Resistance in original Mark I

[guy_incognito]

So I'm finding it very hard to shed light on what the resistance is from a Mark I - 4 ohms? 8 ohms? I gather that, since there are three outputs and everything, some of them are different than others... or are they? I don't see any markings on the amp. Please, enlighten me.

Also, although the stock combo sounds just fine, I've got a Hiwatt cabinet kicking around and I'd like to try pushing some more air out of the speakers, but it's rated at 16 ohms - running these together would be a big no-no, correct? If so, is there any sort of adapter or other procedure that could make this work?

 

[Aart]

the one closest to the AC chord is probably 8 ohm and the other two are probably 4 ohm, thats how it is in all other mark heads/combo's ive seen.

 

[ramalam]

The one that sounds different from the other two is the 8 ohm jack. It will also be louder.

 

[guy_incognito]

Hmm. I don't hear a difference at all... strange.

 

[phyrexia]

Are you playing it at whisper quiet volumes? You'll probably have to turn it up to tell a difference.

 

[guy_incognito]

No, I have it at a pretty healthy blast, though it still could be cranked a lot more... I'd go and try it louder than I had it before right now but it's 12:30 in the morning at the moment.

 

[guy_incognito]

Hmm, something just occurred to me: the speaker in the cab is rated at 8 ohms, yet no harm comes to it if I use the 4 ohm output (whichever one(s) it is). What's the word on using a 16 ohm cab, then?

 

[stokes]

Do you have an ohmmeter?Test the resistance of the output jacks,2 will be the same,one will be different,the odd one is 8ohms the other 2 are 4ohms.You can use the cab you mentioned.Plug it into the 8ohm out and the speaker in the combo in one of the 4ohm outs.

 

[guy_incognito]

I don't believe I have an ohmmeter kicking around, but I'm sure I can scrounge one up, good call.

What combinations of impedence between amp and speaker would you not want to use for fear of frying an amp, then, and why does this happen? I've got some working knowledge of electronics but not a great deal, and I'm finding it hard to get information on this/wrap my head around it.

 

[stokes]

The OT in that amp can stand a lot of mismatch in impedance.A general rule of thumb is that it is safer to connect a larger speaker (8ohm) to a lesser output (4ohm).If you do it the other way around,4ohm speaker to the 8ohm out your tubes will run hotter and in some cases die an early death.

 

[guy_incognito]

Wait, how do I use an ohmmeter in this case? The impedance marked on the output jacks doesn't actually refer to the output, but rather the impedance of the speaker it's designed to go to... right?

 

[kdove]

My '78 Boogie ... looking from the inside ...

• two on the left are tapped are 4-ohm load (so two 8-ohm speakers would work)
• One on the right is 8-ohm (one 8-ohm speaker would work)

Notice the GREEN wire is the 4-ohm feed from the power transformer and the two jacks are wired in parallel ... so two 8-ohm cabs. The YELLOW wire is the 8-ohm feed from the power transformer ... so one 8-ohm cab.

Now from the back on the outside ... obviously reversed ... two to the right are 4-ohm load and the one on the left is 8-ohm.

If you read the MK1 Reissue Manual it shows you the matches and safe mismatches you can use ... starting at page 14 of this manual.

http://www.mesaboogie.com/manuals/Mark1%20Reissue.pdf

You can download this manual, as well.

 

[guy_incognito]

Weird - mine looks a bit different from that - the one on the right isn't separate from the others, but looks, to my amateur eye, like it's in series with the rest. (I believe it's from 77)

 

[kdove]

Mine is stock and matches the manual I linked you to. Now someone may have changed the wiring at some time and placed these all in parallel.

But the important fact is ... which Power Transformer lead (probably only one if they are in parallel) is hooked up to these output jacks. It might be easy to put them back to the original wiring if the PT lead is tied off in there somewhere. You may want to take it to a Tech and let them take a look ... it appears to be modified.

Also, if they have been all wired in parallel that would answer the question of why do they all sound the same.

The first thing I'd look at ... are there two leads coming from the Power Transformer to those output jacks? If only one, is there another lead tied off or clipped? If it's a '77 ... it probably has the same power transformer as mine ... and wire colors. If so, mine is original ... one owner ... nobody's ever touched anything in that photo.

 

[guy_incognito]

That's what mine looks like, originally bought by my dad and not modded.

 

[kdove]

The picture is a bit blurry and the angle is a little different. But it looks to me like you have the green wire (which I think on mine was 4-ohm) hooked to all three.

I'm not a tech but surely someone here could tell. A better picture might be needed though.

Do have have a yellow wire tied off unused from your transformer? You don't think the transformer has ever been replace?

 

[guy_incognito]

I'm fairly sure that everything inside is stock - no, I don't see any tied off wires.

Either way, I have been using it with that cab and nothing has blown up or sounded terrible yet, so...