Tech: Valve Junior head mods

These are the two Valve Junior heads that you hear on Rosetta's track (Temet Nosce). I run them in stereo, each through a 2x12 closed-back cabinet with one Vintage 30 and one G12H30 (made by Avatar, who I highly recommend). On (Temet Nosce), I played my mutant '72 Telecaster through a stereo delay pedal, with each amp/cab set up and mic'ed in a different room. We just panned the tracks left/right and that was it. The outro to Monument was also recorded with these heads, but in 4 takes with my Gibson Sonex-180.

The amps sound significantly better now than they did then, due to further modding. At that time, the only changed components were R1, R2, R6-9, and 10uF cathode bypass caps on the preamp triodes. That circuit was a huge improvement over the stock sound, but I think the current version is vastly superior.

The schematic below (based on stock schematics from SEWATT.com) shows all of my changes in red. You can find an excellent explanation of different components in S2 Amps' upgrade kit instructions. Whether or not you buy their kit (I did not, and doubt it is still available), the instructions are full of useful info. Loads more info can be found in the SEWATT Octal Mod file, even if you have no interest in adding another power tube. The complete repository of Valve Junior info is here.

My 390pf "bright switch" around R6 (switched via large toggle on the black amp, push/pull volume knob on the white amp) is very helpful for humbucking pickups. It's very "Marshall-y" sounding. I prefer it off with single coils. The 47pf bright cap on the volume pot only adds a tiny bit of shimmer or chime when playing clean. As you turn up the volume, the effect gets smaller, which is great because it's not needed once the amp starts to break up --- around 11 o'clock on these heads.

Both amps have Hammond 125ESE output transformers. These are far larger than needed for a nominally 5-watt amp, but the greater dynamics and bass extension are worth it, particularly since I most often play a baritone-tuned telecaster (my '72 Frankencaster) through the VJs.

Not noted on the schematic is that the rectifier on the high-voltage power tap is discrete 1n4007 diodes, in the stock amp. I replaced these with UF4007 diodes and the switching noise was reduced.

For recording purposes, the black amp has a second, smaller 3-way toggle that switches R15 from 10k to 5.6k or 1.5k, for a kind of presence or power amp voicing control. Most of the time it stays on 10k, to match the white amp.

Tubes: both of these amps use a JJ ECC803S (long plate) in the preamp, and a JJ EL84 in the power amp. I specifically asked for power tubes with lower current draw for maximum clean headroom.

Final notes: these amps are LOUD -- much louder than the 5-watt rating would suggest. I can't get anywhere near breakup at "bedroom" levels. If you're interested in controllable preamp gain, I don't recommend adding a master volume. I've tried it and it sounds crappy. The best solution is to use a tube overdrive pedal in front of the amp. The closest thing I've found to adding extra gain stages is the EH LPB2ube, which works in either stereo two-stage or ganged-mono four-stage. I use it in stereo to front end both amps, with a stereo volume pedal. Getting a small amount of distortion from each tube in the overall chain adds up and sounds great.

EDIT 3/2011: I've made some more changes to these amps, detailed in a post on my work blog, Electronic States.