| Author | Message | ||
     jamie danter |
Noticed the output jacks have a fiberboard plate insulator inside the chassis. Also fiber washers under the jack nuts on the outside. Why? The schem for my HD130 shows a ground connection for the sleeve. Internally the main jack sleeve connects to a black wire from the output tranny. Internally the Ext. jack sleeve is connected to pin 8 and 1, on the output tube closest to the driver. Approx 2 ohms above chassis ground. Measured with Fluke 87 III meter. Any comments? | ||
| Steve Kennedy |
Point 1: If you look at the HD-130 schematics, you will notice that the "ground" end of the output transformer connects to the "circuit ground" of the preamp, but is DC isolated from "chassis ground" by a capacitor. This means that Negative Speaker connection isn't directly connected to chassis. Point 2: Also notice that the two speaker jacks are in SERIES, with the sleeve of the first jack connected to the Tip of the second jack. We also don't want a ground in the center of a series connection, so at least 1 jack would have to be isolated from the chassis for this series connection to work properly (even if the DC isolation capacitor wasn't there). Most of the Music Man combo amps have their jacks in parallel (unlike the HD-130 heads) so both of the jacks have their sleeve connected to circuit ground but because they are DC isolated from chassis ground by the capacitor, they too must be insulated from the chassis. The two grounding schemes are ideally tied together at a single point where ground loops can be avoided and chassis eddy-currents and hum from the power supply system can be minimized. Steve | ||
| jamie danter |
Thanks for the concise circuit analysis, Steve. Interesting circuit design. I did notice the symbols for common ground and earth ground. And traced the green wires back to the low voltage filter board behind the pwr xfrm. Just didn't reach the conclusion. | ||
| Matt Sumner |
Hey, all. Just discovered this site and am very glad I did. I have owned my 210HD-One Thirty amp since '85, and I am the second owner. It is the old-style model with the 2275-130 chasis. I use it as a bass amp now and recently started experimenting with different cabs connected to the extension jack in addition to the 2 10's off the main jack. Some mixed and unexpected results lead me to believe I have impedence issues. Here's my question: if it is not explicitly noted on the back of the amp as a "parallel" or "series" connection between the main and ext. speaker jacks, how do I tell which it is? The only writing in this area states "Class II Wiring May Be Used". I pulled the chassis and noticed there is a white wire jumping between the jacks, which leads me to guess these are wired in series. Confirm? Deny? Suggestions? Help! Please! Thank you. Matt | ||
| Steve Kennedy |
Just to recap an e-mail response to this inquiry: The two speaker combos are the units most likely to be wired with Series speaker jacks along with the heads. When 2 8-ohm speakers are used in parallel, the internal speakers exhibit a total impeadance of 4 ohms. This makes series jack wiring mandatory as the amp will not safely tolerate an impedance lower than this long term. Using the internal speakers in series with an external 4 or 8 ohm cabinet is perfectly safe with the amp's output impedance switch set to 8 ohms. Steve | ||
| slej |
Steve Kennedy: I've got a MM HD-130 'head' and speaker enclosures. I once obtained two differing schematics from Ernie Ball/Music Man. On one the speaker jacks seem to be wired in parallel and on the other they are series. How can I tell for sure without going inside?}} | ||
| Steve Kennedy |
1. Make sure the amplifier is turned OFF, AC line cord unplugged and all speakers disconnected from the amp's output jacks. 2. Plug a guitar cable or speaker cable that is open (nothing plugged in at the far end) into the Extension Speaker jack (not the Main Speaker Jack). 3. Then take an ohmmeter or continuity checker and connect one probe to the actual chassis of the amp and the other to the mounting nut on the Main speaker jack. If there is NO continuity, then your jacks are wired in series. If there IS continuity, go to step 4. 4. Remove the open guitar cable plug from the Extension Speaker Jack and plug it into the Main Speaker Jack. Check for continuity between the amp chassis and the mounting nut of the Extension Speaker Jack. If there is NO continuity, then your jacks are wired in series. If there IS continuity then your speaker jacks are wired in parallel. You can only do this test with an open plug or cable plugged into the opposite jack you are testing for continuity to the chassis. Steve | ||
| slej |
Steve: Thanks for the idea. I'm going to try it. Sounds clever; I'll think about it, too. | ||
| slej |
Steve: Which do you think is more likely with respect to my HD-130 'head', series or parallel speaker output jacks? | ||
| Steve Kennedy |
Series wiring is the most likely configuration on all Music Man heads and 2-speaker combos. | ||
| slej |
Steve: Why do you suppose they (EB/MM) provided two differing schematics--one with series jacks and one with parallel jacks? To do the 'test' you described about this, I used a multimeter in the ohms configuration: its first digit flashes when there's no continuity and gives some quantifica-tion when there is. In my application, I was 'getting' about .4 ohms. Does all this seem OK? It seems that mine is in parallel. | ||
| Steve Kennedy |
What you are reading is the DC resistance of the selected secondary winding of the output transformer. The value indicated is not important to the determination of Parallel or Series jack wiring, just the open or continuity (Open/Short) indications are required. The two different schematics are there because the same basic chassis can be used in a head or two-speaker combo (these use series wiring), or in a 410 or single speaker combo (these two use parallel jack wiring). Steve | ||
| dbillups |
Hi all. I once repaired a 112 65, 12ax7 pre, it had series jacks. I wonder if the chassis was swapped? I never thought of single speaker combo's and 410's as the ones with parallel jacks, but it makes sense. I have yet to see a parallel jack unit, I think I have put my hands on less than 10 MM amps. Dick |