Fender Blues Jr Bias setting

[ajb]

Hi,

I wonder if someone could help me with the best way to check my bias setting on my Fender Blues Jr turret board coversion.

Is there a voltage I can read of the board to check it is set correctly?

A have the standard valve set installed.

Thanks.

[sluckey]

Monitor the voltage at pin 3 of each EL84 or at the 1Ω resistors on the board. Adjust the bias pot for 25mV.

The EL84 is good for 12 watts. A lot of people like to set the bias on a fixed bias amp to 70% (8.4 watts in this case) Read this info for details...

     http://www.duncanamps.com/technical/lvbias.html

[ajb]

Thanks for the help, I've seemed to have sorted the problem out by tweeking the bias pot.

I was getting distorted break up and the volume was blooming when the guitar signal peaked, then dropping off at other points.

I also took out the meat control part of the circuit and removed the links on valve two, earthing the other side as the original Fender.

I'm not 100% sure these were'nt part of the problem as I had left the meat control max out one band practice and it started playing up with it at that setting.

It's been good as gold so far, so fingers crossed it's all sorted. The amp sounds great.

[pompeiisneaks]

Just 'tweaking' this pot doesn't necessarily solve the problem, if you bias too far the other way it can be detrimental to tone as well, and cause cutoff in the right environment (no output).  You need to bias as Sluckey mentioned at or near 70% or at least near that 25mV he mentioned.  What did you read on both of the tubes when the volume is at 0?  (the plate current measurement is done then when 'idle')

Just randomly changing that resistance value isn't the best solution. 

Edit:  not only that, but just biasing it out of drive doesn't mean its at a 'good point' for the tube, and it may still be over 70% dissipation, and cause the tubes to burn up from running too hot.

~Phil
 

[ajb]

Hi,

Initally I set the bias so the reading from each valve was 25mV.
The amp worked ok but from cold there was a volume increase, then as it warmed there was suddenly quite an increased hum and the volume dropped back to normal.
Thats when I tweeked the bias pot back so the hum decreased and the volume difference disappeared. Now from cold it's reading 8mV from one and 6mV from the other.
Do I need to monitor the readings as the valves warm?

Thanks for the help.

[ajb]

Hi,

I've set the bias back up to 25mV and it's working ok now.

Just one thing, there's still seems to be a warm up period of a five minutes or so where there is an increased volume.

Then after this the volume drops back to normal and it has played fine for the rest of the evening.

Not a major problem I can live with this, but any clues on what might cause this. Cheers.

[pompeiisneaks]

Not sure, it could be a tube, they're one of the more common things that can behave erratically.  Also, if the filter caps are old, they may be performing poorly, but that seems less likely, due to then usually being pretty consistently problematic, not self fixing after five minutes. 

Phil

[ajb]

Cheers Phil, I've been back in and found the connection from treble to V2, it's shield wasn't earthed properly. So fixed that and also forgot to mention that the amp was giving a tremolo effect at times, so I swapped the OT wires.
I've had it running for a couple of hours today and it's definitely behaving better.
Thanks.