Impedance question...?

[GreyBeard]

I ran mine at 4 ohms (2x15's) both 8 ohm in parallel for probably 30 years before I retired the head.

 

[Lowbrow]

The old Fender bassman amps had the speaker outputs labeled the same way ... that always bothered me because it looks like they want you to run an 8 ohm cab from one and a 4 ohm cab from the other.

Of course it looks that way because THAT'S WHAT IT SAYS!

 

[Lowbrow]

Why wouldn’t you plug your one 4 ohm cab into the jack labeled “4 ohms”. Seems pretty darned straightforward to me. Why would you plug it into the 8 ohm jack when it’s a 4 ohm cabinet?

You’ve been playing bass since 1968 and you’re asking this question?!?

The descriptions of how this amp works and how that does not exactly align with how the outputs are labeled indicate that this is not quite straightforward.

 

[dbase]

I also had the matching cab with 2x15 JBL D-140F's
I gigged with that rig from '68 to '84View attachment 4473612

Loved lugging that amp around from bar to bar ... simple controls, Volume, Treble, Bass what else do you need.

 

[HolmeBass]

The descriptions of how this amp works and how that does not exactly align with how the outputs are labeled indicate that this is not quite straightforward.

Everyone is saying to plug his 4 ohm cabinet into the four ohm output jack... but I’ll take your word for it.

 

[HolmeBass]

I see, not straightforward. My apologies.

 

[Lowbrow]

I see, not straightforward. My apologies.

Honestly, I work as a writer of marketing materials for a firm that does technical work, and the battle to help technical people understand that end-user materials need to be comprehensible to END USERS (not only the tech people) is never ending. Often has to be stressed again and again during a single conversation.
The way that the discussed amps are labeled vs. how they actually operate indicate that engineers had control of how these outputs were labeled.

 

[iiipopes]

Honestly, I work as a writer of marketing materials for a firm that does technical work, and the battle to help technical people understand that end-user materials need to be comprehensible to END USERS (not only the tech people) is never ending. Often has to be stressed again and again during a single conversation.
The way that the discussed amps are labeled vs. how they actually operate indicate that engineers had control of how these outputs were labeled.

I had this same discussion with my son at one of his high school teachers' conferences. The usual son's question, "Why do I have to write term papers and essays. I'm going to write technical as a career." Answer, "Yes, but not everyone who reads what you write is technical. You must write for the broader audience. So you must learn to write term papers and essays so you can communicate with them." My son scowled. The teachers smiled.

 

[iiipopes]

The amp has three transformer taps but only two are connected at a time. The options are (16 and 8) or (8 and 4).

The impedance can be converted, so not a bad idea to inspect how the output jack is wired.

This from the 200S schematic:
View attachment 4473855

The tip of the two jacks are in parallel, and the EXT jack is switching. If you use only the Speaker jack the impedance is 8 ohms. If you use the EXT jack, the switch moves to the 4 ohm tap. You can plug one 4 ohm cab into the EXT jack, or you can plug 8 ohm cabs into both jacks.

Color code from schematic:
Brown = 4 ohm
White = 8 ohm
Yellow = 16 ohm

In order to reconfigure the amp for different impedance selection, all three connections must be moved. However, the 16 ohm tap should always be connected to the FB loop.

THIS! THIS! THIS! END OF DISCUSSION: PLUG THE 8-OHM CABINET INTO THE 8-OHM JACK, AND PLUG AN 8-OHM EXTENSION CABINET INTO THE EXTENSION JACK FOR A TOTAL 4-OHM LOAD. ANYTHING ELSE WILL EVENTUALLY KILL THE AMP.

Oh, where is @agedhorse when you need him!

 

[CosmicRay]

Why wouldn’t you plug your one 4 ohm cab into the jack labeled “4 ohms”. Seems pretty darned straightforward to me. Why would you plug it into the 8 ohm jack when it’s a 4 ohm cabinet?

You’ve been playing bass since 1968 and you’re asking this question?!?

