Bias mod on a 50/50 : how to do

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crane
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Bias mod on a 50/50 : how to do

Post by crane » Mon Jan 19, 2009 3:04 am

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.

Image


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.

Image


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.

Image

Image

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

Image

Image

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.

Image

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

Et voilà !
Last edited by crane on Wed Mar 04, 2015 2:40 am, edited 3 times in total.
PRS McCarty Soapbar with BKP Pigs90, PRS custom 22 with BKP Rebel Yell
Mark V Head, 1x12" Mesa 1/2 closed (EVM-12L)
Triaxis 2.0 noFat - Quad preamp
Deep modded 20:20 - Bias modded 50/50 - SimulClass 295
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Timbre Wolf
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Re: Bias mod on a 50/50 : how to do

Post by Timbre Wolf » Mon Jan 19, 2009 11:36 am

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

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crane
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Re: Bias mod on a 50/50 : how to do

Post by crane » Mon Jan 19, 2009 1:39 pm

Timbre Wolf wrote: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 !!!
PRS McCarty Soapbar with BKP Pigs90, PRS custom 22 with BKP Rebel Yell
Mark V Head, 1x12" Mesa 1/2 closed (EVM-12L)
Triaxis 2.0 noFat - Quad preamp
Deep modded 20:20 - Bias modded 50/50 - SimulClass 295
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crane
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Re: Bias mod on a 50/50 : how to do

Post by crane » Tue Feb 10, 2009 5:37 am

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 ?
PRS McCarty Soapbar with BKP Pigs90, PRS custom 22 with BKP Rebel Yell
Mark V Head, 1x12" Mesa 1/2 closed (EVM-12L)
Triaxis 2.0 noFat - Quad preamp
Deep modded 20:20 - Bias modded 50/50 - SimulClass 295
Home made amps/speakers/pedals

rsanz
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Re: Bias mod on a 50/50 : how to do

Post by rsanz » Fri Feb 13, 2009 4:37 am

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

rsanz
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Re: Bias mod on a 50/50 : how to do

Post by rsanz » Mon Feb 16, 2009 2:54 am

hello again:

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

thanks again

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crane
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Re: Bias mod on a 50/50 : how to do

Post by crane » Mon Feb 16, 2009 2:57 am

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.
PRS McCarty Soapbar with BKP Pigs90, PRS custom 22 with BKP Rebel Yell
Mark V Head, 1x12" Mesa 1/2 closed (EVM-12L)
Triaxis 2.0 noFat - Quad preamp
Deep modded 20:20 - Bias modded 50/50 - SimulClass 295
Home made amps/speakers/pedals

rsanz
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Re: Bias mod on a 50/50 : how to do

Post by rsanz » Mon Feb 16, 2009 6:31 am

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.

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crane
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Re: Bias mod on a 50/50 : how to do

Post by crane » Mon Feb 16, 2009 6:38 am

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.
PRS McCarty Soapbar with BKP Pigs90, PRS custom 22 with BKP Rebel Yell
Mark V Head, 1x12" Mesa 1/2 closed (EVM-12L)
Triaxis 2.0 noFat - Quad preamp
Deep modded 20:20 - Bias modded 50/50 - SimulClass 295
Home made amps/speakers/pedals

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crane
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Re: Bias mod on a 50/50 : how to do

Post by crane » Mon Feb 16, 2009 8:23 am

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.
PRS McCarty Soapbar with BKP Pigs90, PRS custom 22 with BKP Rebel Yell
Mark V Head, 1x12" Mesa 1/2 closed (EVM-12L)
Triaxis 2.0 noFat - Quad preamp
Deep modded 20:20 - Bias modded 50/50 - SimulClass 295
Home made amps/speakers/pedals

rsanz
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Re: Bias mod on a 50/50 : how to do

Post by rsanz » Tue Feb 17, 2009 6:31 am

Maybe your amp is newer than mine .

Thanks again, Crane.

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crane
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Re: Bias mod on a 50/50 : how to do

Post by crane » Tue Feb 17, 2009 7:22 am

june 91
PRS McCarty Soapbar with BKP Pigs90, PRS custom 22 with BKP Rebel Yell
Mark V Head, 1x12" Mesa 1/2 closed (EVM-12L)
Triaxis 2.0 noFat - Quad preamp
Deep modded 20:20 - Bias modded 50/50 - SimulClass 295
Home made amps/speakers/pedals

rsanz
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Re: Bias mod on a 50/50 : how to do

Post by rsanz » Sun Feb 22, 2009 4:53 pm

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.

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crane
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Re: Bias mod on a 50/50 : how to do

Post by crane » Mon Feb 23, 2009 12:36 am

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 !
PRS McCarty Soapbar with BKP Pigs90, PRS custom 22 with BKP Rebel Yell
Mark V Head, 1x12" Mesa 1/2 closed (EVM-12L)
Triaxis 2.0 noFat - Quad preamp
Deep modded 20:20 - Bias modded 50/50 - SimulClass 295
Home made amps/speakers/pedals

Groovenut
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Re: Bias mod on a 50/50 : how to do

Post by Groovenut » Fri Jan 18, 2013 11:15 am

crane wrote: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
You've gotta love obsolete technology...

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