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Valve Jr. Gone Vibey

The idea behind this modification of the Valve Jr was to take the combo version of the amp and turn it into a old school style amp of the 40's/50's. The idea was improve the stock circuit so it wasn't as dark and add some versatility in a voicing switch. The amp also would be quite cool with a tube driven tremolo effect, just like what was the standard on the old combo amps.

The tremolo/oscillator circuit is more or less the same style as found in the Vibro Champ and other such small Fender amps. It uses a single 12AX7 dual triode and can be implemented fairly easily.

Basically the amp would be a VJ but a bit more open sounding with more solid bass and a bit more "grit" here and there. It would not be so dark because of accented highs. The combo can also sound somewhat "stuffy" and constricted stock so that is another issue that needs to be taken care of.

Here is the schematic (the top half is the stock Valve Jr circuit with mods I have implemented, bottom is the tremolo/oscillator circuit and power supply.).

*It should be noted! At this point the project is working but there are some minor issues. This schematic will give an amp  and a tremolo effect but it is not the final revision of the schematic. Build/use at your own will.*

I left out a cathode bypass switch cause there wasn't a ton of room to draw it in. There would be a SPDT (On-Off-On) switch to select between a 220mF cap, 0.68mF cap and no cap on the first triode's cathode. What this does it is it boosts certain frequencies. Basically it will give you a full range/fat boost, no boost/flat response and finally a boost that accents the highs and mid range frequencies. Should add a lot of versatility to this little amp.

So basically Valve Jr. combo with...
- Volume
- Speed
- Intensity
- Bright Switch
- Voice Switch (cathode cap)

There is also a choke being added for for extra filtering/noise reduction. The solid state rectifier can defiantly handle it. I'll report back on how I like it.

Pictures:

Bright switch and voice switches installed.

Extra dual triode installed.

 

As of right now this amp is a in the "debugging mode". Everything works fine except when the tremolo effect is off the intensity pot acts like a volume control. So if you are playing with tremolo and the intensity is at 5 then switch the tremolo off you get a volume loss. You then have to adjust and set the intensity pot to make to get back your volume.

I am looking into the issue and hoping to have the problem resolved soon. When fixed a final schem will be posted along with some pictures and sound clips.

An Update (long time due!)

Well I finally got around to tackling this project. Got busy and distracted with lots of other projects. But I got around to sitting down and figuring things out. Finally got it working and complete!

First thing I did was put a 51K resistor in parallel to the 50K (RA) intensity pot, this brought the value down to approx 25K. The scaling down of the pot value did affect the taper but the sweep of the pot seems smooth and normal, no awkward jumping. This is the value a Champ trem circuit should have. Next I added a large DC blocking cap on the trem line of the intensity pot. A quick tube swap later and the amp was functioning perfectly.

Now that the tremolo effect was sorted out I hooked up the choke I had previously mounted. Removed the first 220R dropping resistor and replaced it with my 170R 9H choke. A bit higher voltages across the amp now, bit more clarity and headroom. A lot less noise and a bit tighter and smoother with the added choke.

Fun amp for practice and recording, easy to get to break up. Actually has a lot of tones from it which is surprising seeing as it's controls are few.