Modifying the
Epiphone Valve Junior Guitar Amplifier
"From least to beast"
Mod 1 -- Re-Tubing Experiment
Gearscore.com's
Modifying the Epiphone Valve Junior
Guitar Amplifier
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The modifications in this chapter are the easiest to do, we will be replacing the stock tubes with some selected aftermarket tubes to change the voicing on the EVJ. But before we do that, it is time to create some baseline sound samples for comparison throughout the entire mod process. This will enable us to check back on a regular basis to listen to the tonal differences each mod may or may not make.
Much of the gear used to make the sound samples is described in the introductory chapter. I'll add some more notes here and then create the first set of samples for later comparison.
In addition to the previously mentioned gear and process to create the samples, I have chosen to use my Carvin CT6M guitar, running on the neck pickup, which is a Carvin Holdsworth H22N humbucker. The tone and volume knobs on each sample recorded are always at the full on position.
A note about speakers: I am using a 2x12 speaker cab rated at 60 Watts. The initial samples will be duller than if you have the combo unit or are using 8" or 10" speaker(s). The reason is, this is a low power amplifier at 5 Watts stock and there just isn’t enough power to drive the cones on 2-12" speakers at this point. I have also chosen to use the neck pickup for all the mod samples -- which will also contribute to a darker tone, reason being, I expect this amp to brighten up considerably before we are done with it, so starting dark seems like a good idea to me.
What all that means is, if you have a 8" or 10" speaker(s) and play on a bridge pickup -- out of the gate, your stock sound will be a tad brighter than what I have recorded as the baseline samples.
At each stage of modification, including the baseline, the samples will be taken with the volume knob at the 9:00, 12:00 and full on positions. The resulting MP3 files will be named Stock-900, Stock-1200, Mod1-900, Mod1-Full, etc., as they appear in the Gearscore MP3 player. Each sample is about 32 seconds long and the same progression/licks will be played for each sample (boring) for comparison. In the event that a mod introduces different switching positions, samples will be generated for each switched position as well, and follow this naming convention for the MP3 files: Mod2-900-S1:1, Mod2-1200-S1:3, and so on, with "S1" being being switch one and ":1" being the lowest position of that switch. Don't worry, I'll try to do a spreadsheet which should make it easier to tell which sample is which, later on.
Included in the MP3 name will also be db (decibel) levels for each sample, the db level represents the sound pressure level at 4 feet from the speaker cabinet -- same position as the recording microphone. For more information on SPL (sound pressure levels), I recommend this site, which also has a calculator: http://myhometheater.homestead.com/splcalculator.html
In the player below -- we now see the first set of samples -- click to listen to a sample,
I found the stock sound in the 9:00 position to be fairly sweet, not particularly thrilled with the nasally/muddy sound from the stock amp once it started into clip. The second sample at 6:00 was barely usable tome and the full position was extremely noisy and completely useless.
I am going to try and give a very basic principle of theory on tubes here, it is by no means a complete discourse and is only presented so that you can understand why some of the changes in this mod.
Let's start with the light bulb, the original filament heater mechanism -- when Edison put a couple more elements in the light bulb and heated them -- he (and others) found that a current flow happened from one element to another. For all intensive purposes, it was also found that the electrons were released from one element with current on it, went in one direction, with the opposing current on it. This only happened from the negatively charged element to the positively charge element. These two element tubes became known as diode tubes. The elements were named the cathode (negative) and the anode (positive or plate).
To control the flow of electrons from the cathode to the anode, a third element was introduced called a "grid." By varying the current on the grid, electrons traveling from the cathode to the anode could be accelerated. This tube type with three elements became known as a triode, and thus, tube amplification was born.
Originally, the cathode was directly heated by filaments, as refinements occurred, the filaments were positioned to indirectly heat the cathode -- it was also found that too much heat could lead to premature failure and that an optimum temperature be found for the filaments, through many a calculation, a filament voltage of 6.3V was arrived at -- and that's the magic number for heating a tube in 90% of all tubes made today.
So, one thing leads to another -- adding another element (grid) for control of the electrons gave rise to the tetrode tube, adding yet another element created the pentode -- which is the tube in common use in tube amplifiers today.
Essentially, what you have in a tube is an electron accelerator, where a base voltage is applied to the cathode, which is indirectly heated to release the charged electrons from the metal material and then flow towards the positively charged anode or plate. Along the way, they are accelerated by traveling through grids that are also charged and the result at the plate is accelerated or amplified signal. Obviously, vary the voltages to any of the elements in a tube affects the electron flow -- and gain/amplitude control results.
