#1
I'm using an '74 Fender Bassman and it has a speaker output and an ext. speaker output.

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Now when I plug a cab into the ext. speaker the volume is significantly less than if i plug it into the regular speaker output. Does anyone know why this is?

Also, if I were to run 2 cabs (I currently have a 1x15, I'm wanting to add either another 1x15 or a 2x10 for more headroom) what would the impedance on the second cab have to be? The 1x15 cab I'm using now is rated at 8 ohms (yes, I'm running mismatched impedances but the cab is higher than the head so its okay - and for those of you who which to argue that point I really don't want to hear it right now). If I were to get another 8 ohm cab and run one into the speaker out and one into the ext. speaker, would that mean that the two cabs would be running at 4 ohms each?


I'm a bit confused here so if anyone can help me out I'd appreciate it


EDIT:

OH ****! Big pictures ftl! I don't know how to make it smaller


EDIT 2:
Nevermind - made it small
Last edited by i_am_metalhead at Sep 14, 2009,
#2
Most amps expect you to use the internal speaker out and the external at the same time- if you're just plugging in the external, with nothing to the internal, you're loading the transformer improperly.

The internal jack for that amp should be 4 ohms, I think, so I'd get a 4 ohm cab to use connected to the internal jack and an 8 or a 4 for the external.
#3
Quote by Roc8995
Most amps expect you to use the internal speaker out and the external at the same time- if you're just plugging in the external, with nothing to the internal, you're loading the transformer improperly.

The internal jack for that amp should be 4 ohms, I think, so I'd get a 4 ohm cab to use connected to the internal jack and an 8 or a 4 for the external.



Okay, I was a bit confused because its a head so I didn't realize that there was an "internal" and "external". As far as the mismatched impedance, I've read in several columns written by techs that it is perfectly safe as long as the cab is rated higher than the head (although a lot of users on here would disagree lol).

But if I ran a cab into the internal and external at the same time, wouldn't that be running the cabs in parallel, thus cutting the impedance ratings in half? I know its different from amp to amp but my Ashdown has a single 16 ohm output for running into a single 16 ohm cab, and two outputs that are switchable between 8 and 4 ohms. If you're running two 16 ohm cabs you're suppose to set the switches both to 8 ohms and run them into the 16 ohm cabs (8 ohm output + 8 ohm output = 16 ohm output into 2x16 ohm cabs running in parallel = 16 ohm input).

So assuming both of the outputs on the Bassman are 4 ohms, wouldn't I want to run each into an 8 ohm cab?
Last edited by i_am_metalhead at Sep 14, 2009,
#4
Ok, I'm even more out of it than I thought-
What I was referring to as "internal" should have been the "speaker" and "external" should have been "extension" - but the rules still apply as I mentioned them. The primary speaker jack should always be loaded when the amp is on.

Yes, you're correct that the cabs' impedances will be halved as they are in parallel when you plug both in- so in retrospect two 8 ohm cabs would likely be the best choice.
However - you need to some legwork to find out the output impedance. I was thrown because you said "bassman" and I immediately thought it was a combo- which would mean the outputs would be two ohms, hence my harebrained 4 ohm cab suggestion. It's most likely a 4 or 8 ohm output for each, but you'll need to find out for sure before buying a cab.

Eek, off to bed for me. No more impedance problems until I get some sleep.
#5
Alright. Thanks Colin!

I've looked around the internet before and haven't been able to come up with much info on vintage Bassman heads but I'll try to see if I can find something out.
#6
Well the only info I could find said it has an output of 4 ohms. Also, if you look at the pic at the top it says "Total load 4 ohms". Is it possible that it has 4 ohm output if one or both outputs are being used (i.e. its 4 ohms if the speaker jack is being used, but it splits each jack to 2 ohms if both the speaker and the extension are being used)? I'm not good with all of this impedance math lol. However, if that is the case I will need to look into 4 ohm cabs instead of 8 ohm ones.
#7
I can't see the picture, so I didn't know what it said.
If it's four ohms total load, it will look for 4 with one speaker and 8 each with two (parallel). Two 8 ohm cabs are your best bet, a 8:4 mismatch when using one cab should be fine.
#8
if this helps at all, my sunn 1200S has a 16 ohm output impedance with a speaker out and the external speaker out wants a 16 ohm load. my guess is so you can run 2 of the stock cabs with the head unit, this would imply the sunn is switched over to an 8 ohm tap off the transformer with 2 16 ohm loads run in parallel. i am guessing this is how it worked 'back in the day' instead of impedance selectors or multiple output. this is a quote from conrad(aka as the dude who made and designed much of the sunn stuff)

Quote by Conrad
The speaker output jacks are switching, that is when you have an 8 ohm
speaker plugged into the 8 ohm jack and then plug in another 8 ohm speaker
into the external speaker jack the amplifier auto switches both jacks to the
4 ohm tap on the output transformer. ( 2 - 8 ohm speakers in parallel = 4
ohms. Hope this helps


i would have to assume this is how it worked for fenders too, as back in the day they were likely to sell 2 straight stock cabs to run with your amp, you didn't get an option of getting a 2 ohm or 8 ohm cabinet option with your bassman. so i would have to assume that hooking a cab to the speaker out and the external speaker out would switch to a 2 ohm transformer tap that would run the 2 4ohm cabinets in parallel for an effective 2 ohm load.

this is really just guess work and comparison though, a little bit of backup would be good. but it feels right in the gut
punk isn't dead, it's always smelled that way.

