With the help of a couple of excellent online resources--including one familiar to us regulars here at TheFret--I finished my final (Really! I mean it!) round of mods on my VJr combo over the weekend. In addition to the previous changes I've made (chronicled HERE), I did a number of tone and feature enhancements that really put the exclamation point on this little tone monster's capabilities!

These additional mods I did are described in one or more of the following three sources:

On Tung's page, pay special attention to his new "Valve Junior Modifications eBook". I was privileged to be able to preview this impressive resource, and I can tell you that he did a comprehensive and thoroughly professional job on it! It's packed with detailed instructions, excellent supporting photos and diagrams, and a lot of useful ancillary information. If you want to undertake modding your Valve Jr and don't have a lot of experience with such things, this is a $20 investment that I'd highly recommend making!

Just one note of clarification here regarding different versions of the Valve Junior: Mine is the original version (i.e. "v.1") of the amp, and the circuitry is somewhat different than the 2nd and 3rd versions, which incorporate significant improvements. If you have the head rather than the combo, by definition you have v.2 or greater. The S2 instructions apply to v.1, though 90% of it is accurate for the later versions, as well. Both Tung's information and the Layboomo Gold/Silver mods apply to v.2 and up. The newest version is v.3, and differs from v.2 only in the fact that it has a better output transformer (OT) and somewhat upgraded tubes. Same circuitry, as I understand.

Anyway, after accounting for the mods I'd already done on the amp, here's a list of the additional things I changed or added:

Reliability & Safety Mods:

  • Added a 20ohm 25W power resistor on the incoming AC to drop the EL84 plate voltage a bit (stock = ~340V, too high).
  • Increased the power tube cathode resistor from 220 to 240ohm to cool the bias and reduce plate dissipation to under 12W (EL84 recommended max.).
  • Replaced the dinky little 16mm stock volume pot with a beefier 24mm pot.
  • Added a 220K "bleeder" resistor between the B+ rail and ground to drain the filter caps after power-down.

Tone Improvement Mods:

  • Upgraded the cheap stock tone caps with 0.022uf Mallory 150's.
  • Replaced stock rectifier diodes with ultra-fast UF4007's to reduce switching noise.
  • Replaced the R6/R7 1M voltage divider resistors with lower ratings to "open up" the tone (used 330K/220K)
  • Replaced 2.2K stock preamp cathode resistors (R8 & R9) with 820 and 680ohm, respectively. This warms the bias of the two 12AX7 triode preamp stages and gives smoother overdriven tone.
  • Also replaced the stock 22uf preamp cathode resistor bypass caps (C3 & C4) with 3.3uf caps. This raises the bass roll-off frequency and gets rid of low-end muddiness in the bass.
  • Stiffened the bias on the EL84 power tube by increasing the cathode bypass cap (C5) from 22uf to 1000uf. This gives the VJr a noticeably punchier sound, more like a fixed bias amp.
  • Replaced the 100pf switchable ceramic "brightness" cap (optional open cap spot on the v.1 combo PCB) that I had previously installed with a larger 220pf silver-mica cap. This gives a stronger emphasis to the trebles and higher-end mids when the cap is engaged.

Added Feature Mods:

  • Installed a standby switch (ridiculously easy mod).
  • Soldered in a switchable 68K resistor in parallel with R6. When engaged, this drops the effective R6 resistance from 330K (per the 3rd tone improvement mod above)to ~56K, and doubles the gain voltage passed on from the first preamp stage to the second. In other words, it's a "gain boost" switch.
  • Added the Tweed tone control shown on Tung's VJr web page.

Some of these mods are very subtle in nature and may not even be noticeable in their impact, while others have a very marked effect. But even the subtle ones are all moves in the right direction and are very inexpensive, so why not do them as long as you're "under the hood" anyway? The cost for all the parts for the 14 mods listed above came to under $16! The main thing these mods take is time and care in doing them. Especially when desoldering the old stock components from the PCB, care must be taken not to apply too much heat, or it's possible to lift the solder pads from the PCB surface. That generally can be repaired if it happens, but it's an added hassle.

The impact of all this work? Well, I really need to get a clip or two recorded & posted here, but together with my previous modifications, this little amp sounds just amazing now. The clarity and punch is light years beyond the stock version, and the gain increase is just scary! I can't even turn the volume knob past 9 o'clock in the small room where I play without deafening myself. Plugged into my 2x12 cab, I'm sure this puppy could be used to gig in a modestly-sized venue. And I really like the new features I've added. I'll post a pic or two of the front panel soon, showing the new switches and tone knob. I know some folks like the simplicity of the Valve Jr and it's single volume control, but I prefer having more tonal control than that. So now I have volume & tone knobs and toggle switches for standby, gain boost and brightness. I can hardly wait to get it into a bigger room and open up the volume so I can get that power tube into saturation! VROOOOOOM!!! :