Valve Junior Mod Project

Epiphone Valve Junior, $119

When I saw Epiphone had introduced an all tube 5w amp for $120 I couldn't pass it up. Even if it wasn't that great I figured it'd be a great little platform to use to experiment with tube tone. Jeesh, you can't buy all the parts to build a simple tube amp from scratch for that, let alone have a nice tolex cabinet.

I actually had been looking for a super low power tube amp to play with. I like to run in stereo and even with my pair of Fender Blues Juniors when I turn 'em up to where I'm getting some genuine tube tone I'm already way louder than I want to be for most casual playing situations. But I figured if I bought a pair of 5w amps I should definitely be able to turn 'em up, and this proved to be just about right. More on my stereo setup below.

I was also attracted to the pure simplicity of the design: no tone control, no master volume, just one knob for main volume. Less between your guitar pickups and the speakers.


Fixing the hum


I'd read on the 'net that they hummed rather badly so I knew I'd need to tackle that first thing. When mine arrived from I plugged it in and sure enough there was a pretty annoying ambient hum even with the input volume all the way down, and if you turned up the volume knob then you got a much louder hum with lots more high harmonics in it. Hum aside, these are nicely made little amps with fundamentally great tone-- not a bad deal for a $100 piece of gear. However, you might seriously want to consider things before you buy one unless you're prepared to do some soldering or pay a tech to do some mod work. Note: yes, I did try some of those tubes that EH says ostensibly reduce hum, but I didn't hear any discernible difference. You gotta solder, kids.

I did three main things to get the hum under control:

  1. Converted the filaments to DC. Sounds complicated but it really only takes two parts. This greatly reduced the high harmonic hum components that I heard when I turned the volume control up.
  2. Added more filtering to the main high voltage supply. They only had 20uf on the first stage of their power supply, and I added two 100uf 450v caps which improved things a lot, eliminating the ambient hum that you hear with the volume all the way down.
  3. Added a jumper wire to the circuit board to make the input jack grounding a little saner and reduce the ground loop component hum. This also helped the situation with the volume turned up.
This still did not completely eliminate the hum, but it got it down to where I could live with it. There's a post on a guitar forum where I guy talks about going to ground-isolated input jacks to get the last shred of hum out. Maybe that's the final step, although most amps can be made quiet without such techniques.

Details are provided below about these mods.


News Flash! 3/3/2006

Just noticed Epiphone is now selling a head-only version of this amp that boasts a DC filament supply. Maybe consider that before you go to all the trouble of doing the hum-reducing mods below.

Tone Tweakage

I really didn't end up doing anything substantial by way of tone mods. I noticed an empty spot on the circuit board for an additional cap in the divider network in front of the volume control and experimented with putting some different caps in there. I tried a .001 cap and that made the whole amp sound overly brittle, boosting mids and high too much. I ended up putting a 100pf cap in one of 'em and in the 2nd one (yes I bought and modded two) I put in a 50pf bright boost. These add just a bit a crispy stuff to the very top of the tone. Makes my Strat and Tele sound a tiny bit bright (I just back down the guitar tone control a smidge) but it helps for the P90 and humbucker guitars.

I like the tone of these amps. It's definitely class A, a little "rattier" than a balanced output stage definitely, but it's a fun sound. And by the time I have my stereo pair going, it's all nice and warm and fat and juicy.

Fun with speakers

It comes with a pretty nice sounding 8" speaker in it, but I bought a pair of 8" Jensens for $30 apiece and tried them. I really didn't like 'em as much. While the stock speaker had a nice Celesion-y kinda mushy warm breakup, the Jensen seemed to bright and brittle. I removed them and put the stock ones back in.

I also added another speaker jack so I could connect the amp to an external cabinet either with or without the internal speaker. I have a Fender Bassman cabinet loaded with Greenbacks and wired stereo and I got a pretty nicely balanced sound with the 4 ohm eights and the 8 ohm twelves together. I know, I know, that's a little under that amps rated output impedance, but tube amps or known to be very flexible in terms of output loads. And speaking of output loads...

Light bulb power attenuator

It sounds ridiculous, but these little 5w amps were still a little loud for bedroom practice if I wanted Real Tube Tone. So I also added an ultra-simple little light bulb power attenuator to the output.
12v outdoor light power attenuator (or "compressor" really)
Now I've honestly never read about anyone doing this before, so maybe I invented it (oops, looks like not, folks have brought prior art to my attention), but I did some experimenting using low voltage outdoor light bulbs as power attenuators and found some really fun sounds. It's a bit dicey and you have the match the bulbs exactly to the power output of the amp, but you can get some really neat attenuation/compression effects.

