Ceriatone Forum

Something that was mentioned by a friend has me wondering. I trust this guy and he probably knows more about this than I do, but I've never heard this mentioned before.

When pulling the 2 outer tubes in a 4 output tube amp to reduce the wattage, he says that its a good idea to reduce the impedance setting from what it was to half of that. So if you had it set for 8 ohms you'd set it to 4 ohms.

I've not heard about this before - anyone know if it's true, and a good idea, or bogus?

My understanding is that you need to plug your speaker into the amp now set to one half of what normal impedance would be when removing two of the four output tubes.

For example if you are using a 16 ohm speaker, set the amp to 8 ohms and plug in your 16 ohm speaker.
I can't fully explain why but many reputable builders state it is the proper thing to do to match the output of the amp and the speaker load.

Sean

I've heard the same thing, and more than once.  Still, I'd like to know what that is....

He is correct.  It's all about ratio.

When you remove tubes, you change the primary impedance. 

- 4x6L6GC amp typically specs a 2000Ω primary. 
- 2x6L6GC amp typically specs a 4000Ω primary.
- 2x6V6GT amp typically specs a 8000Ω primary.

Secondaries are all determined by the winding. 

Without checking with Nik, I'll assume the 100-watt OTS output transformer is wound to 2K:4/8/16. When you pull two tubes, the amp wants to see 4KΩ on the power tubes for optimal operation.  So, to do that you look at the ratio...

2K:4/8/16 = 4K:8/16/32

To run the 2x6L6GC's at their optimal 4KΩ, you'll want to set the switch to 4Ω, but the amp will really be looking for an 8Ω load.  If you don't adjust the switch, you will be handicapping the output and tone of the power amp.

Thanks! That's very helpful!