215 impedance

Old Fart · #1 10-18-2011, 09:45 AM

Warning: Stoopid, simple question ahead.

What is the usual impedance, of a 2x15 cabinet? I'm looking at a head that won't allow 4 ohms. Only 8 ohm loads. Can a 2x15 cab be efficient and 8 ohms?

anonymous02282011 · #2 10-18-2011, 09:49 AM

The usual impedance of a 2x15 is 4 ohms.

The only way to really know is to open the cab, take note of the impedance of the drivers and how it's all wired up, but if I had to guess I'd say 4 ohms (with each driver having an impedance of 8ohms).

BassmanPaul · #3 10-18-2011, 11:04 AM

You can measure the DC resistance of the cabinet with a digital multimeter. While this is not the actual impedance it should read about 60% of it.

billfitzmaurice · #4 10-18-2011, 11:30 AM

Quote:

Originally Posted by Old Fart

I'm looking at a head that won't allow 4 ohms. Only 8 ohm loads.

Look at a different head. Even if you get an 8 ohm 2x15 you might want to expand or change speakers at some point, an 8 ohm only head is too limiting.

RickenBoogie · #5 10-18-2011, 11:40 AM

I'm curious, what head are we talking about that won't handle 4 ohms?

Alex1984 · #6 10-18-2011, 11:41 AM

+1 to getting a new head. It places too much of a constraint on your rig.

Old Fart · #7 10-18-2011, 12:55 PM

Quote:

Originally Posted by RickenBoogie

I'm curious, what head are we talking about that won't handle 4 ohms?

It's a chopped Fender Studio Bass. 200 tube watts. 8 ohms only. There are 8-ohm 4x10 cabinets, right? Seems to me this amp might be a tone monster and (with an 8-ohm 4x10) and volume beast.

anonymous02282011 · #8 10-18-2011, 12:57 PM

Quote:

Originally Posted by Old Fart

It's a chopped Fender Studio Bass. 200 tube watts. 8 ohms only. There are 8-ohm 4x10 cabinets, right? Seems to me this amp might be a tone monster and (with an 8-ohm 4x10) and volume beast.

or better yet with a fEARful 15/6 tube!

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will33 · #9 10-18-2011, 01:12 PM

Never had a problem running a tube head below it's tap. I've run plenty of old fenders on an upward mismatch as well and they took it fine. It may change the overall tone a little but never burned up any of them.

Unless there's something special about that particular one.

I wouldn't think you'd need to chain another cab running a 215 but yes, most of them are 4 ohms.

Stumbo · #10 10-18-2011, 02:09 PM

Can a 4ohm tap be installed? Maybe John K or one of the tube gurus will chime in.

BassmanPaul · #11 10-18-2011, 02:24 PM

Quote:

Originally Posted by Stumbo

Can a 4ohm tap be installed? Maybe John K or one of the tube gurus will chime in.

Only by changing the output transformer. Sorry, there's no free lunch.

billfitzmaurice · #12 10-18-2011, 03:13 PM

Quote:

Originally Posted by will33

Never had a problem running a tube head below it's tap. I've run plenty of old fenders on an upward mismatch as well and they took it fine.

We used to run Fenders to 1 ohm, not knowing any better. Never bothered them a bit. Tube load ratings are always for the maximum load impedance, not minimum. SS is the other way around.

Old Fart · #13 10-18-2011, 04:09 PM

Apparently, this amp has an unusual configuration. The output transformer is "linear" and works in a different way from ordinary tube amp power sections. There is no pushing or overloading output tubes. Input tube distortion can be achieved with this amp, but the output section will not distort. According to my sources. Schematic (if I upload correctly):

Old Fart · #14 10-18-2011, 06:00 PM

What the heck is the difference between a 16-ohm 15" speaker and an 8-ohm 15" speaker? Something about wrappings or wire thickness in the voice coil, or something. And what is the net effect? One is louder than the other? One requires more power than the other? One can travel less far than the other? One has more moving mass than the other, and therefore is slower? What?

billfitzmaurice · #15 10-18-2011, 06:05 PM

Quote:

Originally Posted by Old Fart

What the heck is the difference between a 16-ohm 15" speaker and an 8-ohm 15" speaker?

The voice coil.

Blue · #16 10-18-2011, 06:06 PM

My guess is you'll be fine .. while 4 ohm is kinda a normal load for 2 x 15 - most amps are OK with that. I have an old Lab Series what say's 8 ohms only on the actual amp - but the brochure says 4 ohms is OK.

I also read somewhere that the original Fender Amps had very wide tolerances on the components - and it was neither unusual nor out-of-warrentee to operate way put of spec.

**B-string** · #17 10-18-2011, 09:35 PM

I doubt you will have any major issues with a 4 ohm load.

rodl2005 · #18 10-19-2011, 03:26 AM

I've run that same amp at 4 ohms about 30 times over about 10 years. A mate has one in his rehearsal space (won't sell it dammit).
Never had a problem.
& as I use it most, I paid for it to go to a tech for a service- as a way of paying 'rent'

T-Bird · #19 10-19-2011, 04:25 AM

Hi.

Try the 4Ohm cab and find out.

I wouldn't though, UL can be picky about the impedance, ask any Marshall Major user.

Regards
Sam

will33 · #20 10-19-2011, 04:52 AM

Quote:

Originally Posted by rodl2005

I've run that same amp at 4 ohms about 30 times over about 10 years. A mate has one in his rehearsal space (won't sell it dammit).
Never had a problem.
& as I use it most, I paid for it to go to a tech for a service- as a way of paying 'rent'

Ya. An ad popped up on craigslist here for a guy selling the cab portion of one that had been chopped so I asked him if he'd be interested in selling the amplifier.....the answer was a big fat NO.