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Old 1st December 2006, 11:00 PM   #1
PRNDL is offline PRNDL  United States
diyAudio Member
Join Date: Oct 2006
Default Tone circuit and other guitar amp Q's

I am copying a guitar amp and have some questions.
It's an EL84 push-pull amp - about 18 watts, with a circuit very similar to a Matchless Spitfire.

The tone stack is a little unusual. It's similar to the Fender stack (for the 18 Watt Lite II) but not really.
It works, but you have to turn the knob to about 8 and then at 9 the volume falls off quickly. I've attached a photo.

I'm also considering adding a triode/pentode switch to lower it to 5 watts, which would be nice for bedroom use.
Does this sound good?
I'm asking because I don't see one on the 18 Watt or Matchless.

Another change would be a master volume.
There's only one stage of gain, so there's two choices:
A pot between the output of the phase splitter
(I've seen this in the Vox AC30 circuit)
A dual pot on each side of the phase splitter.

There's also a 1/2 of an 12AX7, which I could add as a boost stage.
I don't really see a need, however, since the amp screams (although it is a bit loud).

Schematics for the 18 Watt Lite II can be found here


PS - If you're interested in helping me fine tune it, I'll send you a schematic by email or private message. Let me know.
Attached Files
File Type: pdf tone.pdf (16.3 KB, 60 views)
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Old 3rd December 2006, 10:26 PM   #2
diyAudio Member
Join Date: Feb 2002
Location: Western Sydney
Are you using the correct taper pots?

5W is still too loud for the bedroom, 0.5W would be more like it;

Check out how the master vol is done in this schematic:

Pete McK
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Old 4th December 2006, 02:47 AM   #3
PRNDL is offline PRNDL  United States
diyAudio Member
Join Date: Oct 2006
I see now ... the only difference is the cap from the tone pot to ground.
Mine has 0.1 MF vs .01 -- perhaps that's why the tone control is at 9.

>> 5W is still too loud for the bedroom, 0.5W would be more like it


What build are you recommending ... the AX84 FireFly?
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