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300b calibration (tubelab)

Hi,
I've had my amp running for about 15 hours a day for the past several days, and so was going to do a final calibration before I case it up.
I must have one of the few Hammond power transformers that actually performs w/in its spec's, because I'm only getting about 310 volts b+. (I expected it to run higher than it should)
Is there a problem running a 300b w/ so little power? Does that effect the sound, or just the power output of the amp?
My output tx's are 3.5k pri and 4/8/16 ohm secondaries. I'm running 8ohm speakers.

Then, if I have lower b+ than usual, would I want to bias it higher than normal, or lower, or just leave it at about 68ma?

thanks!
 

hotbottle

Member
2008-03-16 6:31 am
I am not a 300b expert, but just looking at the specs and eyeballing the plate curves on audiomatica, it seems like you could bias a little hotter.

With 310 volts, and assuming a bias voltage of 50 volts approx, that means 260 volts across the tube. Maybe you could squeeze a little more out of it by raising the bias. It's a balance between hitting zero volts on the negative grid swing along the 3.5k loadline, and getting too far into the non-linear region on the positive swing.
 
With 310 volts, and assuming a bias voltage of 50 volts approx, that means 260 volts across the tube.

The TubelabSE is fixed bias with the filament firmly grounded. You will get the B+ voltage minus the loss across the OPT across the 300B.

I have a TubelabSE that I run NX-483's (a 5 volt 45) or 300B's in. The B+ is 310 volts. I am using 5K OPT's so I get about 1.6 watts with the 483's. When I use 300B's I can crank the tube current up to 100 mA, but I find that the best sound in this amp comes at about 70 mA. The power output doesn't seem to change much for any current above 60 mA. I get about 6.5 watts. The distortion goes down as the current goes up, but my OPT's start showing some saturation effects with currents above 75 mA, so I set it at 70.

The sweet spot for your amp will depend on the particular tubes, and your OPT. You will get more power than me with the 3.5K OPT. Most 300B tubes have no problem at 100 mA and some can handle quite a bit more. Check the max spec for your OPT's. If it is over 100 mA then you can experiment a bit. Start at a low value (50 mA) and increase the current in 5 or 10 mA steps, and listen for an hour or so at each level. You will find that at low currents (like 30 mA) the amp sounds dull and lifeless. As you increase the current the sound comes to life. You will reach a point where the clarity and sound stage stops improving. Listen carefully for interaction between different instruments and instruments interfering with vocals (IMD distortion). Too much current will begin to cause saturation effects in the OPT. These can be heard as distortions on heavy bass notes, and bass notes causing breakup of other simultaneous sounds. You should be able to find a happy medium for your amp and your musical tastes. The sweet spot for a 300B is pretty broad. I set mine to 70 and leave it there. I have a switch on my KT88 SimpleSE to kick up the current when I want to rock!