Modifying the
Epiphone Valve Junior Guitar Amplifier
"From least to beast"
Mod 4 -- Installing the Mercury Mod Kit
Gearscore.com's
Modifying the Epiphone Valve Junior
Guitar Amplifier
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This installment begins the "high" end work on the EVJ with replacement and new install of 2 upgrade transformers and a choke. This section will take more skills to complete than the previous modifications, and it would be helpful to locate an amp tech locally in the event you run into trouble. Still, with average skills and care, the electronic novice should be able to complete these mods.
A transformer is the heart and soul of many electronic applications, using coils, a voltage is introduced on one side of a transformer and a different voltage is output on the other side of the transformer. Coil inductance is the electronic phenomenon that enables two (or more) adjacent coils to transform one voltage to another. There are several types of transformers and many different construction techniques.
Cheap transformers are noisy, introducing eddy currents to the output signal and can be very inefficient, performing the transform at 50% efficiency or less. Efficiency is how well power is transferred through the unit, poor windings, bad materials, loose cores, cheap cores, etc., all contribute to resistance -- which when used in power applications, creates heat -- which in turn, reduces output as power is dispersed by heat instead of at its target, the speakers. By and large mass produced tube guitar amplifiers do not use high quality transformers, the cost is prohibitive and mass production often dictates cheap components to keep costs down and profits up. The transformers supplied by Mercury Magnetics is this mod are billed at 97% efficiency. Efficiency costs money in materials and processes, hence the justification for the higher cost of quality transformers.
Besides the overall quality of build and parts in a "boutique" amplifier, a large portion of the cost associated with boutique amplifiers (besides being limited production built), lies in the transformers used. Being able to handle large current demands, free of noise and high efficiency transfer of audio signal power are the hallmark of transformers used in boutique amps, some of which are hand wound for specific performance curves.
A high quality transformer often equates to brightness, more power, articulation of the audio signal and clarity. This will become evident as we replace the stock EVJ transformers with higher quality aftermarket transformers.
Typically, most tube amps have two transformers, a power and an output transformer. In our project we will be replacing both of these and introducing a third -- which will be used in the form of a "choke" -- basically a single coil winding to smooth out voltage ripples.
Just like the old adage of "garbage in, garbage out" -- the same applies to the power supply section of an audio amplifier. Because the output is an audio signal, and the components used in an audio amp are noise sensitive -- the cleaner the power supply circuitry to drive these components, the less underlying noise and buzziness the audio output will have, thus improving tonality overall.
Our next mod is not a cheap one, but should escalate power, performance and clarity significantly. For this modification, I have chosen the Mercury Magnetics Epiphone Valve Junior Upgrade Kit. The reasons are simple, we are staying within the "kit" paradigm, not only does Mercury Magnetics supply all of the parts to perform the entire mod in one box -- they also supply a 64-page instruction booklet filled page to page with large photographs for each of the way.
Transformers are where the real voice and balls of an amplifier come from -- the tone stack circuitry of an amplifier shapes the audio signal and peaks and/or subdues certain frequencies. The types of tubes used in an amplifier all have their respective colorings to the audio signal. It is the output transformer that actually gets these qualities out to the real world, the better the transformer, the better the tone of any amp will be -- short of boutique amplifiers, which already have the features, we are about to install to the EVJ.
Note: The Mercury Magnetics documentation covers the Version 1 PCB quite well, Version 2 is a little fuzzier and Version 3 is really temporary, being a kind of combination between Version 2 text and Version 3 pictures. It really is difficult to sort out and put this information into the correct by places as was done in the Version 1 section. When this story was being compiled the staff at Mercury were in the process of writing a new manual.
That being said, I will deviate from the supplied manual and try to construct a clear path, using my own pictures. I spent two days alone analyzing the schematics, pictures and text before arriving at this conclusion.
5.2
-- Basic Circuit ChangesThere are several transformer companies that supply replacements available on the market -- most of which are just a simple swap. That is, lead for lead -- change out the old and plug in the new. This is not the case with the Mercury Magnetics Upgrade Kit -- in this installment, we will be physically altering the PCB itself, and re-wiring much of it. In addition, some of the supplied circuit mods from Mercury Magnetics will not be used, as the BitMO mods have already introduced changes into certain circuits that are just fine the way they are.
