provide a chassis with all holes drilled/punched for $85.   A partial chassis kit is also available for $130 US, consisting of:
  • Drilled chassis
  • Volume potentiometer
  • Tone potentiometer
  • Power switch
  • Input jack
  • Output jacks (4)
  • IEC C14 connector
  • Fuseholder
  • Premium tube sockets (4) with brass mounting hardware
    The partial chassis kit takes the research and guess work out of getting just the right fit when you order the drilled chassis.   Note that transformers, filter choke, and pilot light are not included in the chassis kit.   Transformers and the filter choke are available direct from their manufacturers.   The Harmless pilot light is a normal Radio Shack chassis-mount LED.  
  
    (Prices are subject to change without notice. Prior sales excluded.)
  
  

"Harmless" schematic, circuit board layout, component hookup, and drilling template, Rev. 0
"Harmless" schematic, circuit board layout, component hookup, and drilling template, Rev. 1
"Harmless" Bill Of Materials

Rev 0 and Rev 1 differ only in the selection and layout of the transformers and choke.   Rev 0 is for a Z-axis power transformer and the updated Champ output transformers.   Rev 1 is for an X-axis power transformer and upgraded, Edcor output transformers.

"Harmless" project progress:
- Breadboard, monaural "test bed" version was in use for months using various output transformers and tone circuit tweaks without a hitch.
- Beta stereo version is completed and in service.  Photo progress:

• A couple of undrilled Harmless circuit boards (blue, 1/8", G10 Garolite).   Harmless' circuit board is 4" by 14.5".
• Harmless prototype circuit board test fit in its chassis.   The aluminum chassis shown (17" x 8" x 3") is produced by Dirty Dawg Amps.   Harmless should fit in a 15" x 8" x 2" chassis (available from Dirty Dawg), but give yourself more room if you can.
• Harmless fits comfortably in its prototype-sized chassis.
• Close up view of prototype circuit board ready for component addition.   Turrets on the prototype are the larger size, Keystone 1503-4.
• Very close view of ground bus end assembly.   An eye is formed around the groove in a knurled nut and soldered.   The knurled nut is free to turn within the eye.
• Ground bus end installed on circuit board.   A #6 brass screw and lockwasher secure the knurled nut to the circuit board.
• Circuit board with ground bus installed.
• Circuit board with jumpers installed, front view [1.41 MB].   This photo is high resolution because the lower resolution cut out all detail.
• Circuit board with jumpers installed, back view [1.26 MB].   This photo is also high resolution because the lower resolution cut out all detail.   Brown jumper has not been secured to the board yet.
• Four Rubycon snap-ins sporting new pigtails.   Two of these are needed for each Harmless.
• Left side of circuit board is now populated [3.25 MB].
• Circuit board is nearly complete [2.05 MB].   All components mounted between turrets are stress-relieved.
• Circuit board is complete!! [3.89 MB].
• Preamp and Tone Stack close view [3.97 MB].
• Chassis cutout template taped in place.   Ready to start drilling pilot holes.
• But first arrange the components on the chassis to verify template is right.  Measure twice, cut once!!  The Fender Champ upgrade transformers from Triode Electronics are clearly visible.  The fact that they sent me two different transformers is also clearly visible.   : (   The transformers are electrically the same, but the orientations are different.   Part number for the horizontal transformer is TF103-48ULH.   The vertical transformer is missing the "H" on the end of the part number.   They have assured me a second horizontal transformer is in the mail.
• Cutting out a hole for the IEC C14 connector.   Corner radii are 3/16", so the final size for the four holes will be 3/8"; they will just barely reach each other on the short sides.   The holes are gradually drilled out with larger bits until the final size is reached.   The mounting holes for the connector were drilled with the connector in place as a template.
• Sockets, grommets, pots, and power switch are in place.   The chassis has been polished to a high luster.   This took a looong time to accomplish.   If you want to duplicate it (not necessary!), I used two passes with Mothers aluminum polish and a terry cloth bonnet on a automotive random-orbital polisher.
• Hardware is in place for the input section.
• The input section is point-to-point using shielded wire.   The heater wiring for the preamp tubes was done first while it was still relatively easy to get to.   Not shown in this photo is the input jack, which is on the rear of the chassis.
• The mains power, signal input, and heater wiring are done.   Harmless is starting to really come together.
• The first connection from the circuit board to the chassis is the single-point ground wire which goes to the input jack.   A high-power iron is needed to quickly solder to the heavy ground bus.
• Connections running from the tube sockets and tone potentiometer are finished.   Most jumpers are Belden 83006 (22AWG Teflon, 19 x 34AWG strands, silver tinned - awesome stuff).   Note the two orange jumpers (they look kinda brown in the photo); these are the only two solid jumpers in Harmless.   I decided to use this wire because (1) it's cloth covered (i.e., it looks cool), (2) it's the right color for screen grids (orange), and (3) I wanted to see if solid jumper wire could hack it compared to MILSPEC W-16878/4.
• Setting up to install iron.   Harmless circuit board screws are removed to provide access underneath where the filter choke and output transformers come through.
• Both screen grid jumpers broke.   Remember those two orange wires?   One broke right away, the other broke a little later while installing one of the output transformers.   No more solid hookup wire for Harmless!
• Iron's in!   Power transformer, filter choke, and both output transformers are bolted in place, the circuit board is moved back into place, and tightened down.   The broken solid jumpers are replaced with Belden 83006.   Didn't have any orange on hand, but had a short piece of white with an orange stripe.   The ultra-linear screen grid wire coming from the output transformer is white, and orange is the right color for screen grids so...yeah, life is good!
• Low-res gut shot of the finished Harmless project.   There's a nice porn shot below for those with high-speed connections.
• The finished Harmless amplifier still sitting on the bench.   This pic is low-res, too.   See glamour shot below for nice photo.
• ...and here's a shot of Harmless' rear end.   Take a look at the output transformers.   The supplier did not get the replacement, right transformer to me so I had to build it with the vertically oriented one.
• With a sigh and a deep breath, Harmless is powered up for the first time!   Incredibly, no smoke pours out of it's innards.   The LED pilot comes on instantly.   A second or two later the heaters begin glowing.   Fifteen seconds later the volume control is turned full up.   No buzz, no hum.   No nothing..!   With a lump in my throat (is it working at all?), I click the Winamp play button.   Sultans of Swing fills the workshop and the crowd goes wild!!!!!
• Harmless gets a workout.
  - Voltage readings revealed that the NOS Stancor transformer, while rated high enough (90 madc) to supply all of Harmless' B+ needs, nevertheless is sagging about 50 volts with the load of the second channel.   This is about the same as the calculated voltage drop across the first dropping resistor in the power supply (R1, 510 Ohms).   Temporarily operating Harmless with R1 jumpered brings B+ back up to nominal (311 volts), so R1 is replaced with a 2 Ohm resistor.
  
  - The B+ for the preamp stage is also running slightly low in the normal range, 275 VDC.   I may tweak that up a bit by dropping R2's value slightly - after which a revisit to the biasing charts will be in order.
  
  - Harmless is burnt in all day, about twelve hours.   The Stancor power transformer get's warm, but not hot.   It makes a perfect hand warmer.   The filter choke and output transformers don't seem to be heating up at all.
  
  
• Glamour Shot [3.31 MB]  
• Porn shot [3.95 MB]  

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Please use AX84 forums for Harmless discussion.  For contact about this site: support@cuamps.com