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Impedance question...?

#3
CosmicRay said:
Hey folks, so I recently acquired a bucket list amp. A 1968 Sunn 200S.
The same model I had, new in '68, when I began playing bass.
The speaker output indicates an 8 ohm load.
I think (hope) it will be ok to use a 4 ohm cab as well.
Yes?
Click to expand...
Had the same amp in 1968 .. show and tell time''
sunn.jpg
 
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#4
JimChjones said:
The circuit diagram I just found indicates 16, 8, and 4 ohm connections to the output jacks. It might be as well to upload a photo of the jack(s) on your amp so we know exactly what you are looking at.
Click to expand...

Sorry the image isn't better. It has two 1/4 speaker outs on the rear panel.
the left says 8 ohm speaker , the right says 4 ohm external speaker
I have a 4 ohm cab I'd like to try with this amp. But I don't want to make the magic smoke appear.
upload_2021-11-15_16-7-8.jpeg
 
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#9
CosmicRay said:
Hey folks, so I recently acquired a bucket list amp. A 1968 Sunn 200S.
The same model I had, new in '68, when I began playing bass.
The speaker output indicates an 8 ohm load.
I think (hope) it will be ok to use a 4 ohm cab as well.
Yes?
Click to expand...

most amps can handle a 4 ohm load ... I believe that amp was designed so it could drive 2 8 ohm cabs for a 4 ohm load. Don't try 2 4 ohm cabs the results could be bad, also running a 4 and an 8 ohm cab at the same time would probably not be good. running a single 4 ohm cab should work fine for an amp from that time.
 
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#10
abarson said:
Here's the answer from a moderator of the Sunn forum:
https://forum.sunnstillshines.online/index.php?topic=10708.msg36897#msg36897

1 x 8 ohms cabinet - use the 8 ohms output
1 x 4 ohms cabinet - use the 4 ohms output
2 x 8 ohms cabinets - one cabinet per output
Click to expand...
Thank you. Much appreciated!
I too went on that forum, after I discovered it, and received the same response.
It's the "4ohm external speaker" that was making me think that it was to be used only when hooking up a second cab.
 
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#11
I would not do that. I’m no expert on SUNN products, but, if it’s designed to operate at 8 ohms, and you run it at 4 ohms (by using a 4 ohm cab instead of 8) you’re pushing the power amp harder. Maybe it’s designed to handle this- I don’t know, but as I stated, I wouldn’t do it.
 
#13
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#14
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:
upload_2021-11-15_18-34-1.png


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.
 
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#16
CosmicRay said:
Hey folks, so I recently acquired a bucket list amp. A 1968 Sunn 200S.
The same model I had, new in '68, when I began playing bass.
The speaker output indicates an 8 ohm load.
I think (hope) it will be ok to use a 4 ohm cab as well.
Yes?
Click to expand...
Here is what Jess Oliver of Ampeg fame said about speaker load on tube amps. What are the benefits of re-coning the 8-ohm speaker in Portaflex amps with a 4-ohm kit, as you did with my B-18? Jess: After being in the amplifier business for many years, I found out you don't always have to follow the book rules. When it comes to speaker load on a tube amp, I've found you can load a tube amp down to saturation without damaging the amp. You just get more effective power out of the amp. Just as long as the secondary of the output transformer "sees" a path, some resistance, it doesn't matter how low that resistance is, as long as it sounds good, and four ohms is not below the saturation point.
 
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#17
Ric5 said:
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.
Click to expand...
yes, and both at the same time - implied from the word "external". The main 8ohm cab, along with an external 4ohm cab.
Anyway, that's the way I comprehended it to be used. Apparently not, however.
 
#18
IMHO EXT implies extension.

As far has comparing to Fender amps...you have to consider variances from model to model. Most vintage Fender amps only had one output tap. If you ran the recommended cab it provided the expected impedance. If you two of the matching cabs the impedance was half of what the amp expected...this will reduce how much power the amp can make and increase wear, but it is how the amp was intended to be run.

Some silverface Fender amps has multiple output taps and switching jacks similar to the Sunn, but not necessarily exactly the same.

For example the Bassman 135 has both a 4 and 8 ohm tap, but it puts the tips of the jacks in series instead of parallel.

upload_2021-11-15_19-59-49.png


To provide the expected load, plug one 4 ohm cab into the Speaker out. If you plug a cab into the EXT jack, it lifts the connection off the 4 ohm tap and the tip is connected to the 8 ohm tap. Since the jacks are wired in series you need to connect a 4 ohm cab to each jack. If you want to use a single 8 ohm cab, use only the EXT jack. The works because the Speaker jack has a switching contact that shorts the tip to ground unless a plug is inserted.

My point here is you need to know specifically how your amp works, instead trying to rely on a general rule. The output transformers in old Fender amps are considered fairly robust and they supposed to tolerate 2:1 or 1:2 impedance mismatches fairly well. But this is not true of all amps. Some amps work well with 2:1, but it may be risky to run 1:2. Some amps are the opposite and work well with 1:2, but may not tolerate 2:1. Some amps are risky with 2:1 or 1:2. From a reliability standpoint, always best to show a tube amp the expected load.
 
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#19
CosmicRay said:
Hey folks, so I recently acquired a bucket list amp. A 1968 Sunn 200S.
The same model I had, new in '68, when I began playing bass.
The speaker output indicates an 8 ohm load.
I think (hope) it will be ok to use a 4 ohm cab as well.
Yes?
Click to expand...
CosmicRay said:
Hey folks, so I recently acquired a bucket list amp. A 1968 Sunn 200S.
The same model I had, new in '68, when I began playing bass.
The speaker output indicates an 8 ohm load.
I think (hope) it will be ok to use a 4 ohm cab as well.
Yes?
Click to expand...
I would plug my 4ohm external speaker cabinet into the "4ohm external speaker" jack
 

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