Kit Review: MOD 102 DIY Guitar Amplifier Kit

The MOD 102 guitar amplifier is a low-cost ($215) vacuum-tube kit sold by Amplified Parts of Tempe, AZ. The kit uses a 12AX7 preamplifier tube, and a single EL84 output tube with solid state diodes for power rectification. The amplifier is rated to deliver 5 watts into an 8-ohm load.

SCHEMATIC REVIEW

Referring to the schematic below (click for a larger pdf version) we see a very simple amplifier design is used in the MOD 102, with many elements designed to keep the cost as low as possible.

MOD 102 Schematic (Click to Enlarge)

Starting at the input jack we see that the input signal is applied directly to the grid of V1A, the first half of the 12AX7 preamplifier tube. This direct-in arrangement sets the "few parts, low cost" theme of this amplifier design. The V1A tube is cathode-biased, meaning that, in this case, a 1.5K ohm resistor is in series with the cathode, causing the voltage of the cathode to rise to about +0.9VDC relative to ground, with the grid therefore at a negative voltage relative to the cathode.

The plate of this tube is tapped with resistor-capacitor network that forms the tone stack (treble & bass) and volume control. The variable center pin of the volume potentiometer feeds the grid of the second preamplifier stage, V1B. This triode is also cathode biased, and it's output feeds the input grid of the output tube V2. The plate of V2 is fed through the primary winding of the output transformer TR2.

This MOD 102 design is described as "single ended" because it uses a single output tube. This is a good configuration for a low-power and low-cost design, but a single ended design can be prone to hum compared with a push-pull or differential pair design where common-mode noise such as hum is cancelled. The source of hum is typically the heater's AC voltage, and there are generally three ways to reduce it.

The first is to use a center tapped heater winding on the transformer, which is not used on the MOD 102, presumably for cost reasons. The second method is to create an artificial center tap by connecting matched resistors from each lead of the heater winding to ground. This arrangement is commonly seen in many Fender amps, such as a Tweed Deluxe. It adds the cost of two resistors and a higher current draw from the heater winding, so possibly a larger and more expensive transformer. We're guessing, but presumably this is why it was not used in the MOD 102.

The third method is called "DC elevation" and the MOD 102 uses a variation on it by connecting one side of the heater winding to the cathode of the V2 power tube, which is biased to about 5.5 Volts DC. This floats the 6.3 volt AC heater voltage on all three tube filaments (two in the dual 12AX7 and one in the EL84) on top of the 5.5 volts DC. This approach minimizes the hum induced from the heater to the cathode because the induced current becomes close to a constant value when the voltage is above a threshold. In other words, although there is still leakage current, it is constant and therefore the noise voltage appearing at the cathode becomes DC, which is inaudible.

From our testing of the amp, discussed below, the approach is effective as the amp is very quiet. Moving to the power supply, the power transformer primary includes the on/off switch and fuse. The first secondary winding is center-tapped for the high voltage supply, starting with about 209 volts AC and resulting, after full-wave rectification and filtering, in 178 to 264 volts DC depending on the test point chosen. The second secondary winding is for the 6.3 volts AC filament supply, to heat the tube cathodes.

Overall this is a simple, low parts-count design that is effective for a low-power, low-cost kit. We'll now turn the review over to Casey4S for his impressions from building the kit. A video demonstration is also included below.

KIT REVIEW

The editors at Guitar Kit Builder (GKB) asked me to assemble and evaluate the MOD 102 Guitar Amplifier Kit for them and I just couldn't wait to do another MOD project. This is a single channel, single ended amplifier kit complete with chassis, tubes and everything to wind up with a cool little amplifier. This is just 5 watts out so it's a good choice for a dorm room or practice amplifier. The kit is just for the amplifier - no speaker or cabinet is included. Everything in the kit (Photo 1) arrived in just one little box. The test equipment and soldering station are mine, and not part of the kit.

Photo 1 - Kit Components and Tools, Ready to Begin

I've built one other MOD kit, The Persuader, and like that kit the instructions (Photo 2) were excellent - well explained and very well illustrated.

