Impendance taps setting on a tube amp

Pubul57-
How could the tube life be longer by switching taps. FWIW, I feel all amps should have access to the taps on the outside of the amp.

As Roger Modjeski explained it to me, the tubes don't work as hard driving an 8 ohm load from the amp's 4ohm tap as they do from the 8 ohm tap - they last longer because they are not working as hard. You lose 20% of the power with this configuration, so my 160 watt (8 ohm tap into 8 ohm load) Music Reference RM9 feeding my 8 ohm Merlins, only put out 128 watts from the 4 ohm tap into that load. If you google Roger Modjeski and Light Loading, you should find the info, it is also discussed in the 6moons review of the RM10 MKII.

The transformer transforms the 4 ohm impedance to the higher impedance that the tubes have to drive. If you load the 4 ohm tap with 6 ohms, the impedance that the tubes see plate to plate will also go up. You will get slightly less power, but the tubes will make less distortion. OTOH, the transformer will not be loaded quite right and so will ring (distort) a bit more. So you get 6 of one/half dozen of the other. It will not hurt the amp.

Bottom line is you just have to try it. I can't think of a good reason to not have the taps available on the outside of the amp. The last thing a manufacturer wants is for the customer to have to open up the unit and mess with it!

I believe that the amount of power capability that would be sacrificed by "light loading" will vary as a function of the amplifier's output impedance, and in many cases will be considerably more than 20%.

Assuming the amp is designed to present the same loading on the primary side of the transformer when an 8 ohm speaker is connected to the 8 ohm tap as when a 4 ohm speaker is connected to the 4 ohm tap, then as the amp's output impedance approaches zero the reduction in power capability into an 8 ohm speaker as a result of being connected to the 4 ohm tap will approach 50%.

That is because the voltage step-down ratio of the transformer to the 4 ohm tap equals 0.707 of what it is to the 8 ohm tap (0.707 = (1/square root of 2)), so for a theoretical output impedance of zero, and based on P = (Esquared)/R, only half the power will be dissipated by the 8 ohm speaker impedance if the voltage across it is reduced by a factor of 0.707.

With a non-zero output impedance the power sacrifice will be less than 50%, with the per cent sacrifice decreasing as the output impedance of the particular design increases. Of course, placing a 6 ohm speaker across the 4 ohm tap instead of an 8 ohm speaker will reduce that sacrifice as well.

Regards,
-- Al

From Roger M:

"The RM-10 is very tolerant of short circuits and will not be damaged by them." "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."