The Output Transformer is the Final Link

The Epiphone Valve Junior is a small 5W amp head that is first most noteworthy because it is… well, cheap. Especially considering it is a tube amplifier, selling for $129 is frankly a bargain. Because of its simplicity and price, it has had a bit of an underground following being a pure-tube circuit with a single 12AX7 in the preamp stage and an EL84 as its output tube running Class A. The amp has 4, 8, and 16 ohm output jacks to enable a wide use of cabinets.

All in all, with this setup and a single volume control, players can get an idea of how tube distortion and compression changes as the volume is turned up. And it can all be done without the need to put in ear plugs that you would otherwise need when trying similar activity with a 100 watt head.

And the amp on its own sounds o.k. It’s obviously not wildly versatile, but “old school” style players can set an amp like this to obtain as much gain they’d like, then they can clean up their signal by use of their guitar’s volume control. Problem solved. Not enough volume for gigs? Throw a microphone in front of the cabinet.

The Mercury Magnetics Modification

The basics are changing the power and output transformers and adding a choke. There are some additional filtering and component changes. While the amp stock has one EL84 tube, the mod kit adds an additional 6V6 tube that gets blended to increase power.

The documentation that comes with the kit is excellent and really makes performing the mod easy. Our resident amp builder Mike Mullen took the amp further and created a new handwired board for the Valve Junior Kit — just for fun. He also flipped the chassis so that the front of the amp has the mesh screen and makes the amp look a bit more like some of the contemporary designs (you can see this look in the videos).

As one would expect with the change in transformers and the 2nd power tube, the amp is louder. In fact, it’s now loud enough to play with a full band of drums, bass, guitar and singer — unless you have a hard-hitting drummer that is! What you can expect is an overdriven sound great for rock and blues. But if you need clean headroom, you’ll still want to mic it up. The modified amp is very dynamic, with much more clarity and fullness on the lows when the amp is turned up to bring on the 6V6/EL84 combination of tube compression.

Where the modified Valve Junior may now have its strongest application use is for recording. It’s no secret that some of the greatest recorded sounds in rock and roll and blues history were done on smaller amps.

The modification kit sells for $299 through and those with basic soldering skills can do the job with the step-by-step documentation and CD being a big help. While some may balk at the fact that they now have spent a total of about $400 for the amp, transformers are indeed the heart of the amp’s circuit, especially the output transformer. The output transformer is the final link between the tubes and the speakers, and we know from past tests what a great output transformer can do to improve the tone and fidelity of a tube amp. With the Upgrade Kit complete here, you’ve now got a custom boutique amp for about $400 — which would still be considered a bargain for many.

We recorded a couple of basic videos to show some of what the Mercury Magnetics modded Epiphone Valve Junior can do when plugged straight in just playing some basic blues and rock style lead and rhythm (no Charvel shred-style this time folks).

The first demo uses a Les Paul, the second a Stratocaster.


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