Multimeter Ohm Readings for 1960A and B Lead Cabinets Acceptable?

[AliasUnknown]

Hey All,

Usual brief introduction as a first time poster and long time reader here - now onto a question for someone in the know.

Apologies in advance for the long windedness of my explanation following so TLDR version of question (see readings in bold below) ... are both of these readings acceptable in someone knowledgable's eyes or do I maybe have something going on here which I need to have attended to?

As the title of this thread would suggest, I have a 1960A Lead cabinet which I purchased (way?) back in 2005 and also a 1960B Lead which I finally managed to scrape for and purchase brand new, still boxed, to complete the full stack only about 3 months back now.

In an effort to test all my current gear, at this point without going to the extreme of fully opening up my JCM2000 DSL 100 and somehow riding some high voltage, I recently successfully managed to bias the head having never done so and it having never been serviced ... at this point, it seems that I purchased a good JCM2000 also back in 2005 having never had a problem with it even after a full heavy drop from the top of the 1960A when that was my single cab! As far as I am aware, the head has no issues even though I have read about several issues with boards and cold solder issues etcetera etcetera both here and elsewhere for other owners.

Now I am currently looking into testing my cabinets impedance levels and individual speaker functionality (for the latter there, I.e., speaker functionality, especially in the case of the 1960A considering it is now at a legal drinking age for where I reside).

So ... I came across a 12 year old YT video indicating that the 1960B cab at the very least can have somewhere in the range of a 25% less ohm reading as opposed to the rating ... the video can be searched in YouTube using the following search term (provided as is cause I can't post links being first time): Alexander James - Marshall 1960 B 4x12 Speaker cabinet impedance

I tested both cabs with both cables obviously connected to the 4ohm jack considering that is how I must run them as a mono setup and the results were as follows:

1960A READING: 3.5
1960B READING: 5.2

As already mentioned, the YT vid indicates that 25% LESS is quite normal and in the case of the A cab a reading of 3.5 with simple math equates to about 12.5% LESSER reading and therefore I guess that is acceptable.

On the other hand, I tested the B cab and got a reading of 5.2 which would equate to a reading of 30% HIGHER.

Therefore, it seems that one cabinet is running colder and one is running hotter for lack of a better term IMO.

For those that didn't go the TLDR route ... again, ... are both of these readings acceptable in someone knowledgable's eyes or do I maybe have something going on here which I need to have attended to?

Thanks folks

 

[fitz]

I tested both cabs with both cables obviously connected to the 4ohm jack considering that is how I must run them as a mono setup and the results were as follows:

1960A READING: 3.5
1960B READING: 5.2

to the forum.
The 5.2 is odd and may suggest some sort of problem.
I would suggest taking the backs off and verifying what speakers are in the cabs and how they are wired.
Pictures are worth 1,000 words if you want any answers that are more than speculation based on your descriptions.

 

[Dogs of Doom]

to the forum...

my concern is that you're looking at running 2 cab's at 4 ohm, which would be a 2 ohm load on any given head. To my knowledge, there aren't any Marshall heads that will safely run a 2 ohm load w/o running very hot &/or having catastrophic outcomes...

a couple things not included, that might help:

what year are the cab's? do they have the mono/stereo 16/4 ohm & 8ohm stereo modes?
what amp, or amp's?

maybe a better indication is, if you are going to run both on 1 head, you measure the 16 ohm jacks & let us know those readings...

 

[fitz]

JCM2000 DSL 100

Should have speaker jacks like this:


Like @Dogs of Doom said, if the cabs are 4/16 or 8 stereo, they should go in the amp parallel jacks set to 8 ohm and the 16 ohm cab jacks.

 

[AliasUnknown]

OK so I am preparing for someone to call me as dumb as doggy do do at this point about how I have setup wrong perhaps? eeek!

FYI (all) and to reiterate, the Amp is a JCM2000 DSL 100 (purchased 2005 with manufacturer year unknown but if you tell me where to find out I can let you know) connecting to a 1960A Lead cab (again year of manufacture unknown but purchased in 2005 with whatever are the original speakers it came with) and a 1960B Lead purchased three months ago roughly with stock speakers that would be 4 x 75-watt G12T-75 speakers if the SWater site which has specks is to be classed as gospel (purchased in Australia) ... I therefore assume the 1960A to also have the same speaker config unless of course these cabs were released with another speaker config dependant on year or something perhaps?

Anyways ... here is how I have things setup ATM for both cabs and I've just got back into playing after a long hiatus so forgive my stupidity if the setup is off.

Photos:

 

[AliasUnknown]

Just Dr Googled how to find cabs and amp year of manufacture so some more info ...

JCM2000 DSL 100 is a 2005 as per M-2005...
1960A was manufactured in 2005 as per serial sticker M-2005....
1960B is a 2021 as per M-2021...

