More "Impedance" math:

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More "Impedance" math:

Postby Pete B. » Sun Feb 22, 2009 2:06 pm

SarnoMusicSolutions wrote:There was some math bantered about a while back that took a close look at how Jerry used his McIntosh MC2300. As I recall this had to do with how Jerry reportedly (Healy) only used one side of his MC2300, at least for a large period of time, and also mismatched the impedance of his cab and the McIntosh output taps. The McIntosh has an autoformer output so that the power transistors themselves see a constant load. Jerry supposedly connected his 3-JBL cab (approx 2.7-Ohm net) to the 2-ohm taps. The impedance math suggests that this would actually be providing about 225 (or so) watts to his speaker cab. The theory continues to suggest that if he was pushing the McIntosh to where it began to max out and activate its peak limiter, this whole action would be happening with a peak limited max power range in the low/mid 200 watts arena. This would have let Jerry just bang away on that amp setup and it would remain pretty clean and protected, maybe with some limiter dirt.


Hey, I just realized that connecting an 8ohm and a 4ohm speaker in parallel = a 2.7 ohm load.
2x12's can equal 3x12's.
Very interesting.
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Re: More "Impedance" math:

Postby SarnoMusicSolutions » Mon Feb 23, 2009 8:08 am

True dat.

But the 4 ohm speaker will be working twice as hard as it will draw more of the power.


Brad
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Re: More "Impedance" math:

Postby Pete B. » Mon Feb 23, 2009 8:13 am

SarnoMusicSolutions wrote:True dat.
But the 4 ohm speaker will be working twice as hard as it will draw more of the power.
Brad


In my case the 4ohm speaker is a JBL-E120-4!
Yeah Baby!
:cool:
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Re: More "Impedance" math:

Postby Pete B. » Mon Feb 23, 2009 8:20 am

Hey, just another question...
All this stuff about impedance mismatches and damaging your tube amp, etc...
Does any of this matter if you are playing in your basement at a low volume, or if you don't get past 3-4 on your Twin Reverb volume knob at a gig?...
Or is this all just "worst case scenerio" study???... at what power rating does bad stuff start to happen???... just basically idle-ing?... or do you have to crank it to max power to reach failure mode with an impedance mismatch?
Like... "Dude, I'm running my Twin with a 200% impedance mismatch, is that bad for my amp, man?" [re-read as if Chong is saying that statement while passing a spliff].
Last edited by Pete B. on Mon Feb 23, 2009 8:27 am, edited 2 times in total.
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Re: More "Impedance" math:

Postby strumminsix » Mon Feb 23, 2009 8:24 am

Harder working can be a huge benefit depending on the rig...

Consider the tale of my and 68 Dual Showman Reverb.

With it I have 2 cabs:
2x12 at 4ohms with Weber NeoMags (D120f replica)
1x15 at 8ohms with a Weber NeoMag (D130f replica)

Now when both of these plugged in the amp sees 2.7 ohms which is a safe mismatch

However the 2x12 is getting 2/3 of the power if my math is correct.
And that is perfect for my application since it has 2/3 of the speakers!

Regarding impedance and mismatch, I see alot of dudes doing it for convenience.
My recommendation is to know what your result is and then proceed.
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Re: More "Impedance" math:

Postby strumminsix » Mon Feb 23, 2009 8:32 am

Pete B. wrote:Hey, just another question...
All this stuff about impedance mismatches and damaging your tube amp, etc...
Does any of this matter if you are playing in your basement at a low volume, or if you don't get past 3-4 on your Twin Reverb volume knob?...
Or is this all just "worst case scenerio" study???... at what power rating does bad stuff start to happen???... just basically idle-ing?... od do you have to crank it to max power to reach failure mode with an impedance mismatch?
Like... "Dude, I'm running my Twin with a 200% impedance mismatch, is that bad for my amp, man?" [re-read as if Chong is saying that statement while passing a spliff].

Funny! I loved the re-read.

Now when we talk about a 200% mismatch we need to consider a few things:
- does the OT have different taps?
- is it a 200% up or down?
- are you talking about tubes or solid state?

If you have different taps the OT is often less tolerant for mismatches.
200% is a significant mismatch on any OT. Most Fenders are safe 1 up or down. Say it's a 4 ohm tap you are good at 8 or 2.6.
Tubes prefer a mismatch down, SS prefer a mismatch up

It also impacts tone. My ears really prefer an equal out & tap. From there down is kinda opens her up a bit not in a great way and up is just okay.

The classic example is around a hose. and a high impedance mismatch is like putting a garden hose on a fire plug.
the reversal is like trying to get a steady stream by attaching a 100 foot fire hose to your kitchen patio spigot.
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Re: More "Impedance" math:

Postby Pete B. » Mon Feb 23, 2009 8:39 am

I found a broken tube in my '76 VibrOsonic Reverb (A Twin Reverb with an 8-ohm output instead of 4ohm, meant for 1-15" 8 ohm speaker).
I pulled the two inner tubes, and love the tone.
With only two tubes this amp is now a 16 ohm output.
What if I run it at 2.67ohms???... at what point will it fail???.... immediatly upon turning it on?... or after playing it hard for hours at high volume?
Last edited by Pete B. on Mon Feb 23, 2009 8:53 am, edited 1 time in total.
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Re: More "Impedance" math:

Postby strumminsix » Mon Feb 23, 2009 8:48 am

Pete B. wrote:I found a broken tube in my '76 VibrOsonic Reverb (A Twing Reverb with an 8-ohm output instead of 4ohm, meant for 1-15 8 ohm speaker).
I pulled the two inner tubes, and love the tone.
With only two tubes this amp is now a 16 ohm output.
What if I run it at 2.67ohms???... at what point will it fail???.... immiediatly upon turning it on?... or after playing it hard for hours at high volume?