There are two major deviations from the stock layout that requires the bulk of the PCB modifications. The first is the power supply, the stock EVJ PCB uses an unregulated DC filament supply to fire the tube heaters, which is really a no-no. If Epiphone had included a regulator on the circuit, the stock circuit would have been much better on the filament side. All dual-triode tubes operate heaters at peak efficiency between 6V and 6.3V (AC or DC). The filament output of the stock transformer is nearly 8V AC, then rectified with a voltage loss of about a half volt, making the filament supply approximately 7.4V DC -- too high for proper operation.
The wide acceptance of using DC filament supply revolves around noise -- with no oscillation, the result is a cleaner gain stage, but only if performed correctly. However, some claim this shortens the life of the tube itself and that AC filament supply provides a richer, full bodied tone as opposed to the drier effect of a DC supply. The Mercury Magnetics mod changes the existing DC supply to an AC supply -- so it will be interesting to see just what overall affect this has on tone.
Note: That the Mercury Magnetics filament supply is right at 6.3V AC, so the filament wiring will have to go directly to the tube filament leads. This change though, requires abandoning some of the stock circuit to deliver the filament supply directly to the sockets via hookup wiring.
The other deviation in the circuit is the use of a choke to smooth out voltage ripples, which should improve the signal to noise ratio, again -- also requiring modification to the existing circuit.
For this mod, you will need the Mercury Magnetics Upgrade Kit, a clear work area and take the proper safety precautions. The PCB board will be removed in this mod -- so first we will build a shunt to discharge the power capacitors as discussed in the introduction installment of this project. We will also be using a Dremel tool with a carbide cutter to cut the traces on the PCB. A very sharp Exacto knife can do this with a lot of care, but a Dremel tool is cleaner, faster and more efficient.
In Figure 5-1, you see the layout of parts that will be replaced and/or added in this mod.
Figure 5-1: Old and New Parts
First you will need the parts, commonly available at any Radio Shack or electronics supply depot, also see Figure 5-2:
- 6 to 10" piece of 22-24 gauge wire
- 5 or10W 10 ohm power resistor
- 3/8" OD x 4" long shrink tubing
- 2 -- mini alligator clips w/boots
Figure 5-2: Shunt Parts
1. Peel back or remove the boots from the alligator clips.
2. Solder one clip to one end of the power resistor.
3. Solder one end of the wire to the other end of the power resistor.
4. Solder the other end of the wire to the other alligator clip.
5. Roll the boots back onto the alligator clips.
See Figure 5-3.
Figure 5-3: Wiring the Shunt
6. Slip on the shrink tubing over the whole wired assembly, making sure to overlap the connections.
7. Gently and evenly heat the shrink wrap tubing until is has insulated all parts of the wired assembly and shrunk as far as it will go.
See Figure 5-4.
Figure 5-4: Finished Shunt
For the probe, notice that I am using a leg clipped from a resistor, it could be a paper clip or another piece of wire or nothing at all. I built it this way so I could get the tip, whatever that may be, into a variety of locations.
First things first, refer to the Mod-2 installment for instructions to remove the chassis from the box, if it is not already out.
Remove both tubes from the chassis/PCB and set aside.
There are references to the BitMO mods in this section, if you did not perform the BitMO mods, ignore any instruction that makes mention of them.
The easiest way to get the PCB out is to remove the transformers from the chassis altogether and disconnect the toggle switch put in by the BitMO mods. Some of the wiring can just be unplugged from the PCB, others can be cut.
For reference, Figure 5-5 is the stock wiring of a Version 3 PCB.
Figure 5-5: Stock Wiring Version 3 PCB
Figure 5-6: Wire Removal
Reference Figure 5-6 for the corresponding numbered steps:
1. Clip these wires at the loop, be sure to leave green/yellow wire (not in the loop) intact, it is the output jack PCB ground wire. These wires are from the output transformer secondary taps.
2. Unplug these two wires, these are the transformer primary signal wires. When the wires in steps 1 and 2 have been disconnected, remove the two screws holding the output transformer to the chassis from the other side. Withdraw all these wires through the grommet under the output PCB (not visible). Remove the output transformer from the chassis and set aside.
3. Unplug these two orange wires, they are from the power transformer and are the filament wires on the stock PCB configuration.
4. Unplug these two red wires, they are from the power transformer and are the “B” high voltage wires on the stock PCB configuration.
5. Unplug the brown and blue wires, they the power transformer primary wires for other voltages (euro).
6. Clip the green/yellow wire (ground) and the black and yellow wires at the on/off switch. Be sure to leave a few inches for reconnection later on -- DO NOT attempt to unplug these wires from the switch, doing so will actually break the switch. Once these last wires are clipped -- feed the wires through the grommets and remove the screws holding the power transformer to the chassis, then remove the power transformer and set aside.
