1. Home
  2. Electronics Repair
  3. need help with Tube amp mod

need help with Tube amp mod

I have a "legend super lead 100" guitar amp from the 1980s. Very cool looking amp, solid finger-jointed oak cabinet. It has a 12ax7 preamp that sounds pretty good. But the "color gain" doesn't work right. At 0 you don't get any sound. At higher settings, the amp works but no "color" occurs (ie clipping @ CR1/CR2). I'm in a little over my head here. The color gain is not wired exactly according to the schematic (problem, or post-production tweak?).

Schematic (149kb):

formatting link

Actual factory wiring (24kb):

formatting link

The actual wiring prevents the "shorting" of R10 @CCW thereby preventing significant "color"(right?).

R9 goes to ground @full CCW. This acts as a volume control on low settings, which it shouldn't do (signal at the grid of V1(2) to ground). Is this just bad design?

Would this work instead? Do I need to worry about current flow at the grid V1(2)?

formatting link

Any help appreciated.

Reply to
virtualmarc
Loading thread data ...

Although there are some idiots in alt.guitars.amps, there are also some very smart amp techs reading. Post your question there.

Reply to
Meat Plow

These are class A amp stages, and should not draw grid current. A 100K resistor from grid (pin 2) to ground should be good. Don't change C8, or any other components until you try your mod. You probably won't get the same effect from the rhythm drive control as you get now, but it's up to your ears whether it's acceptable. If you can find a copy of the user's manual for this amp, it might explain the intended operation of the controls. Like it or not, what you're experiencing might be the way it was designed to operate.

--
Dave M
MasonDG44 at comcast dot net  (Just substitute the appropriate characters in the 
address)

"In theory, there isn't any difference between theory and practice.  In 
practice, there is."  - Yogi Berra
Reply to
DaveM

resistor

from

might

Thanks Dave. Your "class A" comment has really clarified things. I see now that my mod would completely screw up the volume control in rhythm mode. Based on your comments, I think a better way to get the "color" to work, might be moving R9 to be in between CR2 & Q1, like this:

formatting link
Any thoughts?

Thanks again, and Seasons Greetings!!!

Reply to
virtualmarc

I had to go back and read your original post about this issue. As I understand, the factory wiring doesn't agree with the schematic in the area of V1. I guess my first response to your questions should have been to ask whether you had tried to rewire the circuitry to agree with the schematic. If you have done that, how did it work out?

To your last question and suggestion for wiring change, I don't see any danger in making your mod, but in my estimation, you probably won't get much of an effect from the color gain control except maybe at one end of the control. As I said, won't hurt anything to try it.

It appears the RC network around R9 is a tone control circuit of sorts. It should only affect the frequency response of the circuit, rather than the total gain. Given that, give this suggestion some thought: Just remove the ground from the bottom end of R9. That will allow the level to be maintained, but will alter the frequency response, which affects "colour".

Cheers~!!!

--
Dave M
MasonDG44 at comcast dot net  (Just substitute the appropriate characters in the 
address)

"In theory, there isn't any difference between theory and practice.  In 
practice, there is."  - Yogi Berra
Reply to
DaveM

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.