Connected to 16 ohm jack of each cab the readings are as follows:

1960A = 13.9
1960 = 14.2 (although I don't know if this reading is accurate because it seems to be flittering about between 13.9 and as high as 15 but the steadyest it gets seems to be 14.2)

Internal photos of exact speakers and wiring I can provide given a little time.

Cheers all

 

[Dogs of Doom]

ok, yeah...

you want to use the 16 ohm inputs on the cab's...

16 x 2 - 8 ohm

so, use the 8 ohm out on the DSL...

the speaker outs, as shown are 2 parallel outs. the switch, is between 4/8 ohm.

2- 16 ohm cab's = 8 ohm
2- 8 ohm cab's = 4 ohm
2- 4 ohm cab's = 2 ohm

single cab's are self explanatory & you match:

16 to 16
8 to 8
4 to 4

respectively.

you were doing it wrong & not good. If it still works no problem, you are lucky, but should be ok...

 

[AliasUnknown]

Cheers @Dogs of Doom

Well it's a wonder I haven't killed the head, the speakers or the whole lot then!

I have seriously been running it like this since I bought the second cab a few months back although I haven't been playing for 16 hours a day like the old days but still giving it a reasonable run.

So all I gotta do is keep the cables in the head as is and run them to the 16 ohm jacks of both cabs set to mono correct?

This would certainly explain why I get massive volume drops at unpredictable times then as well ... let's hope I haven't caused myself a very expensive problem and repair costs ... I'm such a ***** unit!

 

[AliasUnknown]

Also ... what is the POSSIBLE damage I may have caused myself here and the symptoms to be on the lookout for?

Last of all, on a slightly unrelated topic, can someone please link me to the mods thread for the JCM2000 DSL 100 as I can't seem to locate it. Dunno whether I will partake in any or whether in fact a 2005 has any of the issues present as in the earlier revision boards but I would like to take a look at what is possible in any case.

Cheers

 

[mickeydg5]

If a cable has less than perfect connections then it may indicate a resistance above say "0" in that case. Make sure cables read near zero from tip to tip and sleeve to sleeve.

 

[AliasUnknown]

If a cable has less than perfect connections then it may indicate a resistance above say "0" in that case. Make sure cables read near zero from tip to tip and sleeve to sleeve.

Both cables tested resulting in 0.2 ohms in all cases - sounds 'bout right then

 

[JohnH]

I'm a bit concerned by that 5.2 reading!. It seems too low or too high for any normal arrangement of 4 speakers! Do all 4 speakers work? and yes it'd be worth taking the back off to see what you have in there. The 3.2 seems credible for the dc ohms reading of a cab wired for 4Ohms. The nominal ohms applies at a frequency of around 400hZ, and we expect less at dc when measured with a meter.

Then switch to 16 ohms for both cabs, for a net 8 ohm load.

 

[RLW59]

Generally speaking, severe impedance mismatches don't do gradual, cumulative damage to the amp. Everything is fine until something in the amp fries. (Extremely high ambient temperatures, unusually long period of running at max volume, unusually high wall voltage or power surges push some component beyond its limits.)

So if your amp is operating normally it's almost certainly fine and healthy. (When something blows due to impedance mismatch it's usually catastrophic enough that the amp stops working., or is fubar enough that there's no question something has gone wrong.)

But avoid running a 2 ohm load off the 8 ohm tap. It's possible that the very next time you do that could be the time some unusual condition pushes your amp beyond its limits.
----------------------
The one type of cumulative damage that can occur is accelerated wear of the power tubes. You may have shortened the life of the power tubes by a few hundred hours.

Or not -- if you generally play at bedroom volume or moderate levels the tubes may not have been stressed very much.

 

[RLW59]

Back to the main question about cab readings -- the new B cab's 16 ohm readings are in the correct range, but they shouldn't fluctuate. It should give you a solid stable reading.

That along with the messed up reading at 4 ohms suggests the switching plate is defective. Oxidation on switching contacts, bad solder joint, conductive PCB, something's not right.

At 3 months old I assume it's under warranty. Marshall should be willing to send you a replacement switchplate (the wiring is all push-on connectors so not hard to replace the switchplate). Or they may want you to take it to an authorized repair place to verify the problem.
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The 1960 switching jackplates are notorious for failures. Many of them (probably most of them) work fine forever, but enough of them go bad that it's a well-known issue.

Personally I'd gut the switching. Hard wire the speakers at 16 ohms to one jack and either block off the second jack hole or wire the second jack in parallel to allow daisy-chaining cabs.

Because I've never had a need for 4 ohms or stereo I'd do both cabs instead of always worrying about the old A cab going bad eventually.