7. Clip the wires from the BitMO toggle switch move out of the way.
8. Once all the wires have been removed or clipped in this section, your PCB/Chassis should look like Figure 5-7 and Figure 5-8.
Figure 5-7: Wiring Removed -- View 1
Figure 5-8: Wiring Removed -- View 2
9. Notice the Brown and Blue wires running from the switch to the power connector, unplug them from the power connector side, twist as shown and replace on the power connector terminals.
10. Remove the knob from the BitMO Tone Control on the front panel, remove the nut and washer securing the pot to the front panel.
11. Remove the six Phillips head screw securing the PCB to the standoffs on the chassis.
12. Taking care to withdraw the BitMO pot, lift and remove the PCB from the chassis by its edges -- remember there may be charges left on those large filter caps, so don’t touch them or any of the trace circuitry on the underside of the PCB.
13. Scrape the silicon goo off of the two plastic connectors on the bottom of the PCB to the front panel and carefully remove the plastic connectors from their wire terminals.
14. Remove the PCB from the chassis and set the chassis aside.
If you have the BitMO Tone Control installed, use a few pieces of tape and support it from the PCB, just enough to keep it from flopping around while we work on the PCB itself, see Figure 5-9.
Figure 5-9: PCB Removed and BitMO Mod Supported
Now is the time to locate the 2 leads for each filter cap and shunt them together, simply flip the PCB over and hold each end of the shunt on the leads illustrated in Figure 5-10 for a couple of seconds (this really is more of a good habit to develop on this amp than a necessity).
Figure 5-10: Filter Cap Leads
The Version 3 PCB is covered in a very dark mask and is very difficult to photograph the traces on the back side -- so I will take a picture of the areas to be modified and outline the traces for visual reference.
As mentioned earlier, you are going to need a Dremel tool with a carbide cutting tip and also a drill with a 1/16" drill bit to complete the PCB modifications. For parts removal, a chisel tipped soldering tool, soldering iron and de-soldering braid will help a lot removing the parts that need to come off the PCB.
Part Removal:
1. Using de-soldering braid, one at a time, remove the solder from each the spade terminal lug and then the terminal lug on the PCB (remove all).
2. Using the same technique, remove capacitor C12 (two leads), you will also have to scrape the silicon from the PCB to remove this capacitor.
3. Using the same technique, remove the large black bridge rectifier (four leads) located next to capacitor C12 that you just removed.
4. Using the same technique, remove resistors R10, R11 and R12 (two leads each).
5. Your removed parts should look as shown in Figure 5-11.
Note: If you are performing only the Mercury Magnetics transformer mods, consult the Mercury Magnetics documentation for additional parts/mods at the other end of the PCB. For those having installed the BitMO mods, these are all the parts that are required to be removed.
Figure 5-11: PCB Parts Removed
PCB Circuit Modifications:
1. Figure 5-12 shows where there are 5 traces to be cut, highlighted by the heavy yellow lines. The same picture also shows where 6 -- 1/16" holes are to be drilled in the PCB. Take care cutting the traces and gentle with the hole drilling, making sure you don’t dive through the board and accidentally destroy a component on the other side. The two holes to be drilled on the thin trace next to the 4 pin of the EL84 socket is tricky, try to drill each hole half on/half off the trace. The 2 pairs of holes to be drilled next to the EL84 and the 2 holes next to the 12AX7 socket is for direct wiring the filaments of those tubes -- is it critical to get a good connection.
Figure 5-12: Modifying the PCB
2. After cutting the traces and drilling the holes -- the PCB should look as pictured in Figure 5-13. Be sure to use an Exacto or sharp pen knife to carefully scrape the black coating off the traces around the holes. While it will be ok to bridge the solder connections of the wires we install later -- it's still good form to get a good firm connection to the PCB traces as well.
Figure 5-13: Traces Cut, Holes Drilled
3. Turn the PCB over and using the wire provided in the kit, twist tightly, insert and solder into the holes on each side of the 12AX7 socket and cut to length/place in a hole at pin 4 and 5 at the EL84 socket -- do not solder these at this time. Figure 5-14.
Figure 5-14: Filament Wiring
4. In the kit provided by Mercury Magnetics, locate the large 22uf electrolytic capacitor and solder in place of C12, which was previously removed.
5. In the kit provided by Mercury Magnetics, locate the 3.3k 1 Watt resistor and solder in place of R12, which was previously removed.
