The Valve Jr transformation is complete....

[tunghaichuan***]

Duhvoodooman,

Thanks for the kind words. It took me about a year to write that book and I sweated a lot of small details.

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.

To be fair, Epi did change some of the circuitry from V2 to V3: they lowered the cathode bias resistors on the 12AX7 from 2.2k down to 1.5k, which is stock Fender value. These resistors bias the 12AX7 warmer and make the amp sound crunchier when driven hard. Unfortunately, they left the 22uF bypass caps on the 12AX7. These make the amp sound blatty when the amp is driven hard.

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:
[*]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.

As an experiment, I converted my V2 VJ to fixed bias. I made a little negative bias supply board and ran it off of the 12-0-12v winding. (Version 3 does not have this winding, it is for powering the opamps on another amp the VJ's power transformer is used in.) It was hard to tell the difference between the VJ using the 1000uf cathode bias cap and the fixed bias VJ. Replacing the stock cap with a 1000uF cap is much simpler, and you don't have to install a board.

[LIST]
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.

Have you considered replacing the PCB with a board from turretboards.com? I believe they make a replacement board for V1 amps. Those stock green PCBs *really* suck.

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!!! :

It really is amazing, isn't it? Take an amp that sounds pretty crappy stock, make a few changes and it turns into a real fire breather. Version 2 was an improvement over version 1, and version 3 takes very little modding to sound excellent. Some have even suggested that the stock version 3 output tranformer sounds almost as good as the Hammond 125ESE. For a Marshally sound, the V3 stock OT may even be better sounding as you get a little core saturation. The 125ESE will not saturate in a VJ circuit at all.

tung

[marnold***]

Version 2 was an improvement over version 1, and version 3 takes very little modding to sound excellent. Some have even suggested that the stock version 3 output tranformer sounds almost as good as the Hammond 125ESE. For a Marshally sound, the V3 stock OT may even be better sounding as you get a little core saturation. The 125ESE will not saturate in a VJ circuit at all.

So would you be happy using a VJ v3 out of the box or would there be a tweak or two that you must needs make?

[tunghaichuan***]

So would you be happy using a VJ v3 out of the box or would there be a tweak or two that you must needs make?

I just pulled the chassis on a V3 head I got a GC a while back. I'd probably do a couple of tweaks:

Replace one of the power supply resistors with a higher value to cut down on the B+ voltage.

Rebias the EL84 with a 270 ohm resistor.

Replace the cathode bypass cap on the EL84 with a 1,000uF cap.

Replace three key resistors with 1W metal film types; the 1meg grid load on the first 12AX7 triode and the two 100k plate load resistors on the 12AX7.

Depending on whether or not I would be cranking it up all the way or not, I'd change the cathode bias resistors on the 12AX7 to 680-820 ohms. I'd also swap out the bypass caps for 2.2uF to 4.7uF caps.

I'm still debating on swapping out the stock V3 OT for this Edcor OT. The one I have is actually the multiple secondary version for use with 4, 8, and 16 ohm taps. Edcor doesn't show it on their webside, but you can order it for the same price as the one shown. The downside is that the OT has exposed solder terminals which is inherently unsafe. You never want B+ exposed outside the chassis. Edcor won't make it with flying leads anymore. As such I wouldn't recommend it for beginners, which is too bad, the price is right.

Edit: having said all that, I've got two V3 VJs that I am going to leave stock. I plan on running a stereo multiFX unit of some kind into them. The only tweak I plan on doing to the stock units is to replace the 12AX7s with a 5751 preamp tubes. The 5751 is like a lower gain version of the 12AX7. Hopefully this will give the amp more clean headroom.

tung