Bias mod on a 50/50 : how to do

[a2dconverter]

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

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

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

 

[HRGplayer]

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.

 

[HRGplayer]

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.

 

[clemford]

I'm not seeing this mod? Was it deleted?
I'd like to do it on my newly acquired 50/50

 

[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.

 

[clemford]

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.

 

[crane]

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 !

 

[m750lux]

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?

 

[Henz]

Sounds to me , first guess, it's the bias resistor for that socket. Do you feel confident enough to open her up and check?

 

[crane]

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.