The
AST, like all other Swarts, is Cathode Biased meaning no user adjustments when
tube rolling. Over the years, we have fooled around with the bias point,
mainly to adjust to different tube runs and types. We have run her pretty
high to that sweet result, but this DOES push the power tubes pretty hard.
We have run as high as 360ohm to as low as 500ohms, to the current stock bias
point using a 430ohm resistor. The higher the bias, the harder the tubes
are pushed, the less tube life you get. But the sonic result can be addictive.
We've found 430ohms to be a happy medium though some still prefer the original
360. Here is a mod on switching this out if you want longer tube life
and a cooler running amp. Or, if a latter 500ohm resistor, you can mod
yours to 360 or 430. The parts are inexpensive and the soldering
quite easy for those comfortable with a little DIY iron work.
As always, unplug amplifier and remove charge from capacitors before work. If
you are unsure of these instructions, please have a certified tech do this mod.
We start this mod with the AST chassis already removed from the cabinet and
on the bench. Avoid cold solder joints at all costs.
1) See
the 360ohm Bias Resistor with Cap. Keep all wires/connections same.
See next step. Click for full.
2) Be extra careful to keep positive and negative
matched to drawing. It will be marked on resistor/cap
Try not to overhead component. And as always,
avoid cold solder joints. Shot below clicks to FULL