6. Make sure to cut the excess legs off of the reverse side and mounting the previous two components.
7. Using one of the trimmed off resistor legs, bend to fit and place as a jumper as shown in Figure 5-15, solder in place.
Figure 5-15: Placing Power Parts
8. Once your parts are all in place, double check placement -- if good, set the PCB aside for now.
The first steps to installing the new transformers will be to drill new mounting holes for them in the chassis. Mercury Magnetics provides a template for locating the holes, but I found it a little tight on the front and given that there is still one more mod to go -- decided to use my own mounting scheme. So, using a 1/4" drill bit -- grab your chassis lets get these transformers mounted.
1. First twist all of the power transformer leads except the orange/white secondary ground lead, leave it by itself.
2. Position the two transformers and choke on the chassis and mark where to drill with a magic marker. Check both of the following illustrations for drill locations in red and placement of the transformers. The power transformer will reuse one hole from the previous transformer.
Figure 5-16: Drill Locations
Figure 5-17: Transformers Mounted
3. After drilling, take care to remove the metal shavings from the inside of the chassis and your work surface before continuing. Debur any holes that may have drill flash left on them.
4. Mount the transformers as pictured with the screws/nuts left over from removing the stock transformers. Mount the choke with the screws/nuts provided in the Mercury Magnetics kit.
Note: The alignment of all units, with wires oriented towards existing grommeted holes.
5. Pull your leads through the grommeted holes leaving a small amount of slack.
Here is a list of the wires and where they come from, that you will have to connect up.
Power Transformer:
Primary -- Black and Yellow
Secondary High Voltage -- Red and Red
Secondary Low Voltage -- Orange and Orange
Secondary Ground -- Orange w/White StripeOutput Transformer:
Primary -- Black and Yellow
Secondary 16 Ohm Tap -- Gray
Secondary 8 Ohm Tap -- Yellow
Secondary 4 Ohm Tap -- Green
Secondary Ground -- BlackChoke:
Input and Output -- Black and BlackI listed these like this to first separate the transformers per their wiring and to help with the instructions for wiring each transformer -- since as you can see, we have several black and yellow wires which come from different places -- and we sure don’t want to mix any of those up!
1. Locate the filament hookup wires you soldered on the preamp tube side earlier, on the EL84 tube side you left un-soldered -- twist the two Orange filament wires coming from the power transformer and insert in the two remaining holes (drilled) beside the EL84 socket. Solder Orange filament supply wires from output transformer to the white filament wires from the preamp tube, on the back side of the PCB.
See Figure 5-18.
Figure 5-18: Filament Wiring
2. Insert a Red wire each, from the power transformer, into the holes left from the terminal lug removals at pads T1 and T2. Solder in place on back side of PCB, see Figure 5-19.
Figure 5-19: PCB Wiring
3. Insert the Yellow wire from the output transformer primary into a terminal lug hole at T3 and solder into place. See Figure 5-19.
4. Insert the Black wire from the output transformer primary into the remaining terminal lug hole at T4 (beside previously inserted jumper) and solder into place. See Figure 5-19.
5. Insert one Black wire from the choke into a hole at one end of the removed R10 location and solder into place. See Figure 5-20.
6. Insert the remaining Black wire from the choke into the hole at the other end of the removed R10 location and solder into place. See Figure 5-19. (make sure it is the R10 location, not the R11 location beside of it).
7. On the back side of the PCB, there are many traces that connect to each other, this is the ground trace of the PCB. Find the location where they intersect, near a component leg coming through the PCB. Scrape the epoxy mask off of the trace at that location until the trace is shiny. Solder the Orange w/White stripe wire from the power transformer secondary to this location -- it helps to also solder the wire to the leg poking through the PCB at that locations as well as to the trace. See Figure 5-20.
Figure 5-20: Orange Ground Wire
8. Strip a ¼" from the wires you previously cut at the front panel on/off switch. Slip some shrink tubing over the wires, connect Yellow to Yellow and Black to Black from the power transformer primary winding. Solder together, slip the shrink tube over the connection and heat to secure in place. See Figure 5-21.
Note: If you cut the Green with Yellow stripe coming from the PCB as I did earlier on, reconnect it at this time, using the same methods described for the power switch -- except wire Green with Yellow to Green with Yellow.
Figure 5-21: Switch Wiring
9. Locate the 4 wires coming from the output transformer secondary and the 4 wires on the speaker output jack board at the rear panel.
10. In the same way that the connections were made at the power switch, I trimmed the wires that were previously cut when the stock transformer was removed, back to about ½" in length and removed the insulation from them close to the speaker jack board.
