Blues junior Mods

[EL34]

Sven Ericsson on facebook has some mods for the Blues Junior that sound pretty good to my ears.

Curious what you guys think about the modded BJ in the video clip

Sven's notes and comments

As I mention those zeners make the amp alot diffrent the powertubes have more linear response and to the good side affect ..the power tubes last 2-3 times longer before its time to change

Sven's mods are in purple on my attached schematic
Check out the Zener Diodes on the Screens

Fender Blues Junior mk2 mod clean soundclip

[tubenit]

I like the way that amp sounds!  It has a good tone, IMO.  Of course, it helps that he is also a reasonably talented player.

With respect, Tubenit

[EL34]

Ever see the screens set up that way before?

[jjasilli]

Ever see the screens set up that way before?

I believe I have, but for not for guitar amps.  For some discussion, see:  http://www.diyaudio.com/forums/tubes-valves/116408-ultralinear-zener-screen.html  and http://www.ax84.com/bbs/dm.php?thread=483446


I think the modded amp sounds good.  Hard to say if it's better than stcok.

[PRR]

> see the screens set up

Odd that he shows the same 321V at point Z and at the screens. Like the Zeners aren't doing anything.

I would not expect a big difference 321V or 296V screens. However the Zener may be doing something more. This is something to try both ways.

[EL34]

PRR,
That is my schematic from my build and those are my voltage readings
I don't have any of Sven's voltage readings

All I did was make Sven parts changes purple colored on my original schematic

[jjasilli]

More info here, skip down to Section 5, and following:  http://www.oestex.com/tubes/screens.htm


What I'm getting is this:  Quoting the old tube handbooks (which did not envision signal distortion as a desirable feature), a regulated screen supply is recommended, especially for Class AB operation.  The reason is that the screen grid, properly charged with a B+ voltage, makes fluctuations in plate voltage almost immaterial, so the tube stays linear.  But fluctuations in screen voltage hugely affect tube current and linearity.  If the screen voltage is held constant, then a smaller input signal voltage produces more gain with more clean headroom and pleasing harmonics.


The zener diode is a simple voltage regulator.  For the above BJr. screen circuit mod:  If if the zeners drop and regulate the B+ supply to the screens enough, they might be able to more or less regulate the screen supply over the range of fluctuation of B+ voltage in the amp.  According to the article I quoted, that should yield more clean headroom with pleasing harmonics. 


EDIT:  Alternatively, the plate & screen supplies could be well-regulated.  But the simple zener mod might yield some of these results.


This would be the exact opposite of the KOC mod which calls for large screen resistors instead of voltage regulation.   

 

[kagliostro]

Mesa Boogie has something similar in the Mark V, the Mark V power section is a bit complicated, but they used those zener in that position
(although not in PP configuration)

EDIT: For those who can be confused by this Mesa power section, this ting was discussed time ago here

http://el34world.com/Forum/index.php?topic=11865.msg110068#msg110068

K

[EL34]

Oh crap, it's a Boogie 

[clyde]

Here's some gobbledygook for you.  Have at it.


http://www.t-linespeakers.org/tubes/triode-trick.html

[jjasilli]

Good post!  I don't think I agree with Explanation 1, that this is a form of triode wiring a tetrode or pentode.  I think Explanation 2 is better in that article.  If the zener (or zener string) is holding the screen 30V below plate voltage, that's a good thing and probably within spec of the tube charts for tetrode or pentode operation.  However, this topic is above my paygrade, and more thought than I've ever given to screen operation!

[MFowler]

Some reading here: http://ampgarage.com/forum/viewtopic.php?t=26502&postdays=0&postorder=asc&start=0

[shooter]

Is this basically setting up a pseudo  UL condition without UL taps?

[EL34]

It would be interesting to hear a before and after only adding the diodes and 56 ohm


I don't have a Blues junior here to test that, but I'll make a note for the future if I get one here again

[jjasilli]

Is this basically setting up a pseudo  UL condition without UL taps?

I think that's a good point!

[bakerlite]

if using the 6n2p-ev  like the mod does is pin 9 of socket soldered to ground and only pin 4 and 5 hooked up (seperately not joined 4+5  like 12ax7 sockets)???


his mods sound pretty  amazing


never heard a blues junior so good!

[Fresh_Start]

It looks like Zener diodes are being used in a couple of different ways in some of those discussions. In Sven's schematic and the Mesa, the Zeners are in series with "normal" screen grid resistors. Some of the others show or talk about tying the screen B+ supply to the plate supply but with voltage dropped a fixed amount (30 volts +/-). In the Ampgarage discussion, they seem to be talking about tying each screen supply to its respective plate supply.

In a normal power supply, can screen grid voltage go over plate voltage?  Is that "bad" or just not clean? 

Two side notes:  Doug's schematic doesn't highlight the fact that Sven has paralleled the two triodes in V2 without changing plate or cathode resistors. It's mentioned in the video though.

The other is that Blues Jr. amps tend to be biased really hot without an adjustment control. My 2001 green board ate power tubes until I added a bias control. I wonder if those Zeners on the screen grid supply might help in a situation where the power tubes are biased on the hot side.

Cheers,
Chip

[EL34]

If screens are higher than plates then the electrons will be attracted to the screens and they will go bye bye
Screens are not made to conduct higher currents like the plates



With the plates higher than the screens, the electrons see the higher plate potential and are attracted to the plates, while just a smaller amount go to the screens as screen current