Amplifier circuitry-4 ohm vs 8 ohm

@atmasphere

What you suggest makes sense. However,  I do recall that the late Roger Modjeski (RAM Labs Music Reference Audio) advocated “light loading” of amplifiers. He recommended using a tube amplifier’s 4 ohm tap to drive an 8 ohm speaker load. I’m not sure what the rationale was for this approach.

Charles

The concept of light loading of tubes and transformers would  seemed very important.  Can someone add some clarity to this concept, and how it relates to the overall delivery of information to a speaker driver and/or impact on the amplifiers Health relating to tube operation. Thanks

@charles1dad regarding light loading, which Roger felt benefited all of his amps. the following is taken directly from the Music Reference RM-10 manual:

"The amplifier is flat within 0.1dB and has low distortion of 0.3% when played below clipping on average level material. At the recommended bias current of 30mA/pair, the idling dissipation is nine watts or 75% of the tubes’ rating. I estimate tube life to be 5,000 to 10,000 hours. Although higher idling currents will reduce distortion, it can also be reduced by light loading. Basically, light loading reduces the output current demand on the output tubes, allowing them to be more linear. It also reduces noise, raises damping factor, reduces distortion by 78% and allows for 80% more peak current when needed. The only loss is about 20% of the power rating or 1dB."

In addition to some neat design features in the circuit, the way the output transformers were wound also contributed to the benefits of light loading as discussed in the manual. The only negative was a 7 watt loss of power, which is inconsequential if you also consider light loading doubled the class A range of the amp to a little over 20 watts.

@clio09 

Thank you for posting Roger’s explanation and rationale for light loading his tube amplifiers. So it seems the way his output transformers were wound made the difference. Ralph made a good case for using an 8 ohm tap for an 8 olm impedance speaker load. 
 

I suppose that with any given amplifier and speaker pairing one could try both the 4  and 8 olm amplifier taps , listen and decide which sounds better. I’m sure it varies from one scenario to another. My speakers are 14 olm impedance. My amplifier has 8 and 16 olm taps. I prefer the 16 olm tap.

Charles 

However,  I do recall that the late Roger Modjeski (RAM Labs Music Reference Audio) advocated “light loading” of amplifiers.

@charles1dad Roger's amps were designed so this technique could be used. Most amps are not. Roger's amps also used feedback and his recommendation relied on this. Your amps are zero feedback so it won't work with them. You will get less distortion from your power tube doing this, but the distortion generated by the output transformer on account of being improperly loaded will be far more than the gains you get from lightly loading the power tube.