#1
I have a 6505 head. I pulled two tubes, so it's now a 60-Watt head. I just moved the head into my office where I have a Marshall MC212 stereo 2x12 cab (instead of the mono 4x12 cab I have in the living room). The two speaker jacks on the MC212 are labeled LEFT 8Ω / RIGHT 8Ω. But the right jack is also labeled, above the jack, MONO INPUT 4Ω.

Because I pulled two of the tubes, I have to set my amp to half the actual impedence value of the cabinet I'm using. I initially had one speaker cable into the right jack, the "MONO INPUT," and the amp set only to 4Ω. If the MONO INPUT is really only a load of 4Ω, then I should've been feeding it only a 2Ω output (which of course my 6505 doesn't have). I've been amp this way for a few days.

Realizing that 4Ω is not half of 4Ω, I just switched to using both of my 6505's parallel speaker outputs, and now use two speaker cables, each going to the LEFT and RIGHT inputs on the cabinet, to attain an 8Ω load (the amp is still set at 4Ω, assuming an 8Ω load). Is this now an 8Ω impedence load on the amp, and is this the correct way to connect it to this particular cab? Did I potentially damage the output transformer by running it the way I was previously?
.
American HM Strat | LP Studio
Soldano Avenger w/DeYoung OT | Mark IV rackmount | DC-3 rackmount | Single-Recto

.
Last edited by LEVEL4 at Mar 23, 2008,
#2
Here's a really quick question: why did you pull two tubes?
I have always heard that that is NOT a good idea, as all power will be dumped on the two tubes.
Unless an amp has a half power switch, and that turns off half of the tubes (which it rarely does) what you've done causes a lot of problems.

Quote by emad
jthm_guitarist
Warned for trolling!


Quote by metal4eva_22
Didn't you say that you had a stuffed fox that you would occasionally fuck?

Quote by Axelfox
It's not a fox,it's a wolf.
#3
those are parallel speaker outputs. Even if each jack sees 8ohm, it's still a 4ohm load the amp is seeing because they are in parallel. You can potentially damage it yes, because you are running a mismatch it was not designed for. If the sound hasn't changed, and it's not blowing fuses, it's probably fine. However, an output transformer can arc inside and still work at lower levels. Once it's pushed is when you notice the damage.

an easy fix would be to break open your cab, and wire the speakers in series instead of parallel. That or only use one speaker.
"The fool doth think he is wise, but the wiseman knows himself to be a fool." - W.S.
amp clips
amp vids
Last edited by Erock503 at Mar 23, 2008,
#4
Quote by jthm_guitarist
Here's a really quick question: why did you pull two tubes?
I have always heard that that is NOT a good idea, as all power will be dumped on the two tubes. Unless an amp has a half power switch, and that turns off half of the tubes (which it rarely does) what you've done causes a lot of problems.
It's fine. It's done all the time without doing any harm to the amp. Ask Jerry at FJAmods, or any amp technician.

Quote by Erock503
those are parallel speaker outputs. Even if each jack sees 8ohm, it's still a 4ohm load the amp is seeing because they are in parallel. You can potentially damage it yes, because you are running a mismatch it was not designed for. If the sound hasn't changed, and it's not blowing fuses, it's probably fine. However, an output transformer can arc inside and still work at lower levels. Once it's pushed is when you notice the damage.

an easy fix would be to break open your cab, and wire the speakers in series instead of parallel. That or only use one speaker.
[Edit: See posts below.]

I still don't get it. Each INPUT (left and right) on the cab says it's 8 Ohms. The specs of the cabinet are: "stereo 8Ω, mono 4Ω." Yes, the amp's outputs are parallel, but I don't know if the 2x12 is in series or parallel. So if I send both of the amp's parallel outputs to each of the L/R, stereo 8Ω speaker inputs, doesn't it see 8 Ohms? Why else would it be labeled that way? It would only see 4 Ohms if I plugged only ONE speaker cable into the MONO input, right?

Sound hasn't changed, and it's not blowing fuses. Also, it's typically "okay" to go into one impedence value HIGHER (e.g., 8Ω amp into a 16Ω cab), but not one impedence value LOWER, right (e.g., 8Ω amp into a 4Ω cab)?
.
American HM Strat | LP Studio
Soldano Avenger w/DeYoung OT | Mark IV rackmount | DC-3 rackmount | Single-Recto

.
Last edited by LEVEL4 at Mar 23, 2008,
#5
Quote by Erock503
. . . is not a stereo amp, therefore the 2 speaker output jacks on the back are wired in parallel. ie. if you used 2 seperate 16ohm cabs, with a speaker cable to each: 16ohm + 16ohm in parallel = 8ohm total load the amp would see.

