View Full Version : mcintosh autobias - how does it work ?

02-18-2013, 07:18 AM
I have ( as usual ) stumbled into an area where i did not know what i was talking about.
(big mouth opens too fast for brain to catch up-syndrome :-)

There is apperently some sort of auto-bias in mcintosh amps such as mc275
The schematic is available but frankly i do not understand how it
will "auto-bias" anything. Google gave me no hints.

Anyone that has analysed it and can make me an hint of it's workings ?

02-18-2013, 07:30 AM
I may be off target here, but isn't that a Blumlein garter circuit implemented with fixed bias ?

02-18-2013, 09:01 AM
I think my MC40's are also auto bias so this topic interests me.

02-18-2013, 10:12 AM
How about self balancing? The screens are tied to the opposite plate, so when one plate is conducting the other screen is drawing current as well. The OT has a tertiary winding connected to the cathode and fed back to the 12AX7 va, an output stage form of feedback. Class B ish?

02-18-2013, 10:13 AM
That's why OT matching isn't very critical in a MC.

02-18-2013, 10:14 AM
Output Tube

02-18-2013, 11:11 AM
Guit is striking near the heart of the matter.

McIntosh 240 and 275 amplifiers to not have any active type of automatic quiescent biasing circuit built into the design, but rather, because they largely operate in Class B mode, the quiescent current of the output stage is generally much lower than that used in the usual Class AB1 output stage. With lower quiescent current, this means that these amplifiers will tolerate a very wide range of output tubes with regards to required bias voltage, and still be well within safe dissipation levels for virtually any example of the tubes installed.

Also, because the bifilar and trifilar would OPTs split the output signal equally between the plate and cathode windings in the OPTs, the output stages are therefore inherently self balancing with regards to dynamic (AC) signals. This, along with the massive core of the McIntosh OPT greatly reduces the need to balance the output stage under quiescent conditions for optimum low frequency performance.

As a result of these design features, the usual DC bias and balance controls are not required on these McIntosh amplifiers, and so were dispensed with. Of course they can be added, but from a pure performance standpoint, the return of such an effort would be minimal.

I hope this helps!


02-18-2013, 11:26 AM
Thanks Dave for the mo betta explanation. Would this be true of other amps using an OT with a tertiary winding (ala Bogen DB20, OT Acrosound TO-350, Dynaco A-441)?

02-18-2013, 01:16 PM
It would depend on how much feedback the tertiary winding in these transformers supplied, and how much imbalance the core of these transformers can withstand before saturating. Obviously they operate quite well with class B operation, but how deeply they can effectively operate in class B to obtain the same qualities as that of the Mac transformers would have to be determined.


02-18-2013, 02:57 PM
Thanks for the insights. It's a very low bias (class B) that does it.
I must say that the schematics looks even more obscure then the D76 i once had.
Is there any description of the "internals of MC-2xx" that describes the features
and why the design is the way it is ?

02-18-2013, 03:28 PM
Hi Peter -- Frank described the basic "McIntosh" output stage in a paper he produced for (IIRC) Audio Engineering back in 1949, and while primitive to the 240, still describes the basic operation of the 240's output stage. I've got it in my files which I'll try to get to if need be, but I would bet it's available on the web as well. The short answer to your question though is that the Unity Coupled Output Stage (Bell Labs) was developed to address the problems with OPTs of the day when used in class AB designs. At the time, they developed quite a "notch" at the push-pull crossover point when moving from class A to class B due to poor coupling in the OPT. Unity coupling resolved that at the time, but of course, as the design and quality of conventional OPTs developed, very good class AB operation was also achieved using traditional designs as well.


02-18-2013, 06:15 PM
Might be on Roger Russell's site

02-19-2013, 12:38 AM
Dave gave some great information here. I'll add that every single McIntosh tube amp uses just about an identical setup MC-30, MC-40, MC-60, MC-75, MC225, MC240 and MC275.... all have the same basic circuit.