I could be wrong but I do not believe removing 2 tubes will change your impedance.

16 to 2.7 is what, 3 steps down? Bad but I don't think that is your case.

8 to 2.7 is 2 steps down and not recommended.

But let's walk through this some. What essentially happens is that there is a relationshiop between OT & Speaker.
When you have it balanced perfect the signal flowing from your OT to speaker is completely optimal and you'll have a years on the OT.

But when you go too low (2 steps down) what you are going is having the speaker creating a draw for more power than the OT can provide.

This will overwork the OT!

Conversely if you are too high (2 steps up) you are chocking the OT causing it to contain power it cannot deliver out!

From all I've read this is not something instantaneous. It takes time to truly damage your OT. But the worse the mismatch the fast.

(ps, cuz you ro c&C reference my coworker, with a cold, is chopping up some old brown sugar for his oatmeal. All I hear is chop, chop, chop, sniff, sniff, sniff)
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Re: More "Impedance" math:

Postby SarnoMusicSolutions » Mon Feb 23, 2009 10:31 am

Pete,

I like these answers. Match closely whenever possible. To restate one key point: placing a higher load than the OT asks for is NOT a good idea with tubes, but it's fine with SS.


Brad
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Re: More "Impedance" math:

Postby tigerstrat » Mon Feb 23, 2009 11:20 am

Pete are you actually running a 4/8 mismatch 2x12 as a stage rig? Or just 'sperimetting?
"There, in huge black letters, was 'The Grateful Dead'. It just... cancelled my mind out."-Garcia
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Re: More "Impedance" math:

Postby Pete B. » Mon Feb 23, 2009 12:25 pm

tigerstrat wrote:Pete are you actually running a 4/8 mismatch 2x12 as a stage rig? Or just 'sperimetting?


So far i am just having fun mixing and matching stuff at home!
fwiw, i have yet to meet ANYBODY who has injured their own amp due to an impedance mismatch on a Twin style amp, so I'm kinda taking it all with stride.
The thing is, at home, I can't crank it without ear protection, and I can't judge tone with ear protection.
I played out with my Vibrosonic with only two tubes last weekend, and had it at about 5, and it was still plenty loud through an EVM-15L, although I did have too much tube didtortion for my taste.
My main Fender Tube amp tech tells me that when you pull two tubes, the output impedance doubles. That would make my Vibrosonic want to see 16ohm, and the EVM-15L is 8ohm.
I need to get my amp over to the Eurotube guy and get a matched quad. If I can get it back to wanting to see 8ohms per Spec, I think I am gloden to try any mismatch I want. I also need to have it un-modded from BF back to "the most clean headroom possible".
Anyone got that link to the most up to date TR Pre-amp Tap mod (that has independant outputs for both channels)?
~pb
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Re: More "Impedance" math:

Postby strumminsix » Mon Feb 23, 2009 12:43 pm

Pete B. wrote:
tigerstrat wrote:Pete are you actually running a 4/8 mismatch 2x12 as a stage rig? Or just 'sperimetting?


So far i am just having fun mixing and matching stuff at home!
fwiw, i have yet to meet ANYBODY who has injured their own amp due to an impedance mismatch on a Twin style amp, so I'm kinda taking it all with stride.
The thing is, at home, I can't crank it without ear protection, and I can't judge tone with ear protection.
I played out with my Vibrosonic with only two tubes last weekend, and had it at about 5, and it was still plenty loud through an EVM-15L, although I did have too much tube didtortion for my taste.
My main Fender Tube amp tech tells me that when you pull two tubes, the output impedance doubles. That would make my Vibrosonic want to see 16ohm, and the EVM-15L is 8ohm.
I need to get my amp over to the Eurotube guy and get a matched quad. If I can get it back to wanting to see 8ohms per Spec, I think I am gloden to try any mismatch I want. I also need to have it un-modded from BF back to "the most clean headroom possible".
Anyone got that link to the most up to date TR Pre-amp Tap mod (that has independant outputs for both channels)?
~pb


I'd take it all in stride too. And if you are only going up or down 1 mismatch you probably won't experience any issues.
Of if it's short term. I was only giving you the text book prescription for amp safety. Do what you will. I cab experiment too!

I'll be damned... Just did more reading and apparently it does. First I EVER heard of this. Odd.
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Re: More "Impedance" math:

Postby waldo041 » Mon Feb 23, 2009 1:27 pm

Image

Image

there is a .pdf at gearheads also.

peace,
waldo
"Tone is in the instruments. Technique in the hands. Do what you will." ~ quote from some guy at the TGP forum
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Re: More "Impedance" math:

Postby RiverRat » Wed Feb 25, 2009 9:52 am

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Re: More "Impedance" math:

Postby tigerstrat » Wed Feb 25, 2009 11:23 am

That's painful. I still dont get the point of trying to mismatch impedances (and/or dB ratings). What's this supposed to achieve again?
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