A lightbulb's resistance varies as a function of the voltage applied. At lower voltages, the lightbulb doesn't light and it has a very low resistance. As more voltage is applied, the lightbulb begins to glow and its resistance increases. When the lightbulb is full on, it is dissipating quite a bit of energy as light and its resistance has gone way up.

So think about this: a lightbulb carefully selected to match the power output of an amp acts as a very interesting organic compressor, with the attack time dictated by the turn on time of the incandescent filament.

I ended up putting a seven watt 12v lightbulb in each of these amps and providing a second pair of output jacks wired with this lightbulb in series with them. This reduces the amp's volume level to a perceived "about half" and also adds this fun gentle compression effect.

Now don't try wiring a single seven watt 12v lightbulb in series with the output of your Marshall. It'll probably just instantly burn out. I did experiment with different numbers and wattages of bulbs with my Fender Bassman head, and got some nice results, but you'll need to tailor a network of these bulbs carefully to the output of each amp. I even found a way to wire groups in parallel/series arrangement so certain groups turned on first and 2nd and 3rd groups turned out at only the loudest levels. I used colored 4w bulbs wired in parallel groups of 2, 3, and 4, so green ones came on first, then yellow, then red. But that's only on more powerful amps, and I digress..
The box the outdoor lights come in

On the Valve Junior I added 3 additional speaker jacks, one in parallel with the normal jack, and the other two in series with the lightbulb (see that schematics below). This way I can connect the 8" and external 12" speaker in a variety of ways, both full power, both attenuated, or combos where one is attenuated and the other gets full power. Some fun subtle differences in there.

And again, if you're obsessing about the changing load on the tube amp, don't worry about it. Tube amps are tough. Only taken to the extremes of very high amounts of attenuation would I think you'd begin to have a problem (and actually you can hear the amp struggling with really high ohm loads).


Details of the mods

So first of all, here's a schematic of the stock Valve Junior:

And here's the modded version, with my additions shown highlighted in purple:

I'll go through the hum reduction in more detail. First I added more power supply filtering. The stock 20uf caps just aren't anywhere near enough for a class A amp. With a balanced output stage you can get away with less filtering on the main power supply output because hum is canceling in the balanced output, but for class A, there's no such cancellation. So I added a pair of 100uf 450v caps. You can see the stock caps on the circuit board at the bottom, and the ones I added up top.

Next I added the ultra-simple DC supply for the tube filaments. I recommend that you buy a bridge rectifier that bolts to the chassis as it will need to dissipate a good amount of heat. This results in a DC supply still with a volt or more of ripple, but it's good enough to take hum components out of the filament injection path. I experimented with more filtering but with only negligible incremental returns.

Finally, I added a jumper to the circuit board to improve the input grounding. They did something kinda odd and let the input only ground through the chassis. This results is quite a "wide" ground return path between the input jack and the first gain stage. By adding this jumper you do break pure star grounding rules a bit, but you dramatically reduce the ground plane hum injection. Note that the resistor you see soldered to the back of the circuit board was not added by me-- that's a factory "mod".

I don't have pictured that extra tone cap I added. But the empty location is pretty easy to spot on the circuit board. Try adding small value caps there, again with 50pf or 100pf being my suggested values. I considered adding some kind of bright switch type arrangement but decided on a KISS approach.

And finally, here's the finished pair of modded amps sitting atop my Fender Bassman stereo cabinet:


Going stereo

So the whole point of this exercise was to have a pair of amps that I could run stereo and turn up to get real tube output stage distortion. Lemme talk a bit about my Grand Theory of Stereo Guitar Amps now.

So I love the sound of a chewy chorus, but I find normal chorus units sound too "electronic" to me. Here's a quick review of what chorus is about.

A typical chorus unit signal routing looks like this:

The input signal is routed to a short delay line where the delay amount is varying slightly, typically in range of 10-40ms for the center of the delay sweep. Then the delayed signal is recombined with the "dry" signal. This yields both the doubling effect of the slight slapback delay plus some interesting comb filtering effects due to certain frequencies being exactly 180 degrees out of phase at any given point in the sweep. The comb filtering effect is the root of the flanging sound, but it become less evident at delay values above 10ms. Still it's definitely there, and it's what I think causes the "electronic" sheen sound. Now I love that sheen sometimes, but often I want the fattening or doubling effect without so much sheen.

About delay times

Many stomp box chorus units use delay times in the 10-15ms range which is barely longer than the delay used for flanging. This aggravates the comb filtering effect and doesn't really get to the point where you're perceiving the doubling effect as much.

I often like the sound of chorusing more in the 30ms range-- this is shorter than the point where you start to perceive it as a "slap back" echo effect, but much chewier feeling than the tight 10-15ms stomp box chorus units. But very few non-studio chorus units go out this far. The Electro Harmonix Poly Chorus however will do really long chorus delay times, the tradeoff being you lose a little high end and you can hear some audible artifacts from the compander they put around the old analog delay chip to reduce the noise. Still I like it.

