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Need help on impedance match for 115-65


Dave R
Posted on Sunday, May 19, 2002 - 05:52 pm:   

I have a stock '76 115-65, and am not an electronics guy. I'm not sure what my speaker
impediance switch should be set to, 4 or 8 ohms. I have a multimeter that measures ohms, and get a reading of about .5 ohms across the the two terminals on the speaker. When measured across the tip and sleeve of the input plug coming off the speaker, I get about 6.1 ohms. Both these measurements jump around quite a bit before seeming to settle at the approximate values stated above.

I've used this amp for a while at low settings with the switch set (as I got it) in the 4 ohm
position and to date haven't had any problems, but does this sound ok? I know ohms and impediance are technically different but also used interchangably sometimes. In case this matters, it says under the two speaker input jacks that they are wired in series. I only use the onboard speaker pluged into the the input jack on the left.

Thanks for any help or advice anyone can provide.
Steve Kennedy
Posted on Friday, July 05, 2002 - 12:48 pm:   

Speaker impedance is only partially made up of resistance, which is what you are measuring with your meter.

An 8 ohm speaker measures about 6 ohms or so. A 4 ohm speaker measures 2.8 - 3 ohms.

The measurement at the speaker terminals should be the same as at the end of the cable IF it is unplugged from the amp! The .5 ohms you saw was the resistance of the secondary winding of the output transformer in parallel with the speaker, not the speaker itself.

With an 8 ohm load, either position of the impedance switch will work and no damage should occur. The 8-ohm setting would be more efficient because of the obvious match.

Of course, things can change dramatically when attempting to use an external cabinet. Since your amp has the speaker jacks wired in series, you can use a 4 or 8 ohm extrnal cabinet and keep the switch in the 8 ohm position.

Amps with tube output stages can typically tolerate a 100% mismatch (keeping above 4 ohms total), which means as high as 16 ohms would be acceptable in the 8-ohm position and 8 ohms in the 4 ohm position.


Dave R
Posted on Tuesday, July 16, 2002 - 09:57 pm:   

Thanks Steve.
Rory Dardich
Posted on Friday, August 23, 2002 - 08:23 am:   

First, is there anyplace on the web I can go to understand ohms/impedance so I don't waste your time with that part of the question?

Second, I just bought a 212HD 130 (chassis 2475) & it has the ohm switch on the back panel (as all do, I guess). Is that just for dealing with extension cabinets or is it also for a "power cutting" feature I've read about in owner reviews?
Steve Kennedy
Posted on Friday, August 23, 2002 - 11:24 am:   

Speaker Impedance & Connection--------------------

There are probably quite a few places you can find information on speakers, speaker wiring and Ohms Law. No particular site springs to mind but searching for these terms should produce more links than you can shake a stick at! Try speaker manufacturers, especially the smaller ones (like Weber).

The Impedance switch on the rear of a tube amp is for matching the load (speaker) impedance you are using (the total of both internal & external loads) to the windings of the output transformer for the most efficient power transfer and least amount of excess heat and energy loss.

Be careful when calculating total impedance. Most of the Music Man combo amp models (single speaker and 4 speaker) have their speaker jacks wired in Parallel. The 2 speaker combos (like your 212) and the heads typically have these jacks wired in Series! (It should be listed on the rear panel somewhere near the jacks on most Music Man amps if yours are in series.)

This is important to know so you can calculate the proper total impedance and know where to set the switch (or even if you CAN safely connect the two speaker loads together).

If you aren't sure you can check this from the outside without disassembling the amp. UNPLUG THE AMP POWER CORD FIRST! Plug an "open" 1/4" plug or an open-ended guitar cord into the extension (external) speaker jack and use an ohmmeter to check for "0" ohms (or continuity) between the body of the two speaker jacks. If there is a direct short between the jack bodies, then the jacks are wired in parallel. If there is NO connection between the two jack bodies this would indicate a series wiring.

Calculating Total Impedance ----------------------

Speakers in series add the impedance together:

4 ohms + 4 ohms = 8 ohms total (Series)

In parallel, it is a bit more complex but not overly so. The formula is "product over the sum", meaning you multiply the two loads (the result is the mathematical "product") and divide this by their sum (adding the two loads together).

When dealing with 2 identical load impedances (i.e. 8 ohms and 8 ohms) the total in parallel is exactly 1/2 of ONE of the loads (in this example, 1/2 of 8 ohms = 4 ohms).

If the loads are different, use "product over the sum". If dealing with an internal 8 ohm load and connecting a 4 ohm external cabinet in parallel:

8 x 4 = 32
8 + 4 = 12

32/12 (32 divided by 12) = 2.666 ohms

This load combination would be too low on a Music Man amp (never go below 4 ohms if you can avoid it) with parallel wired jacks. However, if your amp has these jacks wired in series, the the sum of 8 + 4 = 12 ohms total. This would be perfectly safe to operate in the 8 ohm position since it is only a 50% mismatch to the switch setting (only 4 ohms higher than the 8 ohm optimal impedance).

Speakers wired in series have a total impedance that is ALWAYS higher than the highest impedance speaker you are using.

Speakers wired in parallel have a total impedance that is ALWAYS lower than the lowest impedance speaker being used.

Hi/Lo Power Switch -------------------------------

"Power Cutting" (on purpose) is the function of the Hi/Lo power selection switch NOT the speaker impedance switch. This power selection switch changes the output tube plate voltage to a lower value (low power) which not only cuts output power in half, it also lowers the stress on the output tubes and output transformer and reduces operating temperature.

A tube amp is happiest when the speaker load impedance matches the setting of the impedance switch on the rear of the amp. A mismatch generates more heat in the output transformer, can make the tubes run harder (elevating operating temperature and lowering their lifespan) and places the amp closer to the point of tube and output transformer failure under heavy loads.


Posted on Thursday, July 22, 2004 - 03:58 pm:   

I have an old (70's) Music Man 112/65, with the stock 12'' speaker. Does anyone know if the impedance should be set to 4 or 8? I know nothing about electronics and have no testing equipment. Thank you very much.
Posted on Thursday, August 05, 2004 - 04:58 pm:   

I own a 112-65 I purchased new in 1974 as soon as it hit the store. The owners manual stated that the purpose of the high - low power switch was to lower the voltage to the tubes(low power setting) causing the tubes to overload easier to produce distortion . Better tube life and volume cuting were not the reason for ths feature.
Posted on Friday, August 06, 2004 - 08:17 am:   

I think you are referring to the above posting dated August 23, 2002 regarding the Hi-Low Power Switch.

You can bet that anything stated by Steve K. regarding any aspect of MM amps is right, correct, on the money, well meant, etc...

Considering that lack of distortion ability was one of MM's product shortcomings, even in the mid 70's market, it's no wonder the 'improved distortion' statement made it to press. 'Volume cutting' would not have been a wise marketing ploy at that time, either.

Steve Kennedy
Posted on Monday, August 09, 2004 - 02:08 pm:   

I have played through my Music Man amps on hundreds of occasions in a live environment over the last 25 years. While it is true that you CAN overload the output tubes easier on Low power, it is not the "Distortion" switch the Music Man press releases may imply.

Personally, I never found "easier distortion" to be the BEST reason for having the switch... lower stress on the output stage which leads to increased longevity and reliability is more than enough reason to include this feature all by itself IMHO!

In practice, by the time you reach output stage distortion you can throw the switch to the High position and the distortion all but disappears due to the higher headroom gained through increasing the plate voltage of the output tubes. If you increase the drive to the output stage by increasing the input gain/volume you will quickly return to distorted output stage at a marginally higher volume level than before but at twice the stress on the components.

Twice the plate voltage may equate to about twice the watts, but twice the watts does NOT equate to twice the volume. It takes a 10X increase in watts to achieve and honest 2X increase in perceived volume. Such is the logarithmic sensitivity curve of the human ear.

So, if I increase the stress on my amplifier by 100% in order to achieve that last 10% in volume is it really worth doing all the time? Even live in small to medium-sized clubs, 40-60 watts is really all that is necessary and a 100+ watt amp isn't that much louder. You'd be better off in most cases to stay in low power and use a second cabinet to increase dispersion.

Of course, some people prefer the tone they get with the amp in the high power setting. There are so many variables to this equation that there is no one right way to do any of this. However, I personally will lean towards the direction of increased reliability everything else being equal.



Posted on Saturday, April 22, 2006 - 06:28 pm:   

I have had my music man for 32 years and except for one or two times always ran it on low power. I am still on my original tubes ! They have hundreds if not thousands of hours on them. I had the amp serviced for the first time a few years ago. Had a cap jod and new tubes GT6CA7 . But after one of them burned out quickly I put the old Sylvania tubes back in and they sounded great.Though slighly microphonic they are still in now. So low power does have its benefits. My amp has never had a break down or even blown a fuse come to think of it.
mike kaus
Posted on Sunday, April 23, 2006 - 04:53 am:   

Those old sylvania tubes WERE amazing. I have several sets of them for backups(though I don't know why). The GT tubes will not(in MY opinion) handle the high(700v) voltage of a MM. Try JJ's(my favorite).
Mike Kaus (mm210)
Username: mm210

Registered: 05-2006
Posted on Sunday, June 11, 2006 - 08:34 am:   

twink removal
Edward Solberg (edward_solberg)
Username: edward_solberg

Registered: 05-2006
Posted on Thursday, September 07, 2006 - 07:33 pm:   

remove all twinks; the sooner the better. had I the power I would scourge them from this temple.
of course, that's just my opinion
Mark Lewis (skidmark)
Username: skidmark

Registered: 01-2008
Posted on Friday, January 18, 2008 - 06:12 am:   

I have an early mm sixtyfive head. I matched it with a 4x12 GS cabinet (8ohms). I am adding a 1x15 lopo line cabinet (8ohms). Instead of daisy chaining the cabinets...isn't is best to leave the amp on 8 ohms and use the main speaker out to run the 4x12 GS and the extension speaker out for the 1x15?
Mike Kaus (mm210)
Username: mm210

Registered: 05-2006
Posted on Saturday, January 19, 2008 - 10:25 am:   

If your head is an early one, like with the PI tube, the outputs SHOULD be wired in series so that would gove you a 16 ohm load which would effectively cut your power. Daisy chain(parallel) them and put the switch on 4 ohms. Mike.
Mark Lewis (skidmark)
Username: skidmark

Registered: 01-2008
Posted on Saturday, January 19, 2008 - 12:12 pm:   

Thanks Mike. It is the PI tube version.
Mike Kaus (mm210)
Username: mm210

Registered: 05-2006
Posted on Sunday, January 20, 2008 - 04:01 am:   

Make sure by looking on the back at the little note next to the output jack. There should be a note that say they are in series. Then the advice I gave you is correct.
Mark Lewis (skidmark)
Username: skidmark

Registered: 01-2008
Posted on Monday, January 21, 2008 - 06:08 am:   

It doesn't say anything on the back of the amp in regards to series or parallel. Terry Loose is doing a cap job, tubes, and replacing the old power cord on it right now. He e-mailed me and said there is an extra jack. Apparently it is some type of mod. It all looked factory to me. He thinks it might be some type on line in. I e-mailed him and asked about the jacks, series or parallel. I'm sure he'll check it out. I haven't purchased the 15' speaker yet. Would there be a better ohm value to look for in the 15' ??
Mike Kaus (mm210)
Username: mm210

Registered: 05-2006
Posted on Tuesday, January 22, 2008 - 04:27 am:   

If you are going to use the 4x12, the 8 ohm will be fine UNLESS whoever(whomever?) modded it changed the configuration of the external jacks. If he put them in parallel, then you can still use the same cabs a,d just plug into the ext jack with the 1x15 and the main jack for the 4x12. Still using the 4 ohm setting.

Speaker jack impedance on a 210-65

Holy moly, paragraph after paragraph with tons of technical info, but I don't want to become an electrical engineer - I'd love to just get a "Yes" or "No" answer: Do need to switch to 4 Ohms to connect an 8 ohm external cabinet to run simultaneously with the onboard speakers when the jacks are connected in series?

One word will fix me up. Thanks.

mm210's picture

One word. NO! See other

One word. NO! See other post.

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