Question about impedance.

[László]

I still have to doublecheck all the cab impedance permutations whenever I have to match a cab to an amp. It’s not something that comes naturally to me at all, and that’s after over 50 years of gigging, etc.

We used to do the craziest things connecting amps and cabs, like daisy chaining two amps via their various input jacks, using guitar cables to connect amps to heads, and at some point I connected a Hammond organ output tapped at the speaker connection to the input on my Fender BF Bassman head, and nothing blew up (thankfully!). I actually got sound out of this contraption,

 

[Tucson Thump]

I still have to doublecheck all the cab impedance permutations whenever I have to match a cab to an amp. It’s not something that comes naturally to me at all, and that’s after over 50 years of gigging, etc.

We used to do the craziest things connecting amps and cabs, like daisy chaining two amps via their various input jacks, using guitar cables to connect amps to heads, and at some point I connected a Hammond organ output tapped at the speaker connection to the input on my Fender BF Bassman head, and nothing blew up (thankfully!). I actually got sound out of this contraption,

That what they used to do with the old tweed Bassman and Marshall amps so I thought it was ok, although I had no idea what was happening.

 

[shimmilou]

The fact is, with a tube amp, an impedance mismatch within a 2 to 1 ratio is perfectly safe, always. The reason for impedance matching with a tube amp is simply about getting the most power out of the amp, no other reason. Note that many modern tube amps use shorting output jacks to protect the amp in case no speaker is plugged in. So the impedance can never be too low, as a short circuit is as low as the impedance can get, and that will not damage the amp. Thus, the danger with a tube amp is using too high an impedance, open circuit being worst case, which can and will destroy the OT. If the impedance is mismatched, the amp will not develop the rated output power to begin with, whether high or low. The output of an amp is AC, which is quite different from DC, and therefore AC is not a direct correlation of simple resistance as DC is.

I have been routinely running mismatched impedances for decades (as have many, many others), and never blown an amp because of it.

The myths have grown, as they usually do, but the math and science just doesn’t budge one bit.