ampage Tube Amps / Music Electronics	For current discussions, please visit Music Electronics Forum.

ampage archive

Vintage threads from the first ten years

previous: BWilliamson As my limited mind tells me. You c... -- 2/11/2000 10:04 PM

Re: Two omhs on BF Bassman?

2/11/2000 11:56 PM
SteveR	Re: Two omhs on BF Bassman? Per RGs FAQ (http://www.geofex.com/tubeampfaq/taffram.htm)- You should be able to double or halve the expected impedance without serious or damaging consequences. However if other factors are not at nominal or "design center" values (like bias being way off) then an impedance mismatch might pose some problems. Also remember that the load the power tubes see is a ratio up from the speaker impedance. (the turns ratio of the transfomer, may be in the neighborhood of 1000:1)So that some resistance(impedance) on the secondary may be quit a bit on the primary. So only a real severe mismatch (or, a short!) will cause the primary to look like a short. However, too low a load is still not really a good thing. If you look at the ratings of the output tubes you'll see that there are operating characteristics designated for a number of different loads, the lowest being just over roughly half of the highest. Depending on how your amp was designed, the direction of the mismatch might be important. If the amp was designed right at the high end of the range increasing the load might be worse than too low a load. But everything I just said could be wrong, so here's my rule of thumb: If a 2:1 mismatch up or down will allow you to finish the gig or not cancel band practice, fine, but I wouldn't make it part of my standard setup. Rewire your cabinet, add or remove speakers, or even remove tubes (if a four tube setup) to establish the proper load. SteveR

2/11/2000 11:56 PM

SteveR

Re: Two omhs on BF Bassman?

Per RGs FAQ (http://www.geofex.com/tubeampfaq/taffram.htm)- You should be able to double or halve the expected impedance without serious or damaging consequences. However if other factors are not at nominal or "design center" values (like bias being way off) then an impedance mismatch might pose some problems.

Also remember that the load the power tubes see is a ratio up from the speaker impedance. (the turns ratio of the transfomer, may be in the neighborhood of 1000:1)So that *some* resistance(impedance) on the secondary may be quit a bit on the primary. So only a real severe mismatch (or, a short!) will cause the primary to look like a short. However, too low a load is still not really a good thing. If you look at the ratings of the output tubes you'll see that there are operating characteristics designated for a number of different loads, the lowest being just over roughly half of the highest. Depending on how your amp was designed, the direction of the mismatch might be important. If the amp was designed right at the high end of the range increasing the load might be worse than too low a load.

But everything I just said could be wrong, so here's my rule of thumb: If a 2:1 mismatch up or down will allow you to finish the gig or not cancel band practice, fine, but I wouldn't make it part of my standard setup. Rewire your cabinet, add or remove speakers, or even remove tubes (if a four tube setup) to establish the proper load.

SteveR