Bassman 135 speaker inputs

[JeromeV]

Hi everyone,

I have been sitting with a problem for more than a year now, and hopefully someone can clear it up.

On the back of my Fender Bassman 135, it says minimum loads 4 ohms (instead of total 4 ohm load for both, like on the bassman 100).

Invalid Link Removed

My question is:
-can the amp take a 2 ohm speaker load?
or
-does the amp put both speaker inputs in series when two cabinets are connected? (the previous owner gave me this version of the story but since I haven't seen it confirmed anywhere....)

I have a schematic for the amp which I can post if needed..

Your help will be much appreciated, I have two 4 ohm cabinets and I'm very eager to try it out on both, but I'm not sure wether the amp will be damaged.

 

[Passinwind]

-can the amp take a 2 ohm speaker load?
or
-does the amp put both speaker inputs in series when two cabinets are connected? (the previous owner gave me this version of the story but since I haven't seen it confirmed anywhere....)

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.

 

[JeromeV]

Thank you very much!!! Do you also own a Bassman 135?

When it says under each input 4 ohms, one would expect to be allright, but since this an unusual feature I would rather go safe.

I'll try it out tonight, thanks again. 2x15" Mesa/Boogie + 4x12" Ampeg, this should make the poweramp tubes distort I hope, because this amp is darn clean, it is really hard to get an overdriven sound out of it.

 

[Ben Clarke]

There may be substitutions of the small tubes that would get you where you want to be.

 

[Passinwind]

Thank you very much!!! Do you also own a Bassman 135?

No, but I've been working on Fender amps for around 30 years. I even did a (very) brief stint in the factory in the early 90s. The 135 watt Twin Reverb has the same output arrangement as your amp, and I've lived with and/or worked on a boatload of those amps.

As far as power amp distortion, your amp may be a tall order to get pleasing results from, in my experience. Fenders with the ultralinear output transformer like yours has are generally regarded as being not as graceful when clipping as many tube amps are. They went out of their way to make it hard to overdrive the output section for a reason, I think. The tradeoff is the very nice clean sound that you've experienced. That shouldn't discourage you from trying it for yourself though. You can start with a jumper or "Y" cord to parallel the two channels, for instance. This is a very common approach to driving the output section a bit more. A good splitter or active A/B box will work a little better for this, BTW.

The only common tube sub I can think of that'll drive the power section appreciably harder is a 12AX7 where the 12AT7 lives now. That'll work better with some circuit changes most likely, which you probably want to avoid. I'm not too sure about potential stability issues in that particular amp, you might ask on the Fender boards or Ampage about that.

If you want preamp distortion, any of the standard mods or add-on pedals can get you there readily enough. Good luck with the extra cab, hope it gives you what you want. You could also play with a single speaker that breaks up early if you want speaker distortion, which can be fun in its own right.

 

[JeromeV]

I've tried it last night, damn!

The sound was great. I use a Fulltone Bassdrive and on the bassman 135 the sound is a lot more "musical" than with my Svt-2Pro which I am now selling The Bassman had more of a vintage tube growl than the Ampeg did.....Maybe an older 70's SVT would be even better....

I am now looking for a second bassman 135....

thanks for the advice....

 

[takeout]

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.

Just picked one of these up, and the above confuses me... do I need a speaker-gauge Y-cable to run two 8ohm cabs off one output, rather than running them each off their own output jacks (i.e., one off the "speaker" and one off the "external"?)

 

[will33]

Just picked one of these up, and the above confuses me... do I need a speaker-gauge Y-cable to run two 8ohm cabs off one output, rather than running them each off their own output jacks (i.e., one off the "speaker" and one off the "external"?)

You can access the proper transformer tap there by chaining your 2 cabs together first, then plugging 1 speaker cable in the main out.

So, 1st speaker cable from main speaker out to first cab. 2nd speaker cable from first cab