Thanks Mr. Snarky for your observation. Very astute of you. If it seems straightforward to you, perhaps you should read what the outputs indicate. Or look up what "external" means.

 

[CosmicRay]

I see, not straightforward. My apologies.

I just saw your apology post. Thanks.

 

[el murdoque]

Why wouldn’t you plug your one 4 ohm cab into the jack labeled “4 ohms”. Seems pretty darned straightforward to me. Why would you plug it into the 8 ohm jack when it’s a 4 ohm cabinet?

You’ve been playing bass since 1968 and you’re asking this question?!?

In the OP's shoes, I'd have the same question.
The 4 Ohm tab is labeled as extension cab which kinda sorta implies that maybe it's a bad idea to fire it up without a load present at the other outlet, which seemst to be the 'main' outlet - When someone bothered labeling the other one as extension, the remaining one must be main, no?

 

[iiipopes]

In the OP's shoes, I'd have the same question.
The 4 Ohm tab is labeled as extension cab which kinda sorta implies that maybe it's a bad idea to fire it up without a load present at the other outlet, which seemst to be the 'main' outlet - When someone bothered labeling the other one as extension, the remaining one must be main, no?

Not just maybe. It IS a bad idea to fire up a tube amplifier without a load. You will eventually burn it up:

"Going All-Tube? Connect a Speaker

This is where you need to be careful. If you have an amplifier with power tubes in the output section, then you cannot operate it without a speaker load connected.

Simply put, in the tube amp output section, the transformer performs the task of transforming the speaker load to a load the tubes want to see. For example, on an amp with four EL34 output tubes the transformer makes an 8 ohm load look like a 1200 ohm load for the tubes.

This is also why you should not mismatch your load to the impedance indicated or selected on the amp. If you are on the 4 ohm tap of the transformer it transforms the same 1200 ohm to the tubes. Tubes are happy. If you are on the 8 ohm tab with a 4 ohm load, the same transformer will put a 600 ohm load on the tubes and they will not perform correctly.

Now, if you do not connect a load, the transformer can not reflect a load and the tubes will only see the wire in the transformer and a big coil as a load. This is basically shorting the tubes to the high voltage and it will cause damage to the amplifier eventually. Another way to think of it is the power tubes pull on the transformer which magnetizes the output transformer; this in turn creates a voltage on the output of the transformer to drive the speaker load. If there is no load, all that energy has nowhere to go, so instead it will be fed back to the power tubes causing damage. Usually the tubes will go into a growing oscillation that eventually burns them. Most of the time the output transformer is ok, but if left on long enough it can also be damaged. And playing through an unloaded tube amp will only speed up the road to damaging the amp."

https://carvinaudio.com/blogs/guita...akers-connected-here-s-when-you-can-and-can-t

 

[JimChjones]

To my mind this is a classic example of being too clever. A pair of jacks in parallel and an output impedance selection switch would be a deal more readily understood than messing with switched jacks. In trying to make operation simpler for the unversed user the end result has been more complicated for everyone.

 

[CosmicRay]

Not just maybe. It IS a bad idea to fire up a tube amplifier without a load. You will eventually burn it up:

"Going All-Tube? Connect a Speaker

This is where you need to be careful. If you have an amplifier with power tubes in the output section, then you cannot operate it without a speaker load connected.

Simply put, in the tube amp output section, the transformer performs the task of transforming the speaker load to a load the tubes want to see. For example, on an amp with four EL34 output tubes the transformer makes an 8 ohm load look like a 1200 ohm load for the tubes.

This is also why you should not mismatch your load to the impedance indicated or selected on the amp. If you are on the 4 ohm tap of the transformer it transforms the same 1200 ohm to the tubes. Tubes are happy. If you are on the 8 ohm tab with a 4 ohm load, the same transformer will put a 600 ohm load on the tubes and they will not perform correctly.

