"Light Loading" Amps - Music Refence and others...


On another thread, a discussion started regarding light loading amplifiers. Since it was a discussion on Vandersteen speakers, I thought it merited an new thread, especially since there is some difference of opinion.

The principle of light loading was prposed by Roger Modjeski of Music Reference.

He said (with reference to his RM10, but applicable to all his amps):

"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." Light loading means connecting the speaker on the tap that's one half its nominal impedance rating (i.e. the 4-ohm tap for 8-ohm speakers). For 4-ohm speakers, the he recommends running two RM-10s bridged to 70-watt monoblocks.

In the aforementioned thread, Ralph Karsten of Atma-sphere said:

"If you use the 4 ohm tap on an amplifier with a speaker of higher impedance, the output transformer will be inadequately loaded, and so it will express less of its winding ratio and more of its inter-winding capacitance. This can result in the amplifier no longer having flat frequency response. In addition, the transformer can 'ring' if inadequately loaded, which is another way of saying that it will distort.

The Merlin is an 8 ohm load, with a dip to 6 ohms or so. Its a benign load and an amplifier with an output transformer, if the transformer is designed properly, will likely work best on the 8 ohm tap. This will minimize the artifact of the transformer."

Two views. And different views from listeners, somew thinking light loading works magic, other saying differently.

What do you think of the priciple. The two technical arguments? Your experiences with light loading?
pubul57
Hi Ralph,

Based on a quick look at the schematics at HHScott.com, it appears that the only Scott amps that used 6BQ5's were the 222A and 222B integrateds. The 222C and 222D used the generally similar 7189, as did the 299A and 299B integrateds.

This link for the 222B indicates a continous power rating of 13W/channel.

I have some older literature which indicates that the 222C, using the 7189, was rated at 24W/channel, "per the IHFM Standard." Not sure if it would rate that high under today's standards. Also, according to my tube manual the 7189 appears to be rated for significantly higher plate and screen voltages than the 6BQ5.

Best regards,
-- Al
Not only 35 watts, but Roger claims 10,000 tube life on the EL84s. He also claims light loading will extend tube life.
Ecclectique.
Haven't seen you around since the Supratek days. Hope you have been doing well. Bob
Hey Bob. Life is grand over here. Hoping things are good for you and your loved ones.