Using a valve amp with a 6 ohm cabinet

[bumnote]

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

[Dad3353]

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

[bumnote]

[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

[brensabre79]

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!

[Dad3353]

[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]

[bumnote]

[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

[Dad3353]

[sharedmedia=core:attachments:167485]

[bumnote]

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

[Dad3353]

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.

[bumnote]

I pinged Fender an e mail and they said 8 ohms wouldnt hurt but I would be better using 4 ohms

[Dan Dare]

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).

[Dad3353]

[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

[gelfin]

[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.

[Tdw]

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.

[bumnote]

My Fender 8 ohm 4x10 has now arrived so it shouldnt be an issue any more but thanks for all your input

[Dan Dare]

It's the output transformer, rather than the valves, that will usually suffer from open circuit/no load/too low impedance.