Now, if you do not connect a load, the transformer can not reflect a load and the tubes will only see the wire in the transformer and a big coil as a load. This is basically shorting the tubes to the high voltage and it will cause damage to the amplifier eventually. Another way to think of it is the power tubes pull on the transformer which magnetizes the output transformer; this in turn creates a voltage on the output of the transformer to drive the speaker load. If there is no load, all that energy has nowhere to go, so instead it will be fed back to the power tubes causing damage. Usually the tubes will go into a growing oscillation that eventually burns them. Most of the time the output transformer is ok, but if left on long enough it can also be damaged. And playing through an unloaded tube amp will only speed up the road to damaging the amp."

https://carvinaudio.com/blogs/guita...akers-connected-here-s-when-you-can-and-can-t

Wow, ok, I do know that a speaker load must be connected to the amp. But thank you for the useful information.
My original question was, and for those not familiar with a Sunn 200S -
the two (2) rear output jacks read as follows: Left out says- "8ohm SPEAKER." Right out says- "4ohm EXTERNAL SPEAKER"
So, now this is me thinking, an 8ohm cab must be plugged in, obviously to the 8ohm out - AND only use the 4ohm external out with a 4ohm cab, while an 8ohm cab is also being used at the same time. That wasn't my objective. I want to use one and only one cab. I have an 8ohm cab and I have a 4ohm cab. I know how the 8ohm cab sounds.
I wanted to see how the 4ohm cab sounds. Which freakin' output should the 4ohm cab go into? Can/should I use a 4ohm cab by itself, with this amp, in the first place?
I will go with advice from the Sunn forum site and I thank everyone for their help, comments, opinions and apologies.

 

[wellshuxley]

Thanks to those who chimed in. I recently faced a similar head-scratching moment when I first went to hook up my 6g6 Bassman. It has two speaker jacks: one labeled “SPKR,” the other “EXT SPKR.” Definitely ambiguous labeling that is easy to get wrong if you don’t look up the transformer outputs and how they are wired up!

 

[CosmicRay]

In the OP's shoes, I'd have the same question.
The 4 Ohm tab is labeled as extension cab which kinda sorta implies that maybe it's a bad idea to fire it up without a load present at the other outlet, which seemst to be the 'main' outlet - When someone bothered labeling the other one as extension, the remaining one must be main, no?

My thinking exactly. And since the "main" is labeled 8ohm, would plugging in a 4ohm cab to that 8ohm main, cause any damage?

 

[Wasnex]

Wow, ok, I do know that a speaker load must be connected to the amp. But thank you for the useful information.
My original question was, and for those not familiar with a Sunn 200S -
the two (2) rear output jacks read as follows: Left out says- "8ohm SPEAKER." Right out says- "4ohm EXTERNAL SPEAKER"
So, now this is me thinking, an 8ohm cab must be plugged in, obviously to the 8ohm out - AND only use the 4ohm external out with a 4ohm cab, while an 8ohm cab is also being used at the same time. That wasn't my objective. I want to use one and only one cab. I have an 8ohm cab and I have a 4ohm cab. I know how the 8ohm cab sounds.
I wanted to see how the 4ohm cab sounds. Which freakin' output should the 4ohm cab go into? Can/should I use a 4ohm cab by itself, with this amp, in the first place?
I will go with advice from the Sunn forum site and I thank everyone for their help, comments, opinions and apologies.

I'll try to state this as concisely as possible.

Assuming the amp is wired to factory specs you have three options:

(1) One 8 ohm cab. Use the 8 ohm Speaker jack.
(2) On 4 ohm cab. Use the 4 ohm EXT jack. Inserting a plug in the EXT jack changes the expected impedance from 8 to 4 ohms.
(3) Two 8 ohm cab. Plug one cab into each jack. In other words one cab goes to the 8 ohm Speaker jack, and one cab goes to the 4 ohm EXT jack. The load is 4 ohms.
​

Getting into the weeds a bit more. You could also daisy chain two 8 ohm cabs off the 4 ohm EXT jack.

​

Hopefully this is plugged into a 4 ohm load.

 

[CosmicRay]

​

Hopefully this is plugged into a 4 ohm load.
View attachment 4474636[/QUOTE]
I hope so too. Yep. I get it. And if Sundholm didn't label the outs as he did, this would all be moot.