ot: which kindof bias is an amp?

[marrstians]

i was looking to change the tubes in my amp and i heard that their are other types of bias... some are self adjusting, some need to be adjusted but i'm not sure how to tell which one i have... looking in my amp their is only one trim pot that i can find but i don't know if that is for the bias or not.... btw... it's an ampeg super rocket... i had an old ampeg before and it was self biasing, are most ampegs that way?

[brett]

Most amps need re-biasing with new tubes.  Your previous self-biasing Ampeg is the exception rather than the rule.  Most professional music shops will bias your tubes for about $20.  Unless you are ok with playing near high voltages, it's probably better to let them bias it.  

Otherwise, check your insurance policy and do the following;
1, The best way to measure your bias current is with a 1ohm resistor between the cathode and ground.  Then 1mV drop across the resistor=1mA bias current.
2. Set the bias trimmer so that the bias current x the plate voltage = 70% of the maximum plate dissipation.  
e.g. in my Musicman amp, the EL34s are rated at 25W, and the plate voltage is an unusually high 700V, so the bias point is 0.7 x 25 / 700 = 25mA.  Some people use a lower rating (e.g. 50%), but that may result in running too "cold".  I don't recommend using "standard" tables of bias current.  For example, for EL34s, the rating is often 40mA.  In my 700V amp, that equals 28W in my EL34s that are only rated at 25W (and that's with no signal  :roll: ).  Better to run at 70% in my opinion. :wink:

There's also a link to an article on biasing amps on Aron's homepage, I think.  Or maybe it's over at geofex.com?