| Author | Message | ||
     Rich Johnson |
I have a MM "65" head that I am going to build a couple of 1x12 cabinets to use with it. (Different speaker in each cab) What I am wondering about, is the output power at 4 or 8 ohms. Is it 65 watts @ 4 ohms and something less @ 8 ohms or is it 65 watts @ either setting. I am going to build (2) 8 ohm cabs and I am trying to figure out what each speaker should be rated for. Sometimes I might use both cabs w/ the MM set @ 4 ohms or one cab w/ the MM set @ 8 ohms. Thanks in advance for your help. By the way, I have had this MM head since '89 and haven't used it since around '92. I just had it retubed and biased. I'm looking forward to playing with the thing. I had no idea of the history of MM amps until I dug mine out of a closet and started researching it and stumbled across this site. Thanks to all of you for all of the info. I bet a few of you guys would like to have this thing. Practically looks brand new, just a hint of rust on the corners. I guess that is pretty common from reading many of these posts. Thanks again. | ||
| Rich Johnson |
Hello....is this thing on? A little help please. ;) | ||
| Anonymous |
Read Need Help On Impedance For 115-65 on this thread. | ||
| Steve Kennedy |
Tube amps use an output transformer to couple the amp to the speakers. When the proper impedance output transformer secondary tap is selected the amp will put out the rated power. A tube amp of 65 watts will put out 65 watts into 8 ohms (with an 8-ohm tap selection) and 65 watts into 4 ohms (assuming a 4-ohm tap selection). This not the same situation as on a solid-state direct-coupled amplifier design (where the power output depends on the speaker impedance). The amp will attempt to output more power if the speaker impedance is lower than the selected transformer tap, but this places additional stresses on the output tubes and transformer windings, and generates extra heat and this all adds up to an earlier death and lifespan degradation due to impedance mismatches at high power. I personally would rate the speakers at at least 100 watts, but the rule of thumb is 2X the maximum power they will see to make sure they run cool enough to survive long-term. | ||
| 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. Steve | ||
| Rich |
Thanks people. I'm certainly not a tech. I wish I knew more about amps in general. I already understood series/parallel basically. I knew to get a 4ohm speaker load, I would need to use two 8ohm speakers wired parallel. I was just unsure about the output power of the amp at 4 or 8ohm switch setting. I didn't know output was the same. I assumed it was like my Hartke bass amp, 350 watts @4ohm load and 240 watts @8ohms. I'm glad I asked. Thanks very much. BTW, I ordered a couple of Eminence speakers from Avatar Speakers. (both 8ohms, rated for 120watts) | ||
| Steve Kennedy |
Avatar is awesome! Dave bends over backwards to help and support customers. I own both 210 and 115 Avatar bass cabinets as well as a stereo 212 guitar cabinet, all with Eminence Vintage (guitar) or Delta (bass) drivers and they are awesome sounding cabs and are relatively lightweight! Highly Recommended! | ||
| Rich |
Yes, Dave was really helpful and has great prices. It was cheaper to order from Avatar and get the speakers shipped across the country than any other source I could find in the eastern U.S. I looked at the Avatar 1X12 cabs. I read some reviews on Harmony Central, which were all complimentary. My only problem with them is that I wanted cabs that were dimensionally similar to my "65" MM head. The Avatar 1x12 cabs are considerably less wide and they don't do custom sizes per Dave. The price was great on them, I just couldn't get past the thought of the head overhanging the cabs by a couple of inches on each end. I might be anal. ;) I have recently finished building my cabs out of pine. One closed back and one mostly open. I still need to get tolex,grill cloth,feet,handles,corners and recessed jack plates. I'll get that stuff probably from parts express.com. I think I have made a mistake though. I read in a post here that it would be best for each cab to plug into seperate speaker jacks.(???) The speaker jacks on the MM head are wired in series. Being that I bought 8ohm speakers, this would give me a 16ohm total load if using both of the amps output jacks. A 100% mismatch, which I believe I read acceptible? Being that, stated in Steve's earlier post, a tube amp is happiest with matching impedance load/switch setting, I guess I should just put a parallel out jack in one of the cabs and just use one of the MM output jacks with the switch set to 4ohms. What does it hurt to use only one speaker jack? Which is best, daisy chaining the cabs or the 100% mismatch? Does it matter? Thanks in advance. | ||
| Steve Kennedy |
