Epiphone Valve Junior Modifications

duhvoodooman*** · June 20th, 2006, 08:37 AM

The 5W Epiphone Valve Junior has certainly gotten its share of press here at TheFret.net, being the subject of the longest single thread in this forum's brief history (212 posts and counting as of today, 6/20/06), and deservedly so. This little gem has quickly established a cult following, since it offers genuine class A tube amp vibe for a ridiculously low price ($140 for the combo, $100 for the head). And you can pick up used examples of the combo version in good condition on eBay for well under $100 (I got mine for $92).

But rather than further effuse over its goodness, which Nelskie, Tone2thebone and others have already done in the aforementioned thread, I thought I'd post my experiences, both completed and in progress, with modifying this amp to improve its tone and performance further. Nelksie talked about swapping tubes and changing the stock speaker, but I want to get into some more fundamental changes here, involving making additions and substitutions in the amp's electrical chassis componentry. These modifications are covered in much more detail on some other websites, and I won't endeavor to go into the technical depth they do. But these are the websites where I found all of the information that I base my following comments upon, so check them out if you want more detail:

Erik Miller's VJr Mods at Euthymia Electronics

Dennis Cronin's VJr Mod Site

Svokke's VJr Mod Page

Brian Oddi's VJr. Mod thread at Kluge Music

VJr Mod Thread at Hoffman Amps

As good as the original combo version of the VJr was, it had some shortcomings, too--completely understandable in an amp this inexpensive. First and foremost is it's propensity to hum. In fact, the amp hums in two completely different ways--a low-level background hum that is independent of the amp's volume setting, and a more obnoxious "buzzy" hum that builds as the volume is increased. On the VJr combo I bought, the former was practically unnoticeable to me, but the latter was pretty bad with the volume up past 2 PM or so. I found several different mods that were designed to reduce either or both of these hum types, ranging from very simple to fairly involved. The good news is that none of them are expensive, so I elected to do several of them and see how much improvement I could get. (Note: These hum deficiencies were addressed in the head version of the VJr, and I understand new production of the combo incorporates the fixes, too.)

A second shortcoming that I noticed in my VJr was a rather dull tone, lacking "top end". I found myself maxing out the treble tone controls on all of my guitars when I played though it. Though the mod sites listed above tended to go after the hum issues as the primary improvement need, all of them made changes that boosted the amp's top end to some extent, as well, so I was obviously not alone in my opinion. Since I don't often crank the volume up into the range where the buzzy hum gets bad, I actually considered this need for more brightness to be my first priority. I swapped the stock Sovtek tubes for JJ's from Eurotubes.com, and though the amp's tone improved and brightened slightly, the improvement was nowhere near what I was looking for.

In addition to the above two issues, a number of other mods were described at these various websites, mostly falling into the category of enhancements to the VJr's intentionally sparse feature set. These included things like adding a high gain switch, an additional 8 ohm output jack, or installing a small light bulb to act as a signal attenuator. However, I had no personal need for or interest in any of these, and opted to pass on implementing them.

To deal with the hum issues, I decided to make the following four modifications:

Improve grounding on the PCB by adding a jumper wire between two of the traces. This corrects a grounding design deficiency.
Replace the stock input jack with a nylon insulated jack, to isolate it from the chassis.
Add a larger filtering capacitor (100uf/450V) to the main high voltage supply to supplement the smaller stock caps.
Convert the voltage supply to the tube heater filaments from AC to DC, requiring the addition of a small bridge rectifier and 1000uf/25V capacitor.

To improve the brightness characteristics of the amp, I opted for two additional mods:
Boost the amount of input signal going to the preamp stage and reduce the signal bleed to ground by changing two resistors on the PCB. The resistor change also causes a net increase in the input impedance of the amp, which effectively boosts the highs a bit.
Add a switchable tone capacitor to an open position on the PCB, i.e. install "brightness switch".

Progress as of 6/20/06:

Hum Fix: Finished #1 and implemented a temporary version of #2, basically using a rubber washer and some strategically applied electrical tape to isolate the stock metal input jack from the chassis. In the meantime, the parts for #'s 3 and 4, plus a real nylon insulated jack to do #2 right, are on order from Mouser.com. At this point, the background hum is essentially inaudible, but I haven't implemented the fixes (esp. #4) that will hopefully tame most of the volume-dependent buzz.

Brightness Boost: Completed #5, resulting in a noticeable boost in the high-end, but I still want more. Additionally, the amp's effective gain was increased by changing the resistor to ground to 1M ohm, sending more signal to the preamp. This made the amp noticeably louder at a given volume setting. It makes the volume-dependent buzz proportionally louder, too! I have the parts on order from Mouser.com for mod #6, as well--a simple on/off toggle switch and some ceramic capacitors in sizes of 50, 68 and 100pf. I'll start out by installing the 68pf cap and see how the brightness sounds when switched on. If it's not enough I can go up to the 100pf cap, and if it's too much, I can back off to the 50pf. These caps cost less than a buck each, so I figured why not get a couple to experiment with?

I'll update this thread further after I receive and install the additional mod parts, hopefully by later this week. BTW, the total of all the parts only came to about $22, so these are definitely not expensive mods to make!

**Bloozcat** · June 20th, 2006, 09:10 AM

Good thread, buddy! I'm looking forward to hearing more.

duhvoodooman*** · June 22nd, 2006, 08:56 PM

The parts for the additional mods arrived from Mouser.com yesterday, and I got everything installed during my lunch hour today, with the help of an accomplished solderer. Specifically, the following mods were completed:

Replaced the stock input jack with a nylon insulated jack, isolating it from the chassis.
Added a larger (!) filtering capacitor (100uf/450V) to the main high voltage supply to supplement the smaller stock caps.
Converted the voltage supply to the tube heater filaments from AC to DC, by installing a small bridge rectifier and 1000uf/25V capacitor.
Installed a tone capacitor to an open position on the PCB, and connected it to an on/off toggle on the front panel, creating a "brightness switch".

Here's a simple schematic of the Valve Jr's PCB, showing the mods that were done today on the board itself, plus the resistor change I made a few days ago:

Here's a photo of a portion of the PCB itself, showing some of the mods, circled in red and numbered:

The mods shown are as follows:

The bridge rectifier and 1000uf/25V capacitor. The orange wires formerly connected to terminals T5 and T6 on the PCB, but now connect to the rectifier, whose + and - terminals now connect to those board terminals.
68pf tone capacitor, soldered to one of the two open C3 terminals on the PCB. The other end is wired to the toggle switch in the panel face (#5), and the other side of the switch is wired back to the second C3 terminal.
The two resistors (R1 and R2) that were substituted for the stock resistors, per my original posting.
The back of the new nylon insulated input jack.
The back of the "brightness switch" toggle.

Here's a different view of the PCB, showing the big 100uf/450V filter capacitor , which is wired in parallel with the smaller filter caps on the board. The connection points are circled in red. The capacitor is lashed down to the board with a wire tie fastened to an adhesive anchor pad.

And here's a photo, courtesy of Dennis Cronin's web page, showing where the grounding jumper is installed on the back of the PCB:

Since we're limited to 4 images per posting here, and I have one more, I'll close this posting and continue in the next one....

duhvoodooman*** · June 22nd, 2006, 08:59 PM

Lastly, here's the finished product, all reassembled and ready to run. The only giveaway that the amp has been modded is the brightness toggle at the left side of the front panel:

And what did this all accomplish, you ask?

Well, a whole bunch, actually. The ambient hum is completely gone, to my ear. The volume-dependent buzz is much reduced, and far less "buzzy" in character now. More of a true hum, and no worse than I get with my Delta Blues cranked up. I don't even notice it until the volume is at 3/4, and I rarely play there, anyway! So the four hum fixes have definitely done their job.

As for brightening the tone, mission accomplished there, too. The resistor changes made a difference, and the new brightness switch takes it the rest of the way there. With the switch on to engage the tone cap, the top-end brightens up nicely. Makes a noticeable difference with both single poles & humbuckers. I like it so much I'll probably just leave it "on" most of the time. I'll post an audio clip or two in the coming days, so y'all can hear how she sounds.

So there you have it, Fretters. If anyone has a question, drop me a PM or ask it here. And I've got all the part numbers I ordered from Mouser.com, if you want to try any of these mods on your own Valve Junior! If an electronics ignoramus like me can do it (with some soldering help!), anyone can!

**Nelskie** · June 23rd, 2006, 05:24 AM

DVM - Again, fantastic post. Shows what a little ambition and "can-do" attitude can accomplish. That brightness switch, though, really has me curious soundwise. I'll look forward to hearing your sound clips, so make sure you have one with / without it engaged.

And . . . although you did list the individual parts in each of the descriptions, it'd be really nice to have a working list - just so one can get all the parts at once, and not forget anything. And since you already have the parts #'s - even better.

BTW - what are you running for tubes in your Jr. these days . . . NOS? I have an "inkling" to try out one of those Groove Tubes Mullard re-issues in my amp, just to see what happens.

**Bloozcat** · June 23rd, 2006, 06:50 AM

WOW!

Now that's some great information. You layed it out beautifully, with great detail. I've never had any real electronics training, so I'm still a little "fuzzy" (how about, in a fog?) when it comes to reading schematics. Your photos with the accompanying explanations and details are first rate. I don't think I'd have any problem following those instructions to completion!

Great work!

**Bloozcat** · June 23rd, 2006, 06:56 AM