STEP-BY-STEP HOW TO FOR MODDING AN EPI VALVE JR FOR HARP




Turn that Epi into an Awesome Harp Amp


INTRODUCTION
Epiphone has come out with a winner in its low-cost bare-bones tube amp. The amp is very popular and for good reason. The amp has even been improved upon since its emergence and now has a DC filament supply and improved power supply filtering, making it a very quiet amp that performs well. The amp is truly built in the spirit of the vintage amps with its art deco cab, solitary volume control, and minimum engineering. The amp is very popular as a first-out tube amp for harp players and one that lends itself particularly easy to modding, which is what this page is all about: modding the Epi for harp.

As I have stated before, I believe that an amp has a character or a natural presence, a tone that is fundamental to the components, their age, and the design of the amp. Let an amp be itself, and it will sound its best. Make simple, reversable mods, and if they do not bring the tone to where you like it, sell it and get another. What may not be perfect for you, may be perfect for another who plays differently and has a different mic, cup, etc. If you chase every mod recommendation on every forum, you will never be satisfied, you will spend more time modding than playing, and your amp will be the worse for the wear.

STEP ONE
Compare the schematic to your amp and make any corrections needed. Changes in production are common, and you need to verify that you have a good schematic before you begin. You should treat both stages of the preamp as a whole, considering the input signal level, the bias, the gain, and the level needed to drive the next stage. This is imperative to achieving a good tone, minimizing feedback, and maximizing the performance of your amp. The power supply should be sufficiently filtered; you loose nothing in vintage tone by having a well-filtered power supply and have everything to gain in quietness. Below are both the before and after schematics. You will see here that I offer two mods: Option One with high and low inputs and a simpler Option Two mod with one input.

     


THE PREAMP
To set up the preamp for the Epi, I used a nominal 0.2V peak-to-peak signal input. This is the approximate level generated by an average mic element when playing the harp softly; a hot element might produce a 0.4V signal easily and up to 1V when played hard. I kept the 12AX7 because I want to keep changes to a minimum. The bias of the second stage is about 1V after installing the 2k resistor. I do not want to hit it with much higher than a 3V signal at max (10) volume, which will give us a 1 to 2V working range. The extra 1V is just to have a bit extra on the volume level if needed. To allow for fewer parts to be changed, we will set the 12AX7 for a gain of 25x, so 25 times 0.2 equals 5 volts. The output of my first stage will be 5 volts. 5 volts is too high to hit the second stage, so we need a voltage divider to drop the signal. Since the Epi already has a voltage divider (R6 and R7 paralled with the volume control), we will keep it but remove R7 and just use R6 and the volume control as my voltage divider. Just using R6 and the volume control as my voltage divider will pick the voltage up to about 2.5V max, which will be close enough to the 3 volts I was looking for. I will be using a treble cut tone control, which has zero loss at low frequencies, so no additional gain is needed. But if you use a different tone control, you will need to consider the loss that it will cause.

The second stage will drive the EL84, which is biased at 9.5V. I shoot for a ratio of about three to one, signal to bias, so that I will have plenty of overdrive but still have full use of the volume control while minimizing feedback. Once more we will set up for a gain of 25x, and 25 times 2.5 volts signal input equals 62.5 volts. So we need another voltage divider here, and we will add a 220k resistor in series with the 220k in place (R5) to give us about 31 volts max applied to the grid of the EL84. A little high but close enough to work. In the second schematic with one single input, you will see that a 100k resistor is used, which is just a compromise between the high and low input options. Note that the gain and the voltages do not have to be exact, but we want them to be fairly close.

The coupling caps, C1 and C2, will be replaced with oversized 0.47uF coupling caps to give the amp maximum bass response. (NOTE: I have now changed this to 0.2uF because the oversized 0.47uF did not add any noticable bass response when compared to a 0.2uF; the standard 0.1uF is also acceptable.) The plate voltages will be lowered to 130V by replacing R13 with a 180k resistor (a 200k is ok). The grid resistors, R8 and R9, will be replaced with 2k resistors. The bypass caps, C3 and C4, will be removed to set the gain. On this amp, I removed jumper J1, which takes the second stage bypass cap, C3, out of the circuit instead of removing the cap itself.

OPTION ONE

    

    


OPTION TWO

    


THE POWER STAGE
No changes were made here; voltage and current checks show that the tube is operating within its perimeters, and I see no reason for changes while keeping with my less-is-better approach to modding. On "Option Two" there is no filter cap change.

THE OT AND THE SPEAKER
The OT remains the same, but if possible, a speaker upgrade to a Weber Vintage Series 8" with the "H" dustcap is a great improvement that is well worth the additional cost.

THE INPUT
A second input jack will be added with a voltage dividing resistor, so that we can have a high gain and a low gain input. Also the 68k R1 will be replaced with a 1m and remounted to provide proper loading to the mic, and R2 will be soldered to the leg of R1 as shown. The inputs are wired to give a ground when no jacks are plugged in. I used the existing PC-mounted input jack for the monitor and installed two new input jacks. On "Option Two" there is a voltage divider on the input that was added because there is only one input.



THE TONE CONTROL
The tone control is a simple treble cut using a 0.003uF cap and a 1m linear pot. All tone stack tone controls have loss; this is inherent in their design and unavoidable. Turning the bass all the way up only minimizes loss, it does not provide any gain at all. This is why I prefer the treble cut, which has zero low frequency loss. The hot lead is soldered to the leg of R6 on the C1 side, and the other lead is soldered to the ground (where R7 was removed). See the image below for installation. It is a good idea to glue the cap to the pot. The addition of the tone control is optional for Option Two.
    


THE OUTPUT MONITOR
An output monitor was added by using the old input PC-mounted jack and installing it in the position closest to the side of the amp on the output board. A voltage divider using a 47k and a 4.7k resistor provides the isolation required. The images detail the installation. On the second image, I drew in the wires that are actually on the other side of the board just to get a good understanding of where the wires connect.
    


As a parting statement, I would like to stress that you should treat the preamp as a whole because it all needs to work together, and do not just take mod ideas from different sources because that is really a hit and miss proposition. Whatever you do to one stage will affect the next. Maintaining a proper signal level to bias voltage ratio throughout your amp is critical to good tone, and definitely spend more time playing than modding. This is not meant to be the end-all Epi mod but just a guideline. There is more than one way to skin a cat and more than one great harp amp.






Here is what the owner had to say about the project amp:
"I played through the amp for about an hour over the weekend and I gotta say that I am thrilled with the results. Playing in a small bedroom, I could still get plenty loud without feedback. Can't wait to get it in a bigger room and open it up a bit more. The bottom end honk and breakup are great too. Breakup gets there well before the feedback, very nice! As I said before I've got very little experience with amps, but this sounds good to my ears. I can't wait to get together with some friends in a garage somewhere and rock out!"

Scott Mann writes:
"Randy, just wanted to check in with you and let you know I am loving my Vjr more and more each time I play with it. Thanks again for the great work."



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