Robben Ford Discussion

Phil T · Posted: Thu Apr 22, 2004 6:55 am Post subject: speaking of speakers

In a recent thread I read some comments about speakers used by Mr. Ford. This reminded me of a comment from Daved a while ago where he states: the speakers are Celestion G12 65 WIRED IN SERIES FOR MAXIMUM IMPEDANCE or words to that effect.
My limited electronics background makes me question whether this wiring configuration could be correct or a misprint.
1) Daved could you comment
2) are the individual speakers usually 16 ohm

thanks
Phil

Aeolian · Posted: Thu Apr 22, 2004 3:19 pm Post subject:

From my understanding, the comparitivaly high source (output) impedance of a tube amp means that a higher load (speaker) inpedance is prefereable. This is in contrast to a solid state amp which is a current source and the lower the load impedance, the more power can be delivered.
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Phil T · Posted: Fri Apr 23, 2004 5:56 am Post subject:

The Dumble amp is rated for a 4 ohm load (this is what is indicated on the rear panel of the amp near the speaker connection).
As I understand it, that should be matched when possible (i.e. 4 ohms total of all speakers connected). With tube amps, the range of mismatch should be kept between half and double, the rating. Anything else could have an adverse effect on the performance of the amp, especially output tubes.
I know that two 8 ohm speakers connected in parallel is 4 ohms, connected in series is 16. Two 16 ohm speakers in parallel is 8 ohms, and connected is series is 32.
That the designer/maker of the amp would approve of the 32 ohm speaker load on the amp surprises me.

Just not sure if I'm understanding this correctly. Hope Daved could clear this up.

Of course it may be that I just mis-read somthing somewhere, or memory acuracy is not what it use to be.
Phil

Aeolian · Posted: Fri Apr 23, 2004 7:29 am Post subject:

To muddy the waters further, remember that 8 ohms or whatever is the "nominal" impedance of the speaker. Being a mechanical device a speaker is a "reactive" load. That is it's resistance at any point is dependent on the frequency and how the mechanical aspects of the speaker and cabinet behave at that frequency. At the resonance point most of the sound level is coming from the resonance and the speaker is restricted to very little motion. This raises the impedance signifficantly. Often more than double. Below the resonance point, the speaker decouples from the spring resistance effect of the air and moves as freely as the cone can, lowering the impedance to a fraction of it's nominal value. This effect is more noticable on hi-fi speakers with their soft compliant suspensions than the realitively stiff guitar speakers. Above the primary resonance point the impedance dips and dives a bit at the various secondary resonance points making for a fairly complex behavior. The ability of the amplifier to control the motion of the speaker, to stop it from going in one direction and pull it the other, is called the damping factor. This is basically the ratio between the impedance of the amplifiers output section and that of the speaker. For any given amplifier, a higher load (speaker) impedance will create better control over the speaker cone. This is most noticable in the bottom end and may help to explain how Robben gets such an incredibly tight and solid low end at such high volumes out of what is essentially a Twin Reverb in a slightly bigger box. I would guess that the natural resonance of Robben's 2-12 is around 125-150 Hz. Low E is almost half that so the impedance of the cabinet could drop by half. If he is running two 8 ohm speakers in series, this would give 16 ohms nominal or back around 8 on the bottom end. This would still let the amp be happy, keeping the damping factor reasonable and not drawing too much current though the output transformer which causes it to overheat and do all sorts of other nasty things.

To confuse things further, here's a picture of my 2-12 cab which I built to resemble the Dumble. The speakers are Celestion 65's but they are "15" ohms each!

And the front, for what it's worth. Like the Dumble, and unlike a lot of other cabinets, this is a floating baffle. That is, the baffle is only mounted along two edges. Like a Fender combo amp.

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kirk95 · Posted: Fri Apr 23, 2004 7:52 am Post subject:

What's the amp head to the right just out of the photo...

Sweet cab. Where did you track down the 65's?
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renico00 · Member Joined: 17 Jul 2003 Posts: 32

It's a Fuchs mod.