If you used 2 cables with your cab in stereo, that would be 16ohm at each stereo input jack, so it would be the same thing as using 2 seperate 16ohm cabs in the example above. It would work, but it's using a whole extra cable you don't need. The 2x 16ohm speakers in your cab are already wired in parallel when the cab is set to mono (8ohm).
[Edit: See posts below.]

Erock: I copied your post from another thread . . .

So . . .

1. My 6505 is not a "true stereo" head, but it has two parallel outputs.
2. My cab is a true stereo cab, with each "stereo input" labeled "8Ω." So, the cabinet's two inputs should look like two separate 8Ω cabinets, right?
3. In stereo, the cabinet is 8Ω, and in mono, it's 4Ω, so I guess the cabinet is wired in parallel, right?

So, shouldn't I be "safe" running the two outputs from my "non-true stereo" head to the two inputs on my "true stereo" cab?
.
American HM Strat | LP Studio
Soldano Avenger w/DeYoung OT | Mark IV rackmount | DC-3 rackmount | Single-Recto

.
Last edited by LEVEL4 at Mar 24, 2008,
#6
I think I'm getting it. The above is wrong, right? I HAVE to re-wire my cabinet in series, so that I can double its impedence, from 4Ω to 8Ω in mono, right? [EDIT: THIS IS WRONG. THE TWO 8Ω SPEAKERS, WIRED IN PARALLEL, WILL HAVE A TOTAL IMPEDENCE OF HALF EACH SPEAKER'S RATED IMPEDENCE, e.g., . IN SERIES, IT WILL BE THEIR SUM, e.g., 16Ω] I can't do the two-cables-into-the-stereo-inputs trick, and be able to end up with enough impedence, right?

Okay, I think I got it now. I just opened the cabinet. There are two 8Ω speakers in there (with the mono input wired in parallel, making it 4Ω into the mono input). I'm going to wire them in series to the mono input and delete the second-channel input. So, then, I should have a total impedence of 16Ω. Then, I'll set my amp output to 8Ω (half my actual cabinet impedence), to accomodate my two pulled tubes.

I think I was running an effective 8Ω output (amp was actually switched to 4Ω to accomodate the "half-impedence" deal), into a 4Ω input, if I understood you correctly, Erock. I don't "hear" any damage, but that may be a possibility, as you mentioned.
.
American HM Strat | LP Studio
Soldano Avenger w/DeYoung OT | Mark IV rackmount | DC-3 rackmount | Single-Recto

.
Last edited by LEVEL4 at Mar 24, 2008,
#8
Thanks, Erock! Whew! I just got back from Radio Shack to get some spade connectors and a battery-powered soldering iron.

• Disconnected the jumper from the mono jack to the second input jack.
• Deleted the parallel-wired, second-channel input jack completely.
• Wired both -speakers in series to mono input jack.
• Speaker cabinet total impedence is now 16Ω (8Ω + 8Ω = 16Ω when in series).
• Halved the amp output impedence to to accomodate two pulled tubes.

This is all good, now, right?
.
American HM Strat | LP Studio
Soldano Avenger w/DeYoung OT | Mark IV rackmount | DC-3 rackmount | Single-Recto

.
Last edited by LEVEL4 at Mar 23, 2008,
#9
^ Yes.
You Don't Need a halfstack.

You Don't Need 100W.

Quote by jj1565
i love you slats.
#10
Thanks, Slats! Yup, I guess it's all good, now. Turned it up to about '7,' and the amp sounded fine (I mean, great!). I noticed that the amp seems quieter in the dead room (much smaller room, completely carpeted) with the 2x12, than in my live room, through a 4x12. Hopefully, that's not because I damaged the output transformer, since I played my amp with the mis-matched impedence (an effective 8Ω-output going into only a 4Ω-load) for about a week at low to moderate volumes. Seems to sound fine, though.
.
American HM Strat | LP Studio
Soldano Avenger w/DeYoung OT | Mark IV rackmount | DC-3 rackmount | Single-Recto

.