User-Biasing of Fender "Evil Twin"
neve...@dev.null.mil
find that it has jacks and trimpots for user biasing on the rear
panel.
I did not get a schematic with this purchase, but according to the
instructions one merely attaches DMM leads to J1 and J2 and adjusts
the voltage to 80.0mVDC with trimpot 1 and then attaches the leads to
J2 and J3 and adjusts the balance to 00.0 with trimpot 2. Done.
This is altogether too simple!
I'm not at all clear if this is a real, bench-grade bias/balancing or
not, nor how the circuit works, at least without tearing it open and
looking at the circuit.
The instructions say to use the factory setting of 80.0mVDC "for
optimum sonic performance," or 60.0mVDC "for optimum sonic performance
and increased tube life," or 40.0mVDC "for optimum tube life."
Are these values on a continuum from 80->40mVDC, or does something in
this particular Fender circuit require these specific three voltages?
Thanks for any enlightenment on this business!
Miles O'Neal
|This is altogether too simple!
Feel free to do something more complex. 8^)
|I'm not at all clear if this is a real, bench-grade bias/balancing or
|not, nor how the circuit works, at least without tearing it open and
|looking at the circuit.
It's somewhere between what the designers wanted you to be
able to easily do at home and what their lawyers will let
them let you know about to try.
|Are these values on a continuum from 80->40mVDC, or does something in
|this particular Fender circuit require these specific three voltages?
They're a continuum. It's just dumbed down because
somewhere there's some idiot who'd get it wrong if
they put it any other way, and sue them. That might
still happen, but this way even if there are idiots
on the jury, they stand a chance.
-Miles
Bob Hedberg
This looks like a standard biasing method for push-pull amps.
The single 80 mv. adjustment should be the a voltage representation of
all four tubes , and if it is across a 1 ohm resistor, it would
represent 80 ma total, or 20 ma per tube, which is in the ballpark,
and would explain the mentioning of 60 ma. for longer life, as the
idle current through the tubes is being reduced, prolonging their
life.
Of course, decreasing the idle current also may reduce the dynamics of
the tube as well, soften the bass, ssss the treble, and cause world
unrest.
Or not.
The 0 volt adjustment sounds like a balance adjustment of each side of
paired tubes. If so, it would be used for making sure each side of
output tubes are balance very closely, current-wise.
If this is what these adjustments are, then yes, they are actual bias
adjustment.
Anyone have any knowledge of these amps?
regards
Bob
neve...@dev.null.mil wrote:
Bob H.
Just grab that plate in one hand, the chassis in the other,
and FEEL the power of tube audio!!!
(not literally, of course, just kidding. DON'T DO THAT!)
Gary Gerhart
OK,
Here's the schematic:
Click schematics, fender, evil twin.
The test points are labeled differently on the schem, but the 1 ohm
resistors are there, so you're measuring the current for 1 pair of
tubes. The balance measures across cathodes with no reference to
ground.
So far so good.
Thing is, the power switch can change things.
At the full power setting, B+ is around 460 per the schem.
The current you measure (mv across 1 ohm) includes the screen
grid current. 80mv would be 40 ma/tube including screen current.
That's less than 18 watts. Half that (40mv reading) must sound
pretty sterile.
--
Gary Gerhart
www.GerhartAmps.com
neve...@dev.null.mil
wrote:
>Here's the schematic:
>Click schematics, fender, evil twin.
No got.
I think that the "The Twin" schematic they have there is the one I've
seen elsewhere, for the pre-'94 circuit. Can't say for sure, as it's
still downloading after fifteen minutes.
According to all my sources, there is no online version of the "Evil
Twin" schematic. Fender won't permit it. They'd rather sell them for
$5, I guess.
I've been looking for that schematic for some time.
Gary Gerhart
Sorry then. The bias test points are probably similar, but if
you don't know the plate voltage, the current doesn't mean squat.
Hint: If you can buy it from Fender for 5.00, quit looking for it ;-)
--
Gary Gerhart
www.GerhartAmps.com