Modifying the
Epiphone Valve Junior Guitar Amplifier
"From least to beast"
Mod 5 -- Installing Mercury's 6V6 Mod
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Modifying the Epiphone Valve Junior
Guitar Amplifier
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This additional Mercury mod is pretty much the cherry on top -- it is simple, quick and fairly inexpensive. It does take a little work, though. Mercury Magnetics supplied me with the plans for this mod, which were designed by Alan Cyr of the Amp-Exchange in Los Angeles. Alan is one of those guru tech guys that builds custom amps and is a large proponent of the Mercury Magnetics' family of products.
Note: This mod requires that the Mod-4 transformer kit to already be installed, it is predicated on the Mercury circuit mods as well -- even though I did not implement the Mercury circuit mods -- the BitMO mods do pretty much the same thing, squeezing about as much gain out of this little amp as can be had.
6.1 -- So what is the 6V6 Mod?
Mod-5 involves installing another power tube, running in parallel with the EL84, still in single ended mode. It does three things:
a. it increases output yet again
b. it raises the headroom floor and
c. adds more voicing to the output section of the amp.
A 6V6 octal pentode tube is used in this mod -- giving it some differentiation from the already present EL84. The 6V6 tube is very close in terms of voice to the EL84 and for this installation is also cathode biased. When using this mod, you can pull out the EL84 and run alone on the 6V6 or pull the 6V6 and run as it was before this mod or have both in and run in tandem, so you see, by manipulating the tubes, you can still still more varieties of tone from this little amp.
At the time of this writing Mercury Magnetics did not have a kit for this mod (it should be coming shortly). Because of that, I won't disclose exactly what parts are being used in this mod or going step-by-step on how to install it.
We will be using the heater filament leads installed in the previous mod to heat the filaments in the 6V6 tube, install a little capacitance circuit to keep the tone up and the bias grid charged and running the cathode (output) straight into the output of the EL84.
For this mod, I highly recommend you visit the local electrical supply store and pickup a Greenlee ¾" Knockout die tool. It really will make your life easier as opposed to drilling 30-1/16" holes in a circle, clipping out the center and filing down the ragged hole. This hole of 1" is required to mount the new socket in the chassis for the 6V6 tube. If you know an electrician, they probably have one -- at an electrical store, they are about $8.00. Even though you would probably only use it once, it is still a great thing to have for punching a hole out of sheet metal. The process is, drill a pilot hole, then put one side of the die with the bolt on the outside and the cutter side on the inside, tighten the bolt to draw the cutter through and voila! A nice clean hole.
Note: A ¾" die is used for a 1" to 1-1/8" diameter chassis mount socket.
First, make sure that all the tubes are removed from the chassis, then see Figure 6‑1 for the approximate location of where to drill the pilot hole for the knockout tool.
The pilot hole size needs to be about 3/8" in diameter, before drilling, make sure that wires on the inside of the chassis are out of the way. When drilling, "pop thru" carefully to avoid messing anything up in the inside.
After the hole is drilled, insert the knockout die and cut out you hole -- after its done, it should look as shown in Figure 6-2.
Once you have cut the hole, you will need to drill two screw holes on each side of the cutout. Place the socket into the hole, with the flanges on the chassis, mark your holes through the mounting flanges. Drill two 5/32" holes for the two 6-32 machine screws you will use to fasten the socket to the chassis. You can get these screws/nuts and star washers at just about any hardware store for about $2.00 max. make sure you place the star washers on the inside of the chassis, under the nuts, when you install the socket.
Next up is wiring -- since the schematics supplied seldom if ever show which pins on a socket are the heater filaments -- I'll tell you -- Pins 2 and 7 of the 6V6 socket are the heater filaments. You will want to take two leads from the EL84 socket (I just soldered them in on the EL84 socket fins, 4 and 5), over to pins 2 and 7 on the 6V6 socket, as shown in Figure 6‑3 (Orange wires).
From there, using the parts specified by Mercury Magnetics and the supplied schematic, wire the rest of the 6V6 socket into the circuit, when completely installed, it will look similar to Figure 6‑4.
Figure 6‑4 6V6 Socket Wired
That’s it! Of course, at this point, you will want to install all the tubes into chassis sockets, install all of the tube shields/retainers and get ready to test this mod. Double check your wiring, the first time around, I wired the heaters incorrectly -- thank goodness no harm was done, that's what I get for being in a hurry though.
Once it is all done, sporting a 6V6 tube of your choice (more on this later), it should look like as shown in Figure 6‑5.
After testing it to make sure it works, it is about time we buttoned the whole unit back up in its case and put a sticker or two on the EVJ. Mercury Magnetics supplies a nice metal plate to stick on and BitMO (not shown) supplies decals for both the toggle switch and the tone/boost control. Looks pretty good!
Figure 6‑6 All Finished!
The clean headroom increased a bit, and the cleans are nice and sparkly with the addition of the 6V6 tube, running the volume at around 9:00 to 11:00. The distortion starts around 11:00 and continues the rest of the way up to full volume. At peak volume, this amp now puts out 118db peak through 2-12" speakers, plenty enough to play a small venue. Doing the calculations, this amp is putting out somewhere between 20-22 watts of power now, pretty hefty. The samples that follow are fairly deceptive in that they sound much the same as without the mod, except that they are coming out at higher volume. To cut back on the overhead, just pull a power tube.
I tried two different types of 6V6 tubes in this mod, the two clean and two distorted samples use a TungSol and JJ/Tesla each.
After a lot of testing on these two brands of tubes -- here's my opinion, the JJ/Tesla 6V6 is more powerful, crisper and had a little better headroom on cleans -- it was also very manageable under heavy distortion.
The TungSol 6V6 was much warmer, thicker and darker -- not as "gainy" as the JJ, but not as peaky either, more predictable. Under heavy distortion with a humbucker, it had a tendency to muddy up a little, and feedback occurrence was more often as well.
So, I break it down like this, I think the JJ/Tesla is an excellent match for a humbucker, and though it won't pump out quite as much power… tonally, I think the TungSol is a great match for single coil pickups, sweet bluesy sound out of a Strat. The TungSol thickens up a Strat just right.
Materials: 6V6 tube, kit price unknown (parts -- socket and others, around $10)
Total cost: approx (piece parts) $40.00
Time to complete: approx. 1 hours
Total investment: Amp + Mod-1 + Mod-2 + Mod-4 $40.00 = $506.00
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Modifying the Epiphone Valve Junior
Guitar Amplifier
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