Photo 2 - Kit Instructions

Even before the kit was delivered to me I had already decided to build it in a Hammond 1444-18 (13.5x5x2) style chassis so I could mount it in a small combo cabinet. The 6x10x2 gray powder-coated chassis that came with the kit would require a separate speaker cabinet to make it usable, but it would be less portable and more easily damaged. I had concerns about the vulnerability of the exposed tubes when carrying the amp around - after all, an amp's not much good to you if it's not portable. I chose this chassis because it is inexpensive, readily available and fits nicely in small "Tweed" style cabinets with the controls on top. In actuality I folded my own chassis but kept the measurements within that of the Hammond chassis. Mine actually measures 13.125" end cap to end cap, giving plenty of room to do a neat job. I had to provide a cabinet and speaker, so I ordered a MOD (Jensen) 10" speaker to go into a combo cabinet I built back in 2000. The chassis construction started with a flat sheet of aluminum with the fold points marked (Photo 3).

Photo 3 - Aluminum Sheet Stock Marked and Ready to Fold

Then the chassis was folded on a box & pan brake (Photo 4).

Photo 4 - Sheet Metal in Bending Brake

Here's the finished chassis (Photo 5) without the end caps (added later).

Photo 5 - Basic Chassis

Next, I laid out the controls and drilled the chassis mounting holes (Photo 6).

Photo 6 - Holes Drilled for Controls

The power transformer (PT) and two tubes are mounted on the bottom of the chassis (Photo 7), which changes the layout a little but not the instructions.

Photo 7 - Main Components Mounted on Chassis

I built the kit using the supplied parts and instructions, with just a few deviations:

• As described above, I built my own chassis so I could easily mount it in a cabinet.
• I purchased a MOD 10-35 speaker.
• I used a strain relief for the power cord instead of the knot & grommet provided.
• I used a Switchcraft 12A type grounding jack for the speaker out because the output transformer (OT) can be damaged if you have no load present (nothing plugged in) when the amplifier is on. With a grounding type jack the output circuit is just grounded and doing no harm if there is no speaker plugged into the amp .
• The chassis terminal strips (Photo 8) were laid out identically to the instructions except T5 and T6. They are just moved slightly and are end to end now instead of side by side
• I mounted the 1.5K resistor and 22ufd bypass cap for tube V1 directly to the tube socket instead of mounting them on terminal strip T4.
• I changed the wire used in the chassis to red, white & blue (I built this right around July 4th, Independence Day!). Red was used for the B+ rail, blue for the signal chain and tone stack, and white for the raw guitar signal going to V1(2).

These were very few and minor deviations from the instructions, and most were in regard to the new chassis.

Photo 8 - Chassis with Terminal Strips Mounted and Labelled

At this point in the build I had all the AC wiring, tube heaters and pilot light wired (photo 9). Next I wired the three potentiometers and the input jack.

Photo 9 - Chassis with AC Wiring Completed

When I wire up an amplifier front panel I make a jig (Photo 10) to hold the parts at their exact spacing, so it's easier to reach the pots and jacks.

Photo 10 - Using a Jig to Wire the Front Panel

Then I just remove it from the jig (Photo 11) and slide it onto the chassis and apply the hardware. I find this usually saves me a lot of time.

Photo 11 - Wired Potentiometers Removed From Jig

I rejoined the instructions at page 12 and drawing number 6 and did everything exactly like the instructions from there on except where noted. In Photo 12 I have finished T1, T2 and T3, and a portion of T4 and I have run some of my lead wires. T6 and T5 are next up.

Photo 12 - Partially Wired Chassis

Photo 13 shows the detail of the two tube sockets. Note that both cathode resistors and bypass caps are on the V1 socket instead of on the terminal strips. The plate load resistors could have both been mounted directly on the V1 socket as well, but I stuck to the instructions. Note the .022 coupling cap going between T6 and T5.

Photo 13 - Tube Socket Wiring Detail

Photo 14 shows the detail of the end caps I installed later to keep the chassis rigid.

Photo 14 - End Caps Added for Chassis Strength

The labels supplied (Photo 15) with the MOD kit are, frankly, horrible. They are laser printed on an Avery label and look pretty bad in my opinion. That's just part of buying an inexpensive kit. I could have polished the aluminum chassis to look almost like chrome but the labels just looked inadequate to me for such a nice amplifier.