11. Slip some shrink wrap tubing over all four of the secondary wires from the output transformer and connect them according the list given at the beginning this section (16 to 16, 8 to 8, 4 to 4 and ground to ground) -- solder and insulate with the shrink tubing.
Note: I cut the secondary leads from the output transformer relatively short to minimize on noise. See Figure 5-22.
Figure 5-22: Output Transformer Wiring
12. Double check all of your connections, actually, triple check them at this time. If all looks well, using the screws previously removed -- secure the PCB in place.
13. If you did the BitMO mods, now is the time to rewire the toggle switch that was disconnected -- this time around, I elected to wire the toggle switch outputs to the volume control instead of the PCB to further minimize noise. It is also time to insert the tone pot back into the hole drilled in the front panel and secure in place. Check your BitMO connections thoroughly at this point. See Figure 5-23.
Figure 5-23: BitMO Mod Re-wiring
14. At this point you are done -- make sure you clean up the chassis, that there is no insulation floating around, and any solder splash or wire clippings are removed. Using some canned air can help with this.
Stop!!!
You are about to test your work -- it is highly advised you have an amp tech do the first power up, unless either you are supremely confident your work is correct or you have a variac handy. Using a variac is the correct way to power up the amp after mod work like this, slowly rolling on more power to ensure that no caps pop, transformers don't get hot and the amp doesn’t start smoking.
In whatever manner you choose, your result should be a working amplifier and the transformers should not get hot to the touch. If the amp is working, play it -- and enjoy! Because this transformer mod takes the amp from ok to boutique as expected. Articulation is excellent, clarity is very good, distortions is very bite/smooth to edgy/metal. Experiment with the volume control and tone settings at different places -- don't forget the volume control on your guitar either -- it is an excellent form of drive control, right at your fingertips.
As before, the titles of the samples in the player for this stage of the project will be prefixed with "Mod-4" in their titles. I'll do a random sampling at different positions for the tone and volumes, which will be in the sample clip title names as well.
I wouldn't be at all ashamed using this amp to gig with at this point -- my son, upon hearing this has already told me its 2X over my Mesa F40 amp tone wise, another friend commented that he thought he was listening to a Bogner.
You tell me, compare clips from the previous mods, here is the player...
Of course, all the comparison clips are using the same parameters, guitar, etc. Two things really stand out, especially live, the low end notes are much clearer, the muddiness is all but gone and the power curve has really flattened out -- the db readings between the 9:00 position of the volume control and the full on position is only a 6db differential, peaking at 116dbs. Of course the 116dbs are pushing distortion and sound twice as loud as the cleans at 110dbs,but the difference just is no longer that great. I found the the EL84 tube is pretty much maxed at the 12:00 to 1:00 positions, the preamp now being very strong, turning the amp up higher doesn't produce much more top end. I found my sweet spot at about 10:00 on the volume, pegging at about 111dbs, it's still very loud, but also very manageable. The last clip in this mod is titled "Random...," and it is the same exact settings on the amp, using only the guitars volume and pickup switches to produce 4 levels of distortion/tone.
All I can really say is… it's a joy, still under $500. Without the optional attenuator, this transformer mod just simply rocks. I’d like to thank Mercury Magnetics for:
a) Producing a terrific upgrade kit for this amp and,
b) For all the time they spent with me on this project.
Their products are as billed, and produce a superior sounding amplifier. Of note, on new transformers – its takes about 20 hours of play for them to break in, which will produce yet again, more smoothness as the amp is played.
Big thumbs up for this product!!!
As for amps, if your ear is accustomed to boutique sound and you know what you are looking for in tone -- this poor man's path to a boutique amp just cannot be beat. I'd rank this amp in the top 10% of amps under $1000.00 and in the top 40% of comparably powered amps in the $1000-$1500.00 range.
I think the reigning governor of tone, the honorable Reverend Billy G, would be impressed with this unit as it stands.
Oh yeah -- there is still the cherry to put on top of this pie, Mod-5 is next....
Materials: Mercury Magnetics Epiphone Valve Junior Upgrade Kit
Total cost: approx. $289.95
Time to complete: approx. 8 to 10 hours (taking your time and paying attention to detail)
(1) Total investment: Amp + Mod-1 + Mod-2 + with Mod-3 + Mercury Kit $289.95 = $526.80
(2) Total investment: Amp + Mod-1 + Mod-2 + Mercury Kit $289.95 = $456.85
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Modifying the Epiphone Valve Junior
Guitar Amplifier
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