GNF resistor value?

[Tdale]

I put in a 4.7K resistor and connected it to the 4 ohm tap.

What happens If I need to plug the amp into a 8 ohm cabinet..or even a 2 ohm or 16 ohm? Will it have to run without the GNF, or do I have to open it and re-solder it to the right OT secondary?

Also, can I use the same resistor value for all secondary values (2, 4, 8 or 16 ohm?)

Tommy

[Bob-I]

I put in a 4.7K resistor and connected it to the 4 ohm tap.

What really matters is the ratio of the NFB and tail resistors.

What happens If I need to plug the amp into a 8 ohm cabinet..or even a 2 ohm or 16 ohm? Will it have to run without the GNF, or do I have to open it and re-solder it to the right OT secondary?

No, leave it on the 4 ohm tap. You'll have NFB when using all the taps.

Also, can I use the same resistor value for all secondary values (2, 4, 8 or 16 ohm?)

Tommy

You can, but it'll change the amount of gain reduction.

I'm not a guru on this topic, search the Dumble forum, there was a good discussion on this topic awhile ago there.

[mhuss]

Actually, as long as the OT has multiple output taps (as it typical for all but the weirdo Hammonds and the really old RadioSpares), just leave the NFB where it is.

The two things that matter are the ratio (as Bob mentioned) and also the voltage coming in -- the 16 ohm tap will produce a higher voltage at e.g. 10 watts than the 8 ohm one will. As long as you leave the NFB on the same tap, the voltage produced for a given output level will stay the same, no matter which tap you have a speaker connected to.

Note that all the English amps with output impedance switches (from the early Marshalls onward) work this way,

If you're using a Hammond OT with dual secondaries, it becomes much more difficult, unless you use the "shortcut" two way switch that KOC suggests.

--mark h