Rags to Riches, Cont.

Hello everybody and thanks for all the emails about my last column,

where we modded the Epiphone Valve Junior amp to make it sound like a

Fender Tweed Champ. A lot of you asked for more - more mods and mostly

more gain, so this month we will focus on modding this cool little amp

in the high-gain direction, while next month I will show you how to add

some useful features to this amp - e.g. adding a stand-by switch,

recording out (aka "line out"), adding a gain and/or tone control,

additional 4/8/16 ohm speaker outs, etc. Stay tuned!

Again, all these mods are for the 2nd generation of this amp,

which was sold from approximately March 2006 to today as the combo or

the "head only" version. All the mods will also work for the 1st

generation of this amp, but they require a lot more work because the

circuit suffers from some extra noise and various other unpleasant

things. If you have such an amp, check out this website and follow the instructions to quiet the beast.

Before we start, I''m sure you know what''s coming now:

Do not attempt to perform these mods if you are not familiar with

working inside tube amps or other high voltage electronics. There are

lethal voltages inside the amp, even when unplugged (if the filter

capacitors have not been discharged) and these voltages can cause

serious injury or kill you. If you are unsure, take it to an amp tech.

If you decide to perform these mods, you assume all responsibility for

anything that happens! Whether the amp explodes, you get zapped, or the

amp suddenly increases in value because everyone falls in love with it,

it''s all because of you. The glory, the pain, whatever, they''re all

yours. If you can''t live with that, don''t mess with the stuff here.

Make sure to download the circuit drawing of our Epi amp - if you''ve misplaced it, here it is again. Print it out and put it on your workbench, to follow along with what we''re talking about.

In the first step we will "Marshallize" this amp to make it

sound similar to a vintage Marshall amp, and in the second step we will

take it to the limit and make a high-gain monster out of it - think of

a cranked Diezel or Brunetti amp and you''ll have the idea.

Again, I will divide all the mods into three categories:

general mods, necessary mods and the tone-tweaking mods. You can decide

on your own how far you want to go.

When talking about the Marshall sound, we''re mostly referring

to a Marshall amp head paired with a half or full stack 4x12" or 8x12"

cab. Naturally our little Epi amp can''t hold up with such tone

monsters, but we can get a similar tonal response from it. In addition,

our Epi can drive your full stacks easily through the speaker-out jack,

so don''t underrate this little rascal.

So heat up your soldering irons, unplug the amp, discharge the capacitors and take out the PCB.

Tubes
This is a very quick, easy, inexpensive and effective way to mod the amp. A simple amp like our Epi with only two tubes gives you the rare ability to really hear the differences between different brands and types of tubes with distinct clarity. For a good and strong Marshall tone, pull out the two Sovtek tubes and replace them with a JJ 12AX7/ECC83S and a JJ EL84/6BQ5 "Red Label." Both tubes are excellent sounding and easy to get. Naturally you can also experiment with different tubes to see what you like best, but take care to choose tubes with a lot of gain.
Speaker
I really like the stock speaker and it provides a good and punchy tone for its size. It''s a Weber-designed speaker, so what else can I say? But if you want to change it, try a Weber "AlNiCo Sig 8" or any Celestion 8" speaker. If you can find a Hughes & Kettner 8" speaker on eBay, grab it - they sound excellent for high-gain sounds.
Output Transformer
Many of the less-expensive amps available today ship with low-budget output transformers. These are designed to be cheap to produce, and sound quality often suffers. Inexplicably, the Valve Junior OT is wound with a 7.5K primary, when the ideal primary for an EL84 in SE mode is 5.2K. I did not change my OT because I like the sound, but if you want to replace it, try a Hammond 125CSE, which is twice the mass of the stock OT and can be wired for an almost ideal 5K primary impedance. There are some more OTs that will fit your needs, so feel free to use a different brand.

The mods discussed here should be done to any Epiphone amp to extend tube life, to increase stability and to reduce noise.

1. Connect a 20-ohm/20-watt power resistor between the IEC mains input socket and one of the primaries of the power transformer. This will bring the plate voltage down to approximately 300VDC and will extend tube life dramatically. This will result in a better ("browner") overdriven tone.
2. Replace C6 (stock value: 22uF) with a 120uF/450 volt electrolytic capacitor to quieten the amp by providing a better filtering of the power supply.
3. Replace the four 1N4007 diodes with UF4007 to get rid of the switching noise.
4. If you have an amp with a non-insulated input jack, replace it with a nylon input jack to break the ground loop and get rid of the 60Hz hum.
5. Replace the speaker cable with a good quality, heavier gauge speaker cable and solder it to a rugged Switchcraft "angled" plug.

This is the section where you can influence the tone and tweak it in any direction you want to, so let''s go and Marshallize it in the first step:

1. Replace the stock 1M volume pot with a CTS 1M audio taper pot.
2. Replace C1 and C2 with the yellow 0.022uF "Mallory 150" caps
3. Change R8 and R9 to 820-ohm metal film resistors
4. Change R1 to a 1M metal film resistor
5. Jumper R2
6. Change C5 to a 1000uF/25-volt electrolytic cap
7. Change R4 to a 100k metal film resistor
8. Change R5 to a 500k metal film resistor

If you want to go a step further, here are the mods to convert the amp into a roaring beast, similar to a Diezel or Brunetti amp at full tilt. Let''s unleash the fury - these mods will give you the maximum possible gain out of this circuit:

1. Replace the stock 1M volume pot with a CTS 1M audio taper pot.
2. Replace C1 and C2 with the yellow 0.022uF "Mallory 150" caps
3. Change R3 to a 220k metal film resistor
4. Change R4 to a 100k metal film resistor
5. Change R8 to a 1.5k metal film resistor
6. Jumper R6
7. Remove R7
8. Change R9 to a 680-ohm metal film resistor
9. Change C5 to a 2200uF/25-volt electrolytic cap
10. Change R1 to a 1M metal film resistor
11. Jumper R2
13. Change R5 to a 1M metal film resistor

incredibly authentic sounding Marshall or modern high-gain tone, and a

one-of-a-kind practice/recording amp. You can also do a personal mix of

the two sections to create your own signature amp. I highly recommend

to do all these mods step by step and to listen to the results - you

might find that you don''t need to perform all the listed mods for the

tone you have in mind.

I hope you enjoyed this one and don''t forget to warn your

neighbours before you plug into your modded amp and strum some power

chords! Next month we will add some very useful features to the

Epiphone amp to make it much more versatile. Happy soldering and I''ll

see you all next month.

Dirk Wacker has been addicted to all kinds of guitars since the

age of 5 and is fascinated by anything that has something to do with

old Fender guitars and amps. He hates short scales and Telecaster neck

pickups, but loves twang. In his spare time he plays country,

rockabilly, surf and Nashville styles in several bands, works as a

studio musician and writes for several guitar mags. He is also a

hardcore DIY guy for guitars, amps and stompboxes and also runs an

extensive webpage (singlecoil.com) about these things.