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):
http://www.telusplanet.net/public/beaumac/preamp.gif

Actual factory wiring (24kb):
http://www.telusplanet.net/public/beaumac/actual.gif

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)?
http://www.telusplanet.net/public/beaumac/new.gif

Any help appreciated.

need help with Tube amp mod
 

"virtualmarc" wrote in message
news:A9L9j.19434$UZ4.14016@edtnps89...
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):
http://www.telusplanet.net/public/beaumac/preamp.gif

Actual factory wiring (24kb):
http://www.telusplanet.net/public/beaumac/actual.gif

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)?
http://www.telusplanet.net/public/beaumac/new.gif

Any help appreciated.





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

need help with Tube amp mod
 

DaveM wrote:
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.


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:
http://www.telusplanet.net/public/beaumac/new1.gif
Any thoughts?

Thanks again, and Seasons Greetings!!!

need help with Tube amp mod
 

"virtualmarc" wrote in message
news:%0Aaj.34001$5l3.30068@edtnps82...
DaveM wrote:
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.


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:
http://www.telusplanet.net/public/beaumac/new1.gif
Any thoughts?

Thanks again, and Seasons Greetings!!!




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