bumnote Posted December 11, 2014 Share Posted December 11, 2014 The output settings on valve amps seem to be mainly 4 and 8 ohm. What setting should be used if you are using a 6 ohm cabinet? Thanks Quote Link to comment Share on other sites More sharing options...
Dad3353 Posted December 11, 2014 Share Posted December 11, 2014 (edited) Use 8 ohms, as it's better (in general...) to be presenting slightly less impedance. It's a nominal impedance anyway, and not really as critical as all that. What amp/cab..? That might help more than simply the impedance. Edited December 11, 2014 by Dad3353 Quote Link to comment Share on other sites More sharing options...
bumnote Posted December 12, 2014 Author Share Posted December 12, 2014 [quote name='Dad3353' timestamp='1418335424' post='2629392'] Use 8 ohms, as it's better (in general...) to be presenting slightly less impedance. It's a nominal impedance anyway, and not really as critical as all that. What amp/cab..? That might help more than simply the impedance. [/quote] the cab is a pjb 12b and a fender 100t Quote Link to comment Share on other sites More sharing options...
brensabre79 Posted December 12, 2014 Share Posted December 12, 2014 Some amps say "Minimum X Ohms" on the speaker outputs, so as long the cab is higher than X you are fine. Others just say "X ohms" which is a nominal rating - i.e. in that ball park will be OK, impedance change with frequency anyway. I [i]think [/i]SS amps work the opposite way to valve amps though in whether they prefer cabs to be above or below the nominal rating - but don't quote me on that! Quote Link to comment Share on other sites More sharing options...
Dad3353 Posted December 12, 2014 Share Posted December 12, 2014 [quote name='bumnote' timestamp='1418366427' post='2629539'] the cab is a pjb 12b and a fender 100t [/quote] In that case, I'd confirm the propos; Fender o/p trannies are usual happy at up to 100% 'mis-match', ie: 8 Ohm cab on 4 Ohm setting, 2 Ohm cab on 4 Ohm setting etc. 6 Ohms presented to the 8 Ohm setting won't hurt it. That's a great cab and a great amp; it'll sound wonderful, I'm sure. Nice gear. [size=4] [/size] Quote Link to comment Share on other sites More sharing options...
bumnote Posted December 12, 2014 Author Share Posted December 12, 2014 [quote name='Dad3353' timestamp='1418397978' post='2629912'] In that case, I'd confirm the propos; Fender o/p trannies are usual happy at up to 100% 'mis-match', ie: 8 Ohm cab on 4 Ohm setting, 2 Ohm cab on 4 Ohm setting etc. 6 Ohms presented to the 8 Ohm setting won't hurt it. That's a great cab and a great amp; it'll sound wonderful, I'm sure. Nice gear. [/quote] Why thank you sir, will see how it sounds tonight Quote Link to comment Share on other sites More sharing options...
Dad3353 Posted December 12, 2014 Share Posted December 12, 2014 [sharedmedia=core:attachments:167485] Quote Link to comment Share on other sites More sharing options...
bumnote Posted December 13, 2014 Author Share Posted December 13, 2014 The amp was pretty hot at the end of the night. the front panel and the 2 switches in the back So long since I had a valve amp im not sure if this is normal or not. I see there is a fan inside, need to check if it was working Quote Link to comment Share on other sites More sharing options...
Dad3353 Posted December 13, 2014 Share Posted December 13, 2014 I've a vintage Fender Bassman Export 50, which also gives off quite some heat on a sustained night. No fans in these old ones; check if yours is running, and make sure that there's adequate ventilation all around (nothing near the ventilation grills...). As long as it's not up to egg-frying temperatures, just very warm to the touch, I'd say it's OK. Be prudent, though, and check now and again. Quote Link to comment Share on other sites More sharing options...
bumnote Posted December 15, 2014 Author Share Posted December 15, 2014 I pinged Fender an e mail and they said 8 ohms wouldnt hurt but I would be better using 4 ohms Quote Link to comment Share on other sites More sharing options...
Dan Dare Posted December 22, 2014 Share Posted December 22, 2014 Agree with Fender. Valve amps happy to work into a higher impedance, but less so into lower. Impedance of any speaker is far from fixed - it's frequency dependant and varies a lot (typical 8 ohm cab will vary between something like 6-80). Quote Link to comment Share on other sites More sharing options...
Dad3353 Posted December 22, 2014 Share Posted December 22, 2014 (edited) [quote name='Dan Dare' timestamp='1419276109' post='2638297'] Agree with Fender. Valve amps happy to work into a higher impedance, but less so into lower. Impedance of any speaker is far from fixed - it's frequency dependant and varies a lot (typical 8 ohm cab will vary between something like 6-80). [/quote] Personally, I'd maintain the opposite, but if Fender say it's OK, I suppose they should know. As a general rule, valve amps (with O/P transformers...) work the opposite way to SS amps. For SS, too low an impedance (2 Ohms presented to a 4 Ohm amp...) could lead to damage (magic smoke...), but an open circuit (no load, so infinite impedance...) won't hurt it. Valve amps are the opposite. Too low impedance (up to 50%, as a general rule, or even a short circuit...) won't harm it, but an open circuit (no load, infinite impedance...) will generally end swiftly in damage (magic smoke...). In your case, however, I'd follow Fender's recommendation, even though I don't understand their reasoning. Edited December 22, 2014 by Dad3353 Quote Link to comment Share on other sites More sharing options...
gelfin Posted December 22, 2014 Share Posted December 22, 2014 [quote name='Dad3353' timestamp='1419277164' post='2638320'] Personally, I'd maintain the opposite, but if Fender say it's OK, I suppose they should know. As a general rule, valve amps (with O/P transformers...) work the opposite way to SS amps. For SS, too low an impedance (2 Ohms presented to a 4 Ohm amp...) could lead to damage (magic smoke...), but an open circuit (no load, so infinite impedance...) won't hurt it. Valve amps are the opposite. Too low impedance (up to 50%, as a general rule, or even a short circuit...) won't harm it, but an open circuit (no load, infinite impedance...) will generally end swiftly in damage (magic smoke...). In your case, however, I'd follow Fender's recommendation, even though I don't understand their reasoning. [/quote] Agree with Dad on this one. Quote Link to comment Share on other sites More sharing options...
Tdw Posted December 23, 2014 Share Posted December 23, 2014 I think either would be OK, ive generally heard that a lower impedence setting ( 4 rather than 8 in this case) is better but im far from clear about the details of this, however i think that using either would wear out the power valves more quickly, when compared to using a more accurate nominal impedance amp and cab. Quote Link to comment Share on other sites More sharing options...
bumnote Posted December 23, 2014 Author Share Posted December 23, 2014 My Fender 8 ohm 4x10 has now arrived so it shouldnt be an issue any more but thanks for all your input Quote Link to comment Share on other sites More sharing options...
Dan Dare Posted January 3, 2015 Share Posted January 3, 2015 It's the output transformer, rather than the valves, that will usually suffer from open circuit/no load/too low impedance. Quote Link to comment Share on other sites More sharing options...
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