Bias mod on a 50/50 : how to do

Triaxis, Quad Preamp, Stereo Power series, Recto Preamp

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

Post by HRGplayer » Tue Mar 03, 2015 3:38 pm

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- ... B0087ZCI3W

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

Post by crane » Wed Mar 04, 2015 2:30 am

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

Post by HRGplayer » Wed Mar 04, 2015 3:48 pm

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....

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

Post by crane » Thu Mar 05, 2015 1:13 am

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

Post by HRGplayer » Thu Mar 05, 2015 9:29 am

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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a2dconverter
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Re: Bias mod on a 50/50 : how to do

Post by a2dconverter » Mon Mar 09, 2015 8:40 am

Here's my implementation of External Bias Controls and Cathode Current Test Points....

Image



I disconnected the old bias supply and designed a new bias circuit board allowing separate bias adjustment for each channel...

Image

I also installed 1 ohm cathode resistors and a voltage divider for the B+ to help with getting the bias into Range.

The reason I did all of this was to allow the adjustment of the power tube bias without de-racking the amplifier and to allow the use of any manufacturer's tubes.

It's a little more involved... Please let me know if you have any questions.

more info here...

http://stagecue.com/50_50.html

Seeya

Joe

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

Post by HRGplayer » Tue Mar 10, 2015 8:12 pm

a2d thank you...that's definitely a bit further than i want to take it but very cool!

I managed to install the the bias pot, was very simple to solder that in along with the fail safe (middle/outer lead connected). Unfortunately the bias probing tool i made from a kit i believe has a short, as the tube i was using it on started to red plate, while the others did not. Probably some wires inside the socket made contact or something. Rather than keep tweaking and risk damage or worse, I ate the $20 and threw the kit probe away and set the bias seemingly on the colder side by ear/sight and the amp and tubes seems fine - and tentatively sounds pretty good! (can't really crank it just yet to find out) Have ordered a pre-assembled bias probe to get the amp biased properly before the next gig. Thanks for everyone's help. I'm alive and my amp is still working, bias pot installed = success.

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

Post by HRGplayer » Sun Mar 15, 2015 6:36 pm

Got the bias prpobe in the mail and got the 50/50 biased to crane's spec. I can definitely hear the difference. I'm using a POd HD as preamp. Distortion especially sounds much richer and smoother.

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

Post by clemford » Fri Nov 13, 2015 3:49 am

[quote="crane"][/quote]
I'm not seeing this mod? Was it deleted?
I'd like to do it on my newly acquired 50/50

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

Post by crane » Fri Nov 13, 2015 3:58 am

Yep, just checked it and the first post is empty.... don't know why !

Short version : replace R242 (68k) with a 100k trimpot

WARNING ! Lethal voltages inside.
Unplug before doing the mod.
Put the STBDY switch in the ON position : Check the big caps for zero voltage, they should drain through their // resistors within a few minutes when the switch is in this position.


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

Post by clemford » Fri Nov 13, 2015 10:01 pm

[quote="crane"]Yep, just checked it and the first post is empty.... don't know why !

Short version : replace R242 (68k) with a 100k trimpot

WARNING ! Lethal voltages inside.
Unplug before doing the mod.
Put the STBDY switch in the ON position : Check the big caps for zero voltage, they should drain through their // resistors within a few minutes when the switch is in this position.

thanks very much crane for updating that along with the safety warning. I did read on another forum that it might be a good idea to use a 50k pot in series with a 50k resistor so it can't be accidentally zeroed out. thanks again for the update.

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

Post by crane » Mon Nov 16, 2015 8:05 am

clemford wrote:thanks very much crane for updating that along with the safety warning. I did read on another forum that it might be a good idea to use a 50k pot in series with a 50k resistor so it can't be accidentally zeroed out. thanks again for the update.
Yep better idea to put a 50k trimpo in series with a 50k resistor !
PRS McCarty Soapbar with BKP Pigs90, PRS custom 22 with BKP Rebel Yell
Mark V Head, 1x12" Mesa 1/2 closed (EVM-12L)
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Deep modded 20:20 - Bias modded 50/50 - SimulClass 295
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m750lux
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Re: Bias mod on a 50/50 : how to do

Post by m750lux » Wed Oct 30, 2019 2:06 am

Hello, I have a Fifty Fifty amp. I have noticed that one of the tubes started red plating but sound is still coming through on the channel, no weird noises either. If I swap tubes the tube in the same location red plates. This is happening on channel A. Also Channel B is passing a weaker signal. Any idea what might be causing this?

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

Post by Henz » Wed Oct 30, 2019 10:00 am

Sounds to me , first guess, it's the bias resistor for that socket. Do you feel confident enough to open her up and check?
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crane
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Re: Bias mod on a 50/50 : how to do

Post by crane » Thu Oct 31, 2019 2:48 am

Hi,

The bias is provided to all the sockets through the bias circuit.
For that red plating tube I would check the screen resistor first, then maybe the grid resistor and input resistor.
If their values are correct, could be the power transformer.
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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