View Full Version : Is MC275-IV auto bias?

12-20-2005, 08:09 PM
I just got a new MC275-IV.No where in the instruction book doe's it say anything about biassing if you switch to other tubes or have to replace an original down the road.Doe's anyone know about this?

12-20-2005, 08:20 PM
Definitely it is auto biasing. Plug and play!!! :)


12-21-2005, 05:01 AM
The bias is fixed at -61 volts which works well with any tube that follows the GE 6550 / KT88 curves. Current production tubes like SED KT88, 6550C, Penta Labs KT88, EH KT88s or reissue Tungsol all work well.


12-21-2005, 07:00 AM

Can you tell us if the MC2102 is auto-biasing? The instructions for it, too, are vague on the point.

Many thanks, as always,


12-21-2005, 09:59 AM
Fixed-bias is not the same as auto-bias. I have wondered why the new Macs don't allow the user to easily adjust the bias to optimize tube performance. I'm sure the inveterate tweaker (with some disassembly) can manage it but I don't know if it's necessary or recommended. anyone?

12-21-2005, 04:28 PM

I was told by Ron that the bias on the MC2102 can be adjusted.

You need to open up the unit and measure at pin 4 or 8 of V4, to get a -61 Volts reading. To adjust the bias, use the pot marked R32. The amp needs to be on for at least 1/2 hr, and the volume must be a zero. Just be careful about poking at the wrong places, you don't want to get jolted by 450V.


12-21-2005, 05:07 PM
Should use a variac to make sure the B+ is at 450V on the plate of the output before setting bias.

12-21-2005, 08:08 PM
Good point Terry. The MC275 MKIV should be at -60 volts and the MC2102 is now set at -64 volts on production units. Bias is not supercritical on these unity coupled amps. The MC2102 is internally adjustable but you need to be careful. This is not for the tinkerer to play with. We also have balance pots in the MC2102 which must be left alone.
Other designs do require frequent bias checking and many of these designs are also output tube eaters, BTW.
If an output tube requires a bias change it has a problem not the amp. The outputs should be happy for 5 to 10,000 hours from what I have seen on high hour amps. both the SED KT-88s and 6550C tubes are showing this long operational life.

Merry Christmas (or the holiday of the religion of your choice as we say in California...)