"A perfection of means, and confusion of aims, seems to be our main problem."
-ae
#9
Quote by Roc8995
I can't see the picture, so I didn't know what it said.
If it's four ohms total load, it will look for 4 with one speaker and 8 each with two (parallel). Two 8 ohm cabs are your best bet, a 8:4 mismatch when using one cab should be fine.


Yea it says "Total load 4 ohms" - so alright. I'm not very good figuring out how many ohms an amp will put out when you start adding more cabs and all of that stuff.

Thanks a lot though!
#10
damn, found this on another site too


All through the blackface era Fender amps had an extention speaker jack that was expected to run a second cabinet at the amps rated impedance. That is to say, if the amp had an 8 ohm output, they expected you to run an 8 ohm extention cabinet. This did present a less than ideal load to the output tubes. But I guess Fender didn't feel this was a safety issue. The silver amps mostly used the same or very similar transformers as their blackface counterparts. So I think you'll be fine just plugging another 8 ohms into the extention jack. FWIW that jack is in parallel with the mains jack so another 8 ohm speaker would be parallel to the cabinet speaker creating a 4 ohm load. If you intend to use only the external speaker for an 8 ohm total load, so you unplug the combo speaker, you will need to plug the external speaker into the main speaker jack because it is a shorting jack. the main jack must be in use for either jack to work properly.


this would also explain why plugging straight into the external cab jack didn't work too well
punk isn't dead, it's always smelled that way.

"A perfection of means, and confusion of aims, seems to be our main problem."
-ae
#11
Quote by gumbilicious
damn, found this on another site too


this would also explain why plugging straight into the external cab jack didn't work too well



So basically what Colin was saying - I want to plug an 8 ohm load into each jack, thus creating an 8 ohm parallel load (4 ohm load for each output).
#12
well, i interpreted it as hooking 2 stock cabs into the amp. in the case of the silverface bassman, that would be 2 4 ohm cabs. this should cause the bassman's transformer to switch over to a 2 ohm tap mayhap.

the point i am trying to make is this though, back in the 70's if you were to go to a fender dealer they'd sell you this bassman with a 4 ohm cab. if you wanted another cab for the for the external out, they would have sold you another stock 4 ohm cab. that would be 2 4 ohms cabs, and that would be from the fender dealer. they didn't make 8 ohm cabs to match with this thing, or 2 ohms cabs.
punk isn't dead, it's always smelled that way.

"A perfection of means, and confusion of aims, seems to be our main problem."
-ae
#13
Well now I'm all kinds of confused.

However I would rather have a mismatched impedance of the cabs being rated higher than the head output than to have a head output rated higher than the cabs and fry the OT.
#14
it would make sense if we were in the 70's, cuz that was how they did it back then. but from best i can tell the external speaker jack completes a circuit when a 1/4" cable is inserted thereby bypassing the 4 ohm tap on the transformer and enabling a 2 ohm tap on the transformer. where is slats when you need him. i'll see if he still comes on here anymore.

edit: i checked and slatsmania hasn't been on since january or something, he would have known. try pm'ing my buddy grisky if colin doesn't post with the correct answer
punk isn't dead, it's always smelled that way.

"A perfection of means, and confusion of aims, seems to be our main problem."
-ae
Last edited by gumbilicious at Sep 14, 2009,
#15
I went and checked the schematic for your amp. It looks like the speaker jacks are wired in parallel.
So, Fender gives you two jacks and a four ohm cab, and tells you to add another cab in parallel to make a four ohm load. I guess they didn't plan on very many people using an external cab, or figured the amp could handle running into a 2 ohm load.

Either way, the two 8 ohm cab option is your best bet. You'll be loading the amp properly with two cabs and have a safe mismatch with one.

The shorting switch doesn't engage another transformer tap. It's a common fender design that shorts to a high-value resistor when there's no speaker attached. The idea is that if you forget to plug your cab in, it's better to be playing into a very high load than none at all. When you just plug into the extension cab, the signal gets shorted to that resistor and your volume is much lower because that's all that's left.
#16
damn, i sure helped to make it confusing. cool enough, i'll file this away somewhere as i am sure it will come up again.
punk isn't dead, it's always smelled that way.

"A perfection of means, and confusion of aims, seems to be our main problem."
-ae