Bias mod on a 50/50 : how to do

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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.

DSCN1159.JPG



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.

DSCN1161.JPG



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.

DSCN1166.JPG


DSCN1167.JPG


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

DSCN1163.JPG


DSCN1164.JPG


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.

DSCN1165.JPG


Turn everything off, unplug the multimeter, let your tubes cool down and then remove the adapter and replug the tube.

Et voilà !
 
Very nicely done, Crane!! Though I do not have this amp, I believe that others that do will benefit greatly from your clear instructions. Excellent job !!!!

Though you covered it amply, I think the safety warning bears repeating, since there are lots of opportunities to contact potentially lethal voltages in there. Be careful, folks!

- Thom
 
Timbre Wolf said:
Very nicely done, Crane!! Though I do not have this amp, I believe that others that do will benefit greatly from your clear instructions. Excellent job !!!!

Though you covered it amply, I think the safety warning bears repeating, since there are lots of opportunities to contact potentially lethal voltages in there. Be careful, folks!

- Thom
Love to do the same with the 20/20, not because it's cold biased, but to use any quartet of tubes !

You're right Timber, we'll never repeat ths enough, BE CAREFULL, IT'S NO GAME !!!
 
Note :
See the 17/2.2 factory inscription on the tubes ? it has something to do with the bias. With the original settings they draw 17mA... bit cold no ?
 
I want to thank you for this post. It will be very useful for me. I want to invite you to participate in my post, please visit http://forum.grailtone.com/viewtopic.php?f=5&t=32954 to help me.

Thanks
 
hello again:

did you use a cermet pot? do you recomend installing one of those over a normal one?

thanks again
 
Yes it's a cermet one. I don't know if it it's better than another one, but I've always read that people uses cermet pots for bias.
 
35 mA to 45 mA is not enough to justify the bias intensity...maybe it's because the initial peak intensity when switching on or off.
Could you answer me why your amp has that silver colour transformer (close the output jacks) and mine doesn't have it.

thanks again.
 
Absolutely no idea. Maybe it's a 220v to 110v converter, really don't know if my amp was an european version initially !
I'll take a look at it this afternoon.
 
Ok, it's a mesa tranny.

MESA
550381
606-430

It has only 2 cables, wired across the 1Kohm/5% resistor near the right side of the amp, assuming you're front of it.
Seems to be a choke.
 
Hi Crane:

i also modified the bias current of my amp. I didn't have a tube socket so i put a 1 ohm resistor between pin 8 of the socket and its black cable. My plate voltage was 454 V and the current for the fixed bias was 13.2 mA. I modified the circuit and installed a 22kohm pot in series with a 17kohm resistor where the 68k ohm resistor was. I have set the bias current to 42mA (arround 65%). I haven't heard the results yet.

did you measure the current bias with the volume down? i Did it this way...i guess it's right.
did your sound changed a lot?

thank you again.
 
Yes, measured with volume down, it's an idle current that changes when playing. Don't forget to remove the 1ohm resistor.

Yours was 13.2 ma ! Seems your 50/50 was used as an air conditionner :mrgreen:

You'll notice a great change in your sound, maybe a bit more hum, but definitely more power, punch, warmth.
Hope you'll like it !
 
crane said:
Ok, it's a mesa tranny.

MESA
550381
606-430

It has only 2 cables, wired across the 1Kohm/5% resistor near the right side of the amp, assuming you're front of it.
Seems to be a choke.
Sorry I know this is an ancient post but someone may find this info useful.

The choke you are referring to is a modification Mesa issued for older 50/50 users who wanted to use the 50/50 with the then newly released Triaxis. I believe the 50/50 built after the release of the Triaxis came from the factory with the choke. I purchased my 50/50 in 1989 and had it retrofitted with the choke by Mesa after I purchased my Triaxis. The retrofits were mounted on the side of the rack chassis, whereas the factory mounts are on the floor of the chassis.

There's more info on the mod here http://forum.grailtone.com/viewtopic.php?f=5&t=55963
 
Sorry to resurrect this old thread but I have a question. Is there only one bias pot needed for the entire amp? i was thinking one for each side would be needed, but apparently not.

Looking at grabbing this potentiometer below to perform this mod..seems like it should work...or if there is a better product/source i'd love to be pointed in right direction!
http://www.amazon.com/3386P-1-104-100K-Variable-Resistor-Potentiometer/dp/B0087ZCI3W
 
The bias circuit act on both channels, so only one pot is needed.
It could be interesting to mod the circuit and have two separated bias pot, so two totally different tube pairs could be used together. However it's a more complicated mod that need to alterate the board or add another.

The pot you linked is okay (but it's a 50 pieces pack).
If I have to do the mod again, I'll replace the 100kohm pot with a 50kohm or 100kohm in serie with a 20-30kohm resistor, so if the pot has a short the bias would not be so hot that it burn the tubes.
 
Thank you much Crane! So must of the variable resistors i'm seeing have 3 lugs - do i need to use the ground lug or can i just use the two in/out lugs assuming grounding is handled elsewhere in circuit - obviously i'm far from a pro here....

Edit: ok sounds like i had some bad info. I understand only the center and either side lug are needed, and the unused lug can be connected to the center lug as a fail safe. Getting there....
 
Right, this how to mount a pot as a variable resistor.
Depending which lug you solder the center tap with, the rotation can be reversed for increasing/decreasing the value. That's all.

Again, be very carefull when openig a tube amp as there are lethal voltages inside.
For the 50/50, unplug the power cord and let the amp with the standby switch ON (like you were playing) for a few minutes. This will drain the power caps. Test it with a voltmeter to be sure before doing anything else inside.
 
Thanks again crane, especially for the safety tips. The clarification on wiring the pot either direction helps, i was gathering that from reading up but nice to have it confirmed.

I have the pot and bias probe coming in the mail, along with a new multimeter as the radio shack one i have has never worked at all, just shows jumbled digital readouts no matter what i do - can't even get a 0.0 on it. I also have a tech in mind in case things get hairy! I'm used to soldering, just not on PCB.
 

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