Bassman 135 ohm load

[jmacbass]

I have a 1980 or 81 Fender Bassman 135 head that I picked up recently and it's in extremely good condition. I talked to a couple of folks about the ohm load for the cabinet - the head has two 4 ohm speaker jacks out. I've been told that I can run two 4 ohm cabinets - one out of each 4 ohm jack for basically a 2 ohm load. Since I have a couple of 4 ohm cabinets, this scenario would be great.

I've also been told that I would actually need to run an 8 ohm cabinet out of each jack for a total 4 ohm load rather than hooking up the 4 ohm cabinets. Is anybody familiar with the Bassman 135 head that can clarify if I could in fact run two 4 ohm cabs out of this head? Thanks.

 

[billfitzmaurice]

The instructions printed on the back of the amp have far more credence than those of well meaning but uninformed advice givers.

 

[DMX]

The instructions printed on the back of the amp have far more credence than those of well meaning but uninformed advice givers.

The 'instructions' on the back can be quite confusing though. If I remember correctly from my old 135, it has '4 ohm minimum load' written on the back with arrows pointing to both speaker outs, and it's not exactly clear if it means 4 ohms for each out or in total.

It was always my understanding that the 4 ohm load is the total load, and you should connect an 8 ohm cab to each output

 

[The 0x]

The Bassman 135 is confusing. The speaker jacks are actually wired in series when both speaker jacks are plugged in. That means with two 4-ohm speakers plugged in, you're actually getting an 8-ohm load. If you have a single 4-ohm load, plug it into the left jack. Single 8-ohm load? Plug it into the right jack.

Do NOT plug a 2-ohm load into this amp.

 

[Groovehunter]

Hi Ox, I wonder if you could clarify that point. I've just picked up a 135 and I've got a 4ohm cabinet. By saying the left jack, do you mean the left when you're actually facing the back of the amp? Don't want to blow my amp up so I thought i'd better double check that thanks

 

[DMX]

The Bassman 135 is confusing. The speaker jacks are actually wired in series when both speaker jacks are plugged in. That means with two 4-ohm speakers plugged in, you're actually getting an 8-ohm load. If you have a single 4-ohm load, plug it into the left jack. Single 8-ohm load? Plug it into the right jack.

Do NOT plug a 2-ohm load into this amp.

Ah, that's right - I remember my amp tech telling me mine had been modded at some stage so that the jacks were in parallel. The 'hum reduction' pot (or whatever it was called) had also been removed. Any idea what that actually did?!

 

[T-Bird]

Hi.

I didn't remember that from the 135 I had. I automatically was going to say the usual 4 Ohm minimum and parallel jacks.

The 'hum reduction' pot (or whatever it was called) had also been removed. Any idea what that actually did?!

On the schematics:

http://www.schematicheaven.com/fenderamps/bassman_135_schem.pdf

you'll see a pot marked as "hum balance" in the filament supply winding circuit in the power supply section. By changing the potential of the filament supply one can reduce the 60Hz or 50Hz hum. Yours was most probably modded for DC filament supply that doesn't have as bad a hum problem.

Regards
Sam

 

[jmacbass]

Thanks for the responses. I appreciate being able to ask a question without getting the typical juvenile nonsense back that are part of so many other forums.

The config on the back panel makes it look like you can run two 4 ohm loads, hence the confusion. It refers to the minimum loads (plural) as 4 ohm each. I spoke to Dave over at Avatar Speakers, and according to him, the old Bassman heads could run 2 ohm loads by using a 4 ohm cab out of each jack. I'm still a little nervous about chancing it since the modeling and acting career hasn't taken off yet and I don't want to fry anything that I can't afford to replace. Anyway, this is what's on the rear panel. Thanks again for your responses.

Speaker >> Ext. Speaker
Minimum Loads
4 OHMS 4 OHMS
135 WATTS RMS

 

[billfitzmaurice]

It refers to the minimum loads (plural) as 4 ohm each.

With tube amps 'minimum load' is usually a misnomer. While SS amps don't like too low a load, tubes are the opposite, and don't like too high a load. This Fender is an oddball, but appears from the schematic to be best suited for a 4 to 16 ohm load. To be sure of what it will see for a load you'll have to make sure whether the output jacks have been changed from series to parallel.

 

[anderbass]

Here's a quote from a very knowledgeable guy about this very amp/subject:

Neither one of these, but the previous owner was close.

The external speaker output switches in a separate output transformer tap when you plug into it. Each tap is meant for a 4 ohm load, so you're good to go. The Fender schematic is kinda hard to suss if you haven't seen it before, but that little square thingie connected top green and green/yellow is actually a switch on the external jack.



Oh, yeah: you must use the "speaker jack" first or the external spkr jack won't work, due to the other switch on the speaker jack, which creates a short when nothing's plugged into it. This a a transformer saving feature, BTW.

Here's the thread - http://www.talkbass.com/forum/showthread.php?t=178538

Some of these other threads may have more info on this subject:
http://www.google.com/custom?domain...;S:http://www.talkbass.com;LP:1;FORID:1&hl=en