Hello Tom!
On 20 Apr 98, Tom Scioscia wrote to Rich Lockyer:
 TS>        Ahhh. And I remember you saying something about most overdrive
 TS> being within the midrange frequencies, so I guess "fat" is still a good
 TS> term for it.
It seems to be most useful at lower volumes, just to give the preamp a kick 
in the seat.  If you start getting it near output distortion, it starts 
getting "farty".
 TS>        But you said that you almost couldn't hear it, though. Is this in
 TS> comparison to what a 30 watter should sound like, or you could barely 
hear
 TS> it? I find it odd that the tone stack would add so much resistance to 
he
 TS> signal even when bypassed...
Not a 30 watter... 30 MILLIwatter.
 RL>> This one uses two EL84 output tubes driven at about 15vdc, but the
 RL>> preamp and PI are all TL084 quad op amps.
 TS>        BLASPHEMY!!! :)
 TS>        Only 15v? Are you talking about the heaters? That doesn't sound
 TS> high enough to drive a tube... ??? :) Of course, I'm an idiot trying to
 TS> learn from you. :)
It drives 'em shore 'nuff.  Surprised the hell out of me too.  Through the 
internal speaker, you could sit 10 feet away from someone and they'd hear the 
strings and not the amp.  Through my 4x10, it's quite usable for practice and 
sounds good.  I never did revise the circuit to allow the use of the tone 
stack, simple because the volume and tone controls on the guitar are more 
than adequate.  It was designed for those of us who live in condos or 
apartments and hate headphone amps and the little solid state jobbies that 
can be turned down low enough to use at "conversational volume".
I probably spent about $200 on it, but I modded the circuit to use 4 tubes 
instead of 2 (didn't make a difference in volume), added the tone stack (4 
pots at $3-5 each), and bought new transformers (not having a junkbox with an 
old output or filament transformer).  I also built a nice "Matchless"-looking 
cabinet (birch ply, dovetailed) for it covered in black tolex with a Fender 
style grille over the 4" speaker.  I also added an impedance selector switch 
(which required some VERY creative wiring on the Hammond tx).  Of course, as 
usual, I overengineered the power supply to be able to put out 10 times the 
wattage actually needed and fully regulated it.  Using regulators is the 
easiest way to get a solid +12/-12 without futzing around with bleed 
resistors... the 24vct transformer puts out about +18/-18 after filtering, a 
12vct tx wouldn't be enough, and I haven't seen any 15-18vct transformers.
If I were to do it again, I'd build it exactly as it was designed, on a 
simple cage-style chassis.
The schematic is available at http://www.netaxs.com/~lxh2, however, the power 
supply is NOT included.  He assumes that if you don't know how to build a 
regulated power supply, you have no business building his projects.
That said, here's how I built the power supply:
NOTE: Ground on low-voltage side is isolated from the chassis...
      __|__ symbol denotes this ground.  Only AC ground connects to
      / / / the chassis.  Jacks for the amp are black nylon Marshall-style
            jacks (available from Mouser).
                                       _____    _____________        ___+vdc
                                      |     )||(             |      |
  ac+____________ Fuse ____dpdt_______|     )||(           full     |
 gnd ___chassis                             )||(___     -  wave  +__|
  ac-______________________dpdt_______      )||( __|__ |  bridge
                                      |     )||( / / / |     |
                                      |_____)||(_______|_____|    ______-vdc
                ___B+ to output tubes                  |_________|
               |        _______
+vdc___________|_______| 7812  |_________________________+12v
          |+           |_______|     |+          |
         === 2200mfd       |        === 2200mfd  V LED
    ______|-_______________|_________|-__________|_______dcgnd
 __|__    |+               |         |+
 / / /   === 2200mfd    ___|___     === 2200mfd
    ______|-___________| 7912  |_____|-__________________-12v
-vdc                   |_______|
There also needs to be a tap just after the switch to feed the filament 
transformer.  Unfortunately, the fuse has to be sized for the filament 
transformer, and is far too large to protect the 1a regulators in the DC 
power supply.  It wouldn't be a bad idea to add a secondary fuse of about 
150ma (slo-blo) between the switch and the power transformer to protect the 
DC circuit.  Pay close attention to the pinouts on the 7912 regulator.  The 
7812 is simple... looking at the face of the TO-220 case, it's in-gnd-out.  
The 7912 is (I think), in-out-gnd, but be sure to check.
The tube can be fed from the +12v supply rail, but I chose to feed them from 
the unregulated side to give a slight boost in power... the amp has plenty of 
gain to drive the tubes into distortion, even running B+ above 15v, and I 
think I sweetened the tone up a bit when I fed the tubes a little more 
voltage.
C-ya! Rich
--- GoldED 2.40
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