Re: Valve Jr. version 2 vs. version 3

Mark Bedingfield <atari030@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
RichL wrote:
RS <RS@xxxxxxxxxx> wrote:
On Sun, 28 Dec 2008 00:03:13 -0500, "RichL" <rpleavitt@xxxxxxxxx>
wrote:

yowie <yowie@xxxxxxxxxx> wrote:
The schematics are here:
http://www.sewatt.com/files/sewatt/StockVJ_1-3.pdf

You need to sign up to sewatt.com if you haven't already.
Thanks, that's very helpful! (I was able to download the pdf
without signing up by clicking directly on your link,
interestingly enough).

I'm tempted to go ahead and buy a V3 to keep stock and tinker with
the V2.
Rich, Going by those schematics:

The low input impedance of the V1 and V2 versions doesn't make sense
for a guitar amp. The low impedance will also load down a humbucker
more than it would a single coil (though both would lose top end),
so that's probably what you were hearing. Also, the two 68K
resistors on the input form a divider--that will lose half the
signal before it even gets to stage 1. That's bad for
signal-to-noise.

Another difference V2->V3: The cathode resistors are changed from
2.2k to 1.5k. That will change the operating point of those tubes,
but not drastically. I'd say that's optional.

The third difference: Output transformer changed from 7.5K primary
to 5K: Probably a good thing. But I doubt that you'd want to buy
another output transformer just to get this changed.

So... The biggest factor is probably just the 1M input resistor. So
I doubt that you need to be cautious about downtime. Just open it
and change the resistor.

If you're thinking of other mods, there are a few things I could
suggest if you're interested. The divider formed by R6, R7, and VR1
(volume control) will lose quite a bit of signal. This is probably
intentional, to keep the amp clean at lower settings of the volume
control. But it affords a convenient spot to boost some frequencies,
should you choose to do that.

Yup, after looking over the schematics I realized that the input
resistor was #1 in the sound difference. As I mentioned, my
particular VJ gets into distortion territory pretty quickly so I
probably don't want to up the signal level.


Is there any variation between pups and speakers that affect the
headroom in your setup Rich?

A couple of things I noticed, 1, with the Big Muff guitar volume
control is essential to get good tone, Boss OS-2 and Ibanez TS-10 not
so difficult. I'd assume other Muff/Fuzz pedals would be similar. 2,
EMG's drive the VJ pretty hard and also require taming with the
guitars volume control. With my Greenback clone I get around 50%
volume with my passive Strats before it will start to break up. About
35-40% with the EMG's.

Mark

I don't have any guitars with actives. There's some headroom
differences among the different guitars I've used with it; more clean
headroom with the Strat and Gretsch than anything else, less with the SG
than anything else.

I've only used the VJ with two speaker sets; the closed 4 x 12 Marshall
1960 cab and the greenbacks in my AC30. The greenbacks definitely sound
better but I don't think there's a significant difference in headroom.

I haven't used many pedals with the VJ other than the Keeley Java treble
booster; I need to fool around with some others.


.

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