Bias Adjustment - About How Often SE40 ?

0 Members and 1 Guest are viewing this topic. Read 1237 times.

David Ellis

  • Full Member
  • Posts: 1029
    • http://www.ellisaudio.com
Bias Adjustment - About How Often SE40 ?
« on: 22 Feb 2016, 07:50 pm »
Jeff,

About how often should I set/check bias in my SE40SE ?

Thanks for taking the time to post herein :) 

Some background...

My SE40SE continues to sound glorious :thumb:, and has been very kind to the vacuum tubes too.  I suppose I have 3-4 years of use on the old stock 6SN7 tubes and perhaps slightly less time on the Phillips double halo new old stock tubes.  These tubes were expensive, but they have lasted a good long time - much longer than any El34 in my older Jolida 302b  :roll:.  That thing would "eat" a Svetlana winged "C" every 3-4 months :nono:.

I set my SE40SE bias perhaps 5 months ago and noticed some glowing red plates (runaway  :oops: ) in my Phillips tubes a few days ago.  I immediately turned off the amp and checked the bias.  Some of the tubes were low - about .520v, and some were high - about .650.  I reset the bias to .610v and allowed the amp to cool.  After about an hour I turned on the amp and checked bias during warmup.  Generally, the bias was within +/- .015v during warmup and this is perfectly acceptable.

I suppose some of the bias wandering is rooted in the vacuum tubes and perhaps my tubes are getting old.  Nonetheless, what is your experience in this regard ?

I hope all is well with you and yours.

Sincerely,

Dave

Jeff

  • Facilitator
  • Posts: 164
  • Test...
    • Sonic Craft, Inc.
Re: Bias Adjustment - About How Often SE40 ?
« Reply #1 on: 25 Feb 2016, 02:45 am »
Dave,

I am well, thank you.  I trust you and yours?

When to bias?  There are four occasions:

1) When the amps starts to sound dark.
2) Sound stage starts to collapse.
3) Replacing, or moving the position of any output tube.
4) Tube issues.  Sparks, crackling, red plating, etc...  This includes low or no volume in one channel.

1 and 2 pretty much go together.  I do not know how robust your NOS tubes are, but red plating is bad news for current production tubes in a SE40.  My fast and dirty bias instructions go like this:  Before powering the amp, rotate the bias pots all the way clockwise.  After powering up the amp,  allow at least 3 minutes for warm up.  You will begin the bias process rotating the pots counter-clockwise as necessary for increased bias and vice-versa.  Bias 610-615mV.  Only allow a seven minute window of adjustment.  If the unit is on longer than 10 minutes collectively, shut it off until cool to the touch (about 30 mins.).  Try again.  Ignore the first sentence if you are not replacing tubes.

 This is the bias recommendation on page 3 of my SE40SCX kit instructions: Before powering the amp the first time after modification, rotate the bias pots all the way clockwise.  After powering up the amp,  allow at least 3 minutes for warm up.  You will begin the bias process rotating the pots counter-clockwise as necessary for increased bias and vice-versa. The main thing you are looking for in biasing this amp is that the tubes never exceed 640mV. Notice if you start the amp from cold and monitor the tubes one at a time every ten seconds or so for the first 15 minutes (90 measurements), that the tubes climb to a peak and then settle out. They may peak once or twice, but they should be biased low enough (610-625mV is not uncommon) that they never exceed 640mV. This is a real pain, but it is the best way.  If the bias exceeds 640mVs on a tube rotate the pot a bit clockwise, and start again with a cold amp. Continue if necessary.  The ground wire in the bias harness is the wire furthest to the Left as it plugs into the board with the amp upside down, and the front facing you. The pot closest to the front panel on the side of the amp which you are adjusting will correspond to the wire closest to the ground wire in the harness. Next wire out......next pot out. Last wire in the harness....pot closest to the back of the amp.