Photo 15 - Front Panel Labels Supplied by MOD Kits

I decided to have a professional faceplate made, and purchased one from BNP Lasers of Washington, PA. I sent them a sketch of my design and after a few emails and a PayPal transaction I had this (Photo 16) chocolate brown faceplate by the end of the week. It cost me a little more but I think it was more than worth the investment. The folks at BNP Lasers are great to work with and understand amplifier faceplate needs and concerns.

Photo 16 - Custom Front Panel

Here's a front view (Photo 17) and rear view (Photo 18) of the finished amplifier in the cabinet.

Photo 17 - Front, Top View of Completed Amplifier

Photo 18 - Rear View of Completed Amplifier

I added a plexiglass back panel (Photo 19) to make the interior viewable and safe.

Photo 19 - Plexiglass Back

TESTING, SPEAKER SELECTION AND OVERALL RESULTS

After finishing the initial build, I carried the new amp into my family room and "fired it up". I put all three controls on 50% (or 12 o'clock) and plugged in my Epiphone Sheraton guitar. After a couple of minutes I heard nothing - no hum, buzz or even the usual inherent hiss with tube amps. I thought for a minute I had a "no sound out" problem but I struck the strings and I had plenty of sound. This is just a super-quiet amp at idle. At around 75% or so you do pick up a slight bit of tube amp hiss, but it is acceptable by far.

I tried the tone controls and played with the volume and found I could obtain a decent clean sound. But the MOD 102 loves to kick butt, and when it's cranked it is a fantastic rock/blues amp. I tried two other speakers that I had on hand before I got delivery of the MOD speaker. I tried an Eminence 10" AlNiCo, and a Fender 10" generic ceramic, neither of which were inspiring in this amp.

The MOD 10-35 beats them hands down. I ordered the MOD speaker in part to stay with a MOD theme, and the price was great at $35~. But mostly I went with this because of a recommendation by my friend Bob in Louisiana, who is building custom amps with his son. He started using these and was happy with the overall tone. The catalog describes it as.... " Warm, straight forward tone and tight bass frequencies. Presented with overdrive distortion, takes on a gritty edge". I would agree with this description and there was no real break-in time for the speaker. The response is 70Hz to 9kHz, which is a really broad range and lets the amp show off it's "edge" a bit. There were a variety of styles played through this combination and the speaker never failed to measure up. I'd score this as another winner.

But this was just my opinion, so I took it to the local School of Rock in Vienna, Virginia to shoot a video and get some unbiased opinions from several knowledgeable students and staff. Everyone seemed impressed with the tone and flexibility of the MOD 102 amp and the MOD 10-35 speaker combination. So it wasn't only my builders pride - this little 5 watt amp does the job! Check it out for yourself in our video demonstration:

I only have tweed Champ, Princeton, and Gibson Skylark amplifiers to compare this with, but it is leaps and bounds ahead of those amplifiers in tone, flexibility and overall mojo. I have been searching for a simple amp design for first timers to "pull the trigger" on and get a decent product in the end, and this MOD 102 certainly fits the bill.

PROS AND CONS

Here are some of the positive things that stood out for me about this kit.

• Turn-key complete (except speaker and cabinet)
• Wonderful instructions with great illustrations and some soldering tips as well.
• Not too complex (but sounds great)
• At $215, the price is definitely hard to compete with.
• An excellent, well thought-out design. Kudos to the Engineering staff.
• The simple layout of the chassis makes it an excellent teaching aid and relates easily to the schematic.
• All of the parts and components are of great quality, no loose jacks or crummy pots.
• Did I mention it sounds terrific?

• Most of all, I did not like the chassis provided. That is the 10"x6"x2" gray painted (or coated) chassis. It isn't practical as a small practice amplifier if it isn't comfortable to transport and it really needs a closure or case. And you'll still need a speaker cabinet of some kind.
• I want a strain relief for my AC cord, not a grommet and knot system.
• The output jack should be a 12A type which will switch the circuit to ground if no plug is in the jack while the amplifier is turned on.

CONCLUSION

I don't see how you could go wrong with this as a kit for a novice. If you can read, follow instructions and are able to solder well enough, you can successfully build this amp. You'll need a speaker, either as part of a combo, or as a separate speaker cabinet. The ONLY difference between my build and the original MOD 102 is simply the container. The circuit is identical. If you like the tones in the video, you can build it yourself.