Jeff

Phil T · Posted: Fri Apr 23, 2004 6:07 pm Post subject:

Aeolian,
Wow thanks man, for the comments and picts. (my head will stop spinning soon :-))

I guess the reason I bring all this up is, I was contemplating .... if I have an amp rated for 4 ohms ... if I could find a couple of G12-65s ... what woulld be the best way to go? ... do I need 15/16 ohm speakers/ ... do I need to find 8 ohm ones?

I seem to recall when I exceeded the amp rating in the past, it sounded not as loud, not as bassy and less 'head room"/clean power.

My current major concern is, if I find a G12-65 or two that is 16 ohm, could I use it?
Phil

Daved · Posted: Fri Apr 23, 2004 9:12 pm Post subject:

When I posted about maximum impedance, we were going thru an experimental period of different speakers and their configurations.

We have since gone back to matching our speakers to the rated impedance of the amp. The Dumble is 4 ohm, so I use two 8 ohm speakers wired in parallel to match to the 4 ohms (two 8 ohms in series gives you 16 ohms).

Using speaker impedances greater than what the amp is rated for tends to, for lack of a better term (I'm not sure how you would describe it in electronic 'langauge'), load down the amp and soften the sound some. We've gone back to allowing the amp to operate as it was designed instead of trying to force a specific sound with speaker manipulation.

The key thing to always remember when experimenting with impedance matching is that speaker impedance should NEVER be less than what the amp is rated for (if the amp is 16 ohm, don't configure your speakers to 8 or 4 or 2 ohms.... if the amp is rated at 8 ohms, don't configure your speakers to 4 or 2 ohms... etc.). Speaker impedance at LESS then what your amp is designed for can seriously damage the amp.
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tonemaster2_11 · Member Joined: 07 Dec 2003 Posts: 37

YEAH, WHAT HE SAID.

Aeolian- Dude! Good Answer!!

What Daved said is also what I have found to be the case. I have a Fender Dual Professional rated (like a twin) for 4 ohms and it sounds best and has the most headroom when running into a 4 ohm load. There's no shortage of tightness and at times it can be a little "too present" when you are just wanting to bang out some clean rythmn and would like it to "give" a little.
I hope to try the Weber knockoffs of the G12-65's since I don't particularly like the way the Vintage 30's I have handle the highs. Can't spend the money right now though or I'll be posting on the "how I lost my house" message board.

Aeolian · Posted: Sat Apr 24, 2004 8:24 pm Post subject:

scottl · Posted: Sun Apr 25, 2004 5:55 am Post subject:

Aeolian,

Nice sounding amp you have there!!! I had the chance to sample it during burn in, while at Andy's shop a couple of weeks ago.

Does it have the half-power pull switch on the reverb?? Also, you could have Andy add the triode/pentode switch to further reduce the power.

If you use 50w mode, you need to use the 4 ohm tap (8 ohm load still)

Scott
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davidS · Member Joined: 17 Feb 2004 Posts: 34 Location: Dallas

Scott,

I have a Fuchs/Traynor 50 watt that I use with G12-65s wired parallel to 8ohms. When I run in the cathode bias setting (approx 35 watts) should I use the 4 ohm tap or just when I use the triode setting (approx 25 watts)?

David

scottl · Posted: Mon Apr 26, 2004 4:11 pm Post subject:

Dave,

It is my understanding that you only change taps when running less tubes. On my 100, I use the 4 ohm tap when in 1/2 power mode (two tubes) and not in cathode or triode mode.

Email Andy to confirm, please...

Scott
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juspasinby2003 · Senior Member Joined: 18 Dec 2003 Posts: 255

scottl · Posted: Sat May 22, 2004 4:57 pm Post subject:

No need to confirm. I am correct. Leave the 8 ohm cab connected to 8 ohm tap when running cathode or triode mode. You only run into the 4 ohm tap if you have a 4 ohm cab OR an ODS100 in 50 watt mode.

Hope I helped!

Scott
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