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Fender Blues Junior - Extra Tube Stage

5/26/2000 7:59 PM
Frank Clarke
Fender Blues Junior - Extra Tube Stage
I've used the unused 1/2 12ax7 in Fender Blues Junior to implement a suggestion by Steve Ahola.  
V2a in the schematic below was previously unused.  
You do get more preamp drive and the distortion is resonably smooth.  
I lifted one end of a resistor and capacitor, added a 100k resistor and 4 bits of wire. I had to cut the PCB tracks shorting pins 1,2 and 3 to ground.  
I'd like to add a switch to restore normal operation, but I'm a little leery of the voltages involved.  
Any better ideas? (this is my first tube amp mod)  
Has anyone done something similar?  
5/26/2000 9:12 PM
Frank Clarke

Excuse me. The .gif is on the ampage site, the .txt is on the spaceports.
5/27/2000 7:24 AM
Steve Ahola
Question on dc-coupled pair using separate tubes...
    Your drawing showed the slope resistor before the treble cap so I edited it and uploaded it as:  
    I've never tried a 22uF cathode resistor bypass cap on the first half of a dc-coupled pair so if your amp has too much bass response you might try using a lower value (like .68uF, 1uF, 2.2uF or maybe 5uF). I also usually use the traditional 5F6A Rk value of 820 ohms.  
    On the Mesa Boogie Maverick rhythm channel the Bright/Fat switch would select the signal either from the plate of V1A or the cathode of V2A. (The Fat mode also places a ~750pF cap across the treble cap.) I'd recommend a full-sized toggle switch for that (since I don't like to have B+ on a mini-toggle- the terminals are too close to the grounded frame!) Hmmm... I just noticed that your dc-coupled pair uses two different tubes- if someone was going to build this CF Blues Jr from scratch they would want to use a single tube for the dc-coupled pair. Maybe someone could let us know if there are any problems with building a dc-coupled pair with two separate tubes... (It is a real drag reworking traces on a pcb amp so I can see why you wired it up as you did. ;) )  
--Thanks for your post!  
Steve Ahola
5/27/2000 8:46 PM
Ken Gilbert

"Maybe someone could let us know if there are any problems with building a dc-coupled pair with two separate tubes... "
no problem at all--done it many times. the tubes don't know who their neighbor is, and even if they did they wouldn't care.  
triodes is triodes, no matter what envelope they come from!  
5/28/2000 11:36 AM
Steve Ahola

    I was just reading that the plate voltage of the second half of a dc coupled pair should be ~2.1 times the plate voltage of the first half... is that true or just another suburban myth? (I just checked my notes on my HRT; the first plate is running at 181vdc while the second is running 79% higher- or 324vdc, This is with the standard 5F6A wiring for a dc-coupled pair.) This article also mentioned that there is an upper limit to the voltage between a cathode and the filament... what would that limit be? (It suggested "biasing up" the CT of the filament windings with ~50vdc to stay within those limits...)  
Steve Ahola
5/28/2000 12:53 PM
Carl Z

Sounds like urban myth to me. Now, having said that, In a perfect world you want the plate voltage of the first half running at approximately half the rail voltage. Given that the grid of the second half is directly coupled to the plate AND the plate of the CF is directly connected to the rail you're going to see the numbers you described.  
This will give you the optimize your clean signal output, which is not necessarily what we want in a guitar amp.  
As for a maximum limit, yes there is one but I can't remember it off the top of my head. Just go look it up, it's in the tube manuals.  
Putting a positive bias on the filaments is a great idea no matter what circuit you're running. This is a very old trick that used to be used for noise reduction in laboratory grade instruments in the wayyy old days. Works great! Check out my site and read up on the theory of how this works. It's stupid simple.  
If you want to get really anal about it, you can run the first tube off dc and the rest of the amp off the elevated AC. You'll need to filter out the positive bias with a cap and add in a rectifier which might make things a little crowded. But oh man you'll have a quiet amp!!!! It's actually a little nerve wracking because you can't hear a damn thing till you've got the amp cranked to ear splitting levels.  
Carl Z
5/28/2000 8:15 PM
Steve Ahola

    While dc for the heaters on the first preamp tube will reduce the 60hz hum, elevating the ac heaters on the other stages will reduce the "sh-h-h-h-h" noise (from stray electrons hitting one of the elements inside the tube?)... or something like that?  
    Rather than try to hook a dc filament supply *and* an elevated ac supply from the same tap on the PT it might be easier to use a separate tranny for a dc supply for the first tube and whatever relays or ss switching devices you might be using; wire the dc filaments in series if you need to use 12vdc instead of 6vdc...  
Steve Ahola

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