crane
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Some people asked me if I could show some pics and explanations about the bias mod in my 50/50. So here it is !
This mod works only for the 50/50, I will not be responsible for bad manipulation, shock hazards, even not if it fry your amp. Do this on your own responsibility or leave it to a tech !
Of course this mod void your warranty
So here we go.
FIRST OF ALL you have to understand that there are lethal voltages in a poweramp, even unplugged ! Don’t work with some jewelry on your finger, no necklace, etc…
Be carfeull where you put your fingers. If you’re not, then leave it to a tech !
1) Replace a resistor in the bias circuit with a variable pot
There’s a 68 kohm resistor (blue one, check value on the internet, there are some pages dedicated to the stripes and their values) that you want to replace with a 100 kohm pot, mounted as a variable resistor.
Before soldering the pot, turn it to 68 kohm and check it with a multimeter.
EDIT : it will be safer to replace the 68kohm resistor with a 50k or 100k pot in serie with a 20k - 30k resistor, so if the pot has a short the bias couldn't be sooooooo hot that it instantly burns the tubes.
2) Make an adapter for bias measuring
Ok, this was simply made with and old tube socket and an… old tube, yes.
Carefully break the glass of the tube (inside a box is preferable, so no glass pieces will be thrown on the floor, your eyes, mouth, dog, etc…).
Remove the glass, and gently cut all the electrodes inside, leaving the pins as long as possible (about 8mm in my case) for soldering.
Solder some wire point to point from tube socket to old tube (1 to 1, etc…) except for point 8. I’ve used rigid copper cable, this way the new tube will be well maintened (see below). In any case use wire that could support 500V.
Between points 8 solder a 1 ohm/ 1% resistor. This way when measuring voltage it will be easilly convert to current using the ohm law (U = R * I ; R = 1 --> U = I )
If you experienced problems soldering the copper cable as I do, use old string’s little ring for assembling before soldering.
3) Install the adapter and new tubes, measure and adjust bias
Plug your new tubes in, using the adapter for one of them.
Don’t forget to plug your speakers for both channels !
If nothing is inside the amp or can fall in, turn it on, leaving it on standby for warm up.
You can consider plate voltage is 450V, but I recommend you to check it. For this turn standby on and take measure between one of the chassis screw (not painted) and point 3 of a tube (plate voltage, about 450V). Point 3 is the brown or blue cable soldered to the socket, located at ~7 hour when you see the socket from the solder points. Turn standby off when finished measures.
Okay now a bit mathematics. We consider plate voltage was measure to 462V
Max power of a 6L6GC tube is 30W. But this is too hot, tube’s life will be very short.
Generally, power is set between 60% and 80% of the max power of the tube. More power means a more « hotter » tube.
P = U * I (U = plate voltage (V), I = bias current (mA)).
So if we set the power to 60% we have :
0.6 * 30 = 18 = 462 * bias --> bias = 18/462 = 38.96mA
There is another factor, grid resistor. With 6L6GC it is negligeable (make the bias change about 1mA) so we don’t bother with that.
Now we kown that bias current must be 39mA for the power we want.
Set your multimeter to mV, and put it accross the 1 ohm resistor on the tube adapter
Turn standby on, the adjust the pot until the multimeter show 39 (remember ? 1 ohm resistor, so mV = mA). Let the amp getting warm for 10 or 20min, the adjust again if necessary.
Turn everything off, unplug the multimeter, let your tubes cool down and then remove the adapter and replug the tube.
Et voilà !
This mod works only for the 50/50, I will not be responsible for bad manipulation, shock hazards, even not if it fry your amp. Do this on your own responsibility or leave it to a tech !
Of course this mod void your warranty
So here we go.
FIRST OF ALL you have to understand that there are lethal voltages in a poweramp, even unplugged ! Don’t work with some jewelry on your finger, no necklace, etc…
Be carfeull where you put your fingers. If you’re not, then leave it to a tech !
1) Replace a resistor in the bias circuit with a variable pot
There’s a 68 kohm resistor (blue one, check value on the internet, there are some pages dedicated to the stripes and their values) that you want to replace with a 100 kohm pot, mounted as a variable resistor.
Before soldering the pot, turn it to 68 kohm and check it with a multimeter.
EDIT : it will be safer to replace the 68kohm resistor with a 50k or 100k pot in serie with a 20k - 30k resistor, so if the pot has a short the bias couldn't be sooooooo hot that it instantly burns the tubes.
2) Make an adapter for bias measuring
Ok, this was simply made with and old tube socket and an… old tube, yes.
Carefully break the glass of the tube (inside a box is preferable, so no glass pieces will be thrown on the floor, your eyes, mouth, dog, etc…).
Remove the glass, and gently cut all the electrodes inside, leaving the pins as long as possible (about 8mm in my case) for soldering.
Solder some wire point to point from tube socket to old tube (1 to 1, etc…) except for point 8. I’ve used rigid copper cable, this way the new tube will be well maintened (see below). In any case use wire that could support 500V.
Between points 8 solder a 1 ohm/ 1% resistor. This way when measuring voltage it will be easilly convert to current using the ohm law (U = R * I ; R = 1 --> U = I )
If you experienced problems soldering the copper cable as I do, use old string’s little ring for assembling before soldering.
3) Install the adapter and new tubes, measure and adjust bias
Plug your new tubes in, using the adapter for one of them.
Don’t forget to plug your speakers for both channels !
If nothing is inside the amp or can fall in, turn it on, leaving it on standby for warm up.
You can consider plate voltage is 450V, but I recommend you to check it. For this turn standby on and take measure between one of the chassis screw (not painted) and point 3 of a tube (plate voltage, about 450V). Point 3 is the brown or blue cable soldered to the socket, located at ~7 hour when you see the socket from the solder points. Turn standby off when finished measures.
Okay now a bit mathematics. We consider plate voltage was measure to 462V
Max power of a 6L6GC tube is 30W. But this is too hot, tube’s life will be very short.
Generally, power is set between 60% and 80% of the max power of the tube. More power means a more « hotter » tube.
P = U * I (U = plate voltage (V), I = bias current (mA)).
So if we set the power to 60% we have :
0.6 * 30 = 18 = 462 * bias --> bias = 18/462 = 38.96mA
There is another factor, grid resistor. With 6L6GC it is negligeable (make the bias change about 1mA) so we don’t bother with that.
Now we kown that bias current must be 39mA for the power we want.
Set your multimeter to mV, and put it accross the 1 ohm resistor on the tube adapter
Turn standby on, the adjust the pot until the multimeter show 39 (remember ? 1 ohm resistor, so mV = mA). Let the amp getting warm for 10 or 20min, the adjust again if necessary.
Turn everything off, unplug the multimeter, let your tubes cool down and then remove the adapter and replug the tube.
Et voilà !