Think of the cathode like a handful of sand, the releasing some of that sand into a fan (grid) which is directed at another fan (grid) and again, until the sand arrives at a bucket on the other side -- then you have a pretty good basic visualization of how electrons are accelerated, as the sand will pickup speed with which each fan (grid) it encounters. That’s a very rudimental analogy, but for the purposes of this primer, works well enough.
Figure 2-1 illustrates how tubes are represented on a schematic, note that the heater/filament is traditionally considered to be one object for pictorial use, represented in the schematic symbols as the pointed set of lines under the cathode. Grids are represented by dotted lines between the cathode and anode. In many schematics, where you have a dual triode, which means you have two triodes in one bottle (e.g. 12AX7), you will often see the circular tube split in half on a schematic, with wiring going to each half of the dual triode tube. When reading a schematic with dual triodes, note the reference designator of each half (V1a, V1b) so as not to confuse one half of the triode with the half of another triode in a circuit that has two or more triode tubes.
Figure 2-1 Vacuum Tube Schematic Symbols
Of course the very first thing we must do before installing new tubes is to unplug the amp!
There is one power tube, an EL84 and one preamp tube, a 12AX7 -- used in this amplifier. The stock tubes I found in the amp were an Electro-Harmonix 12AX7 and a Russian (SovTek) EL84. In talking with a few folks doing these mods -- some have reported a Chinese tube in the preamp section and/or a JJ tube in the power section. I think what Epiphone shipped out in terms of stock tubes, depends on the version of the PCB and whatever happened to be in stock at the time of build.
That said, check to see what kind of tubes you have before ordering! If you already have a JJ/Tesla tube in there, it would be pretty foolish to buy another one!
Before we can change the tubes, the back panel of the EVJ must be removed for access. There are 7 screws/washers holding the back panel on -- remove the screws and washers, set aside and remove the back panel. For this project, I will be leaving the back panel off until the final mod is completed.
On to changing the tubes, the EL84 has a spring retainer on it -- just lift the retaining ring off the end of the tube and wiggle the tube out of its socket.
The preamp tube is a bit more involved, it is covered by a metal shield which must be removed first. To remove the shield, turn it (either direction) until you feel it start to pop out. The shield has two nubs on it that fit through two ear holes within the main socket hole. So you have to line the nubs on the shield up with the ear holes in the chassis -- once you have done this, you will feel the shield come free. Remove the shield and note that there is a spring inside the shield -- this is to keep the tube from rattling and causing microphonic noise, more on that spring in a bit. With the shield removed, wiggle the preamp tube out of its socket as you did the EL84 tube.
The tubes I have chosen for this project are a JJ/Tesla EL84, for its punch and a Tung-Sol 12AX7 preamp tube for its creamy gain/distortion. The Tung-Sol tube is based on the old Mullard tube design found in British amps of the sixties, it is also very durable and can take a beating.
I ordered the tubes from http://thetubestore.com, they have an excellent selection of tubes at very reasonable prices, excellent customer service and fast shipping.
To place the new tubes in, be aware that they will only go in one way, so look at the pins on the tube and notice that there is a bare spot in the circle of pins, the sockets are the same way -- so find the bare spot, line up the pins and insert the tubes. It will probably be handy to have a penlight so you can see into the sockets to find the orientation of them.
Before replacing the preamp tube shield, extract the spring and cut about 3 coils off the bottom of it. The spring is way to stiff and could cause PCB issues later on. Put the spring back in the preamp tube shield and reinstall the shield to the chassis.
Lift the retaining rings and stretch the springs out so that you can slip the retaining ring over the EL84 tube, securing it in place.
You are now done with Mod-1!!!
In the player below -- we now see a new set of samples added to the baseline/stock set of samples, click to listen to a sample.
Overall -- I found that the Tung-Sol tube did what I thought it would, although is seems to have smoothed the gain out, it also is less bright than the original stock tube. I still like the rhythm at the 9:00 position and I also really liked the lead tone at the 12:00 position, starting to "sing" -- a very nice lead tone. The overall output of the amp came up a few decibels and the noise went down for each sample as well. The tube swap was worth the cost for me, +1 on the tube swap.
Materials: 1 -- JJ/Tesla EL84 tube, 1 -- Tung-Sol 12AX7 tube
Total cost: approx. $24.95
Time to complete: approx. 10 minutes
Total Investment: amp $92.00 + tubes $24.95 = $116.95
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Modifying the Epiphone Valve Junior
Guitar Amplifier
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