 

[Gene Ballzz]

@AliasUnknown ,
I too am especially concerned that the 1960B that showed 5.2Ω in the 4Ω configuration that should have shown 4Ω or less, is also the same cabinet that showed a reading of 14.2Ω that was "flittering about" "between 13.9 and as high as 15!" This concern also goes along with the described scenario of "I get massive volume drops at unpredictable times then as well" appears to have come about at the same time as adding the 1960B into the mix! If it hasn't already been tried, I would swap speaker cables between the cabinets and see if that "flittering about!" follows the cabinet or the cable.

It should be noted that those horrid jack plates are notorious for failures that have likely been the cause of more amp failures than actual problems with the amps themselves. You might try sliding the switch firmly back & forth a few times, to see if that "cleans" the contacts and gives a more stable reading, for now!

This all begs an important question: "Are the cables you are using for your speakers 'actually' speaker cables, or are they simply shielded instrument cables?"

My $.02,
Gene

 

[Gene Ballzz]

Back to the main question about cab readings -- the new B cab's 16 ohm readings are in the correct range, but they shouldn't fluctuate. It should give you a solid stable reading.

That along with the messed up reading at 4 ohms suggests the switching plate is defective. Oxidation on switching contacts, bad solder joint, conductive PCB, something's not right.

At 3 months old I assume it's under warranty. Marshall should be willing to send you a replacement switchplate (the wiring is all push-on connectors so not hard to replace the switchplate). Or they may want you to take it to an authorized repair place to verify the problem.
------------------------
The 1960 switching jackplates are notorious for failures. Many of them (probably most of them) work fine forever, but enough of them go bad that it's a well-known issue.

Personally I'd gut the switching. Hard wire the speakers at 16 ohms to one jack and either block off the second jack hole or wire the second jack in parallel to allow daisy-chaining cabs.

Because I've never had a need for 4 ohms or stereo I'd do both cabs instead of always worrying about the old A cab going bad eventually.

^^^^^^^ Wise words, right there! ^^^^^^^

My ,
Gene

 

[AliasUnknown]

Hey All - in the interest of not being a hit and run poster without appreciating the time that others have put in to respond I'm back to report some results.

I don't know exactly what the "remedy" was but the worry has now subsided and it seems that all is good.

I am now running cables correctly into the 16 Ohms in back of both cabinets and I got my fingers moving yesterday for an hour and a half or so with zero volume drops ... so far so good ... yep I'm a dope but hopefully that gave someone a laugh at the very least!

What I have also done though is open up both cabs.

Following is an internals image of the 1960B, and as is also the case with the 1960A, both cabs are loaded with the standard Celestion G12T-75's and wired in exactly the same stock config.

Note: The red arrow points to a negative connector that was apparently only placed on a quarter of the way direct from Mr Marshall's factory so it is now pushed on all the way by my own hand:



Both of the cables used to connect both cabs are indeed speaker cables as supplied in the box both for the M-2005 1960A purchased back in the very same year and the M-2021 1960B purchased a few months back ... as a matter of interest, here is an image of the cables compared side by side where IMO the 2005 cable seems a lot more heavy duty than that supplied with the 2021 1960B:



Anyways ... in trying to cut a long story short, I am assuming at this point that the issue pertaining to the 1960B Ohm readings is resolved due to one of two things:

1) The previously mentioned negative connector to the speaker not being on properly I.e., quarter of the way as suggested and now on all the way

OR

2) My usage of a cheap multimeter with only pin type probes which I placed directly onto the sleeve (positive) and tip (negative). After attaching an alligator clip to the positive to grip at least the sleeve properly the readings seemed good and were not "flittering about" as I previously described it

Cheap multimeter:



ACHIEVED READINGS FOR 1960B WITH SAME SUPPLIED CABLE
---------------------------------------------------------------------------------------------
16 Ohm = 13.9
4 Ohm = 3.5

Please correct me if I am wrong but I believe these readings to be quite within the normal range?

The JCM2000 DSL 100 is one tough mutha for putting up with my Dookie in dropping it and plugging it into speakers wrong and all ... seems to still work perfectly for me after all these years even with the original tubes although I've never dimed the thing fully for extended periods. The Reverb still seems rubbish IMO but I deal that out with a stomp anyways.

I would like to thank everyone here who contributed your time and input and accepting me to the community with such quick and helpful responses - truly appreciate it

 

[Gene Ballzz]

@AliasUnknown

That half connected push on could indeed have easily been the culprit of the wonky readings! Still a good idea to keep a close awareness of those jack plates for any potential and/or future anomalies!

And thanks for being a good and considerate sport about the "hit & run" thing! Members like you are nice to have around and banter with!

Don't Be A Stranger?
Gene

 

[AliasUnknown]

Cheers @Gene Ballzz

 

[Gene Ballzz]

Cheers @Gene Ballzz

If it ain't broke, fix it until it is!