About chorus sweep smoothness

Another component to a really good chorus sound is the smoothness of the sweep. Ideally the delay should vary linearly with time, yielding than sweet Eventide Harmonizer type sound. But designing a circuit that yields a linear varying delay has proved a bit tricky for stomp box makers. If you listen to most chorus units, the sweep sounds kinda blurpy or irregular. A true linear sweep with speed turned up and depth turned up will create a note above the main note, a note below, then a note above, and below again. The pitch of the above/below notes is steady with a relatively immediate transition between the above/below note. Turn this down to sane slow sweep rates and adjust the depth right and to my ear this gives that gorgeous Eventide sound.

The Boss CH-1 does a very nice job of this, although they are in the shorter sweep ranges. And Electro Harmonix has always been King of the Sweep in both their flanger and chorus units. They designed a super clever little clock circuit using a single transistor constant current source into a comparator that generates a true voltage-to-period clock oscillator. Hmmmm... I'm getting a little too electronic geek here I guess, but the bottom line is: if you want a good stomp box chorus, cough up for the EH Poly Chorus or for a good shorter chorus sound, the Boss CH-1 is my pick.

Now about those "stereo" chorus units...

Another rant of mine is about the current so called "stereo" chorus units. Here's the way most stomp box makers wire up their alleged stereo chorus units:

Output A is a normal wet/dry mix output as shown above. But for output B, all they do is invert the delayed signal 180 degrees and mix it with the dry signal. Sure, this produces an inverse comb filter at that output, but there's nothing else stereo about it, no delay difference, nada. And for me the real feeling of stereo comes from the varying delay time spread across the stereo separation. In fact, with this phase flopping approach, if you remix that stereo sound back to mono, the delayed component will cancel out completely. *Poof* no more chorus. Not ideal I say.

Why do they do this? Well a number of the original analog chorus units of the days of yore used to, when you plugged in stereo, give you "dry" from one output with "wet" from the other. But I suspect they had too many clueless musicians call up the service department and say they didn't hear any chorusing from one or the other output. Correct. If you only listen to one output at a time, you don't hear any chorus effect. But if you listen to both then you get a really beautiful stereo separation fat chorus. But to prevent these service calls, the fake "stereo" chorus was invented. This way regardless of which output the musician listens to there is always some evident chorusing going on. But the resultant stereo effect is, to my ear, well, almost just mono.

Unfortunately, even Electro Harmonix has fallen into this same trap with their revised new Poly Chorus. Their old Poly Chorus units had the wet/dry outputs, but the new ones use the phase inverted fake stereo. So I promptly rewired mine. It's an easy mod, involving cutting one trace and running one wire to an output jack. And now I have that stereo split I know and love with my new Poly Chorus. Or the Boss CH-1 is the only stomp box chorus unit I know of today that comes stock with the good ol' stereo delay split. Plug into one output and you get mono chorus, but when you plug in that second cable, it goes to a true delay-based stereo split. Check it out. Although it is a shorter delay time chorus, it sounds darn good in stereo.

Mixing in air

So if you've read this far, you have probably figured on that I'm a stereo chorus freak-azoid. Here's how I really like to do it.

Rather than have the signal electronically mixed, I like to run the stereo split into separate amps and let the chorusing "mix in air". This way you get both the true stereo split I describe above, and also you reduce the comb filtering sheen. Particularly when you run into two different amps which are each contributing their own slightly unique distortion signatures, when the signal mixes in air you get great juicy fat warm doubling, with none of the fakey "electronic-ness".

And this was the ultimate goal of the dual Valve Junior mod project-- a pair of amps that I could turn up to the point of real tube distortion and then drive with my true stereo chorus units.


Finally, a look at my effects:

Note my use of the venerable Arion Tubulator distortion unit. I love the sound of these little fellers. And although they're not a sturdily constructed as most stomp boxes, heck, you can buy several to have as backups for the price of one normal distortion unit. I've found 'em for as little as $13 apiece, although they usually go for $20. Check 'em out.

And that concludes my look at my latest guitar setup. Maybe I'll get around to recording something with this so you can actually hear it instead of just read about me obsessing about the components. But it's a nice all tube class A sound, trust me.

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Duh Voodoo Man's Epiphone Valve Junior Mods Page - hum reduction, gain and brightness boost, very nicely done
Another Valve Junior mod page - this guy doesn't worry about hum as much and instead goes for more gain.
Yet another Valve Junior mod page - similar approach, very nice detailed photos and info on exactly how to connect up the bridge rectifier, some more tone mods
Original VJ thread at - long thread about various efforts to mod the VJ.

Dennis Cronin