header image
Home arrow Sopht schematics arrow Good Girl
Good Girl
Written by Administrator   
Tuesday, 04 July 2006

So, after building a few amps, I am seeking something new. I already have a few amps that produce nice distortion. Now, I want something with a nice, rich and fat clean sound. I also want to try new ideas, break outside the normal Marshall/Fender/Vox thing. The idea is to use 2 paralleled triodes for 2 gain stages. Each stage should be nice and fat because of the paralleled triodes (such is the premise). I also want to use a different type of tone control. I want a single tone knob that would do the job well (and yet be simple). I have looked at a few different single knob tone controls and decided that I wanted to try one crafted by Garnet. After a bit of tweaking, I was unable to get the range of tone that I wanted with the Garnet tone knob. Therefore, I settled for a princeton tone stack. Finally, I want to use a big octal for the output (and I have a few 6550 lying around, so....).

Please note that a 6550 in a single ended design such as the Good Girl is not a bedroom amp!  This thing puts out close to 18 Watts! 

This amp has a very warm sound. Perfect for a bit of jazz and some nice, smooth blues.

Here is the version 2.3 of the design (click on image to see a full size version). This is the 'final' version.

With the gain from 0-50%, you get nice lush cleans. With the gain at about 2/3, you get nice blues dirty-ish sounds (especially with a neck humbucker). With the gain wide open and a P90, you get very nice classic rock sounds.

Sound clips

Two quick sound clips:

a clean clip with a fender strat (middle and bridge pickup)
a dirty clip with an es-335 clone (neck pickup - paf reissue)

Both clips are with the gain at 100%, so, the fender clip is as dirty as it gets from the preamp with this amp. Same with the es-335, this is as dirty as it gets from the preamp.

NEW sound clips of the latest version

Allright, here are three quick clips of the last version of the Good Girl:

Clean clip is with Gain: 50%, Tone: 50%, Volume: 25% and neck pickup
Dirty clip is with Gain: 80%, Tone: 60%, Volume: 25% and neck pickup
Classic rock clip with Gain: 75%, Tone: 60%, Volume 75% and bridge pickup

Here is a status of the project:

Activity Status # hours
Design Completed 0.5
Layout Completed 1
Building board Completed
Drilling chassis Completed
Installing board in chassis Completed
Debugging / Tweaking Completed

Some notes

  • With humbuckers or a hot P90, clean to about 2/3 of the gain and then, gets a bit dirty. With single coils (Fender strat), nice chimy cleans all the way, very little dirt!
  • All resistors 1/2 watt except R9 and R11: 5 watt, R3,R4 and R10: 1 watt
  • All three pots are audio taper
  • you can use a 6550C or a KT77 without any bias change

Voltage chart:

Wall : 123.6V
B+: 388
B+1: 376
B+2: 361
b+3: 260


Populated board:

Inside the chasis:

Picture of the outside:

More pictures to come...


This faceplate was created with Inkscape ( http://www.inskape.org/ )
Right clik to save this file: faceplate_good_girl.svg
The pdf file: faceplate_latest_4.pdf

Version History

1.0: Initial draft
1.1: Reduced cap C4 to .001
1.2: Added bleeder resistor R13 for safety
1.3: added fuse and pin numbers
1.4: Change C4 to 470pF, Change tone to 25k, Change R6 and R8 to 620 to reflect my build, Change R4 to 22k (increased b+3 from 206v to 260v), Change R9 to 250ohm (I have a 220 and 6550 is biased a bit too hot), Changed the power transformer to reflect my build
1.5: Added bypass cap C6, this adds depth to the sound. Changed tone stack from a garnet style to a princeton style tone stack. Sadly, I wasn't able to tweak the garnet tone stack to my liking.
1.6: Changed both cathode resistors from 620 ohm to 750 ohm. Now that is a nice change! The distortion (in the last part 1/3) is nice and smooth (it was a bit harsh at 620 ohm).
1.7: Change bypass cap C6 from 0.1uF to 1uF to add some bass. It now has the perfect balance of warm bottom while not being muddy when overdriven.
1.8: Changed the bias resistor from 220ohm to 330ohm. The tube was bias too hot. If you don't have a 330 ohm, go for a resistor in the 330 to 360 ohm range.
1.9: Added resistor R14. Without it, the amp would emit a loud buzz when the gain was at 0, the volume at 100% and the tone anywhere between 0 and 100%
2.0: The hum is gone! Thanks to a lot of people over at ax84, several things where suggested to tame the hum. Paul Ruby accurately identified the problem. By removing the typical 68k from the input jack to the first triode stage, the hum completely disapeared!
2.1: Re-added the grid resistor R2. Made this considerably smaller than usual (10k instead of 68k). The reason I re-added it is because I could hear some radio signal getting into the amp. Adding this made sure that no radio interferance made it into the amp. Change R5 to 220k and R6 to 2.7k to get cleans AND nice dirty tones with the gain control. Changed C3 to 0.01uF because with 0.022uF, the distorted sound was too muddy. Change C10 back to 0.0047uF. This gives a nicer range to the tone stack.
2.2: Modified to tone stack so that it is more independant of the gain knob. Changed coupling cap C3 back to 0.022uF.
2.3: Changed the first stage plate resistor to 100k and the cathode to 860ohm. With the previous values, it was very pedal un-friendly.


Last Updated ( Saturday, 17 November 2007 )