The ONLY thing that really matters to the amp is the TOTAL output impedance. The amp would be happier with a 4-ohm load and the impedance switch set to the 4-ohm position. | ||
| mburduck |
Hi, Guys, So it would NOT be good for me to run my MM HD One Thirty with an 8-ohm MM 115 RH cabinet AND a 4-ohm MM118RH cabinet (it's loaded with a 4-ohm JBL 18) together, do I have that right? You'd think after 34 years of playing bass, I'd know more about impedance!!! Thanks! Mike | ||
| Steve Kennedy |
Since most tube amps (and MM amps in particular) can run with as high as 100% impedance mismatch, you should be OK IF your head has its speaker jacks wired in Series. The the two cabinets together would be 12 ohms in series (a 50% mismatch which should be fine) and as long as you had the switch in the 8 ohm position there should be no problem. If your speaker jacks are wired in parallel then I would be concerned because the total impedance falls below 4 ohms. Check the labeling on your jacks. If they are in series then no problem! | ||
| mburduck |
Thanks a bunch, Steve! All the best, Mike | ||
| michael murphy |
a few questions ,the speakers in my m/m 210 75 have a m/m logo on theam ,can you tell me what kind of speakers these are and second i have a fendar vibrolux it runs on 4 ohms,the m/m has me a little confused t/y mike | ||
| Steve Kennedy |
Like most of the 2-speaker combo models, the 210 series typically has 2 8-ohm speakers wired in parallel for a 4-ohm total load and the amp should be set at the 4-ohm position when operated this way. When using a 4-ohm or 8-ohm external cabinet in addition to your internal speakers, set the switch to 8-ohms (the two speaker loads are wired in series inside the amp). I am not sure who made your 10" drivers... I believe the earlier 210 and 410 models had CTS/Utah drivers and the later models (like yours) may have had Eminence drivers. Steve | ||
| dennis v |
I have a MM 130 212 HD should the switch be set to 4 ohm or 8 ohm? | ||
| Steve Kennedy |
If your speakers are the factory standard units or exact replacements (8-ohms) and wired the same way as the factory did (in parallel), then the total impedance of the internal speakers is 4 ohms and that is where the amp should be set to. Steve | ||
| Glenn Shirey |
I do believe that this was covered below, but I wanna be sure (don't want anything bad to happen to my amp). I have an HD-130 with 2 12's in it. I want to add an extension cabinet with 2 additional 12's that I pulled out of a Wulitzer organ. The 2 wurlitzer speakers are 8 ohms (so I believe), so if I wire them parallel for a 4 ohm load I can plug this cabinet into the HD-130 after I flip the impedence switch to 8ohms? | ||
| carl |
yep 2 X 4 ohm load = 8 ohm hope the speakers sound rad | ||
| Steve Kennedy |
I wonder if the Wurlitzer speakers were made by Jensen? They could sound pretty good! It sounds like an 8-ohm total load on a 212 amp to me too! Just make sure that it says your jacks are wired in SERIES on the back to confirm that the chassis in your amp conforms to the normal situation for a 212 model. Steve | ||
| Doug Miers (doug_m) Username: doug_m Registered: 05-2006 |
I hate asking dumb questions, but I'm turned around with all of what I've read here about how MM are wired with the output jacks in series instead of parallel. I have a 112RP-100 with a single 8 ohm EV speaker. Recently I've started adding an extension cabinet which also has a single 8 ohm EV speaker. My years of playing Fender amps tells me I ought to set the impedience switch to 4 ohms and the amp will be happy. But, if the jacks are wired in series, then I could set the switch on 8 ohms and be fine OR set it at 4 ohms and the amp would see the load as 16 ohms; and again the amp should be happy. BUT, tonight I replaced an almost fresh pair of Phillips 7581As because one of the tubes was burned out and dark. These tubes had less than a 100 hrs on them. SO.... let me ask the brain trust; Should I set my amp on 8 or 4 ohms. Thanks. | ||
| Mel Hadden (merkle) Username: merkle Registered: 01-2007 |
From the schematics, the RP-100 output jacks are wired in parallel. Using two 8 ohm cabinets, set the impedence switch to 4 ohms. The 7581a's are not what are specified for the MM RP-100. 6CA7 or EL34L tubes should be used instead. Unless your amp was setup to use the 7581A tubes, they will burn out fairly quick. | ||
| Doug Miers (doug_m) Username: doug_m Registered: 05-2006 |
I appreciate your writing back. According to the tubechart in the back of the cabinet, my amp should use 6L6GC power tubes. (It's 2100RP) The 7581A tubes are the military spec 6L6s. You're right though, setting the amp on 4 ohms is the right thing to do. I felt kind of dumb after asking the question on the forum, I noticed that it says right on the back of the chassis that the output jacks are wired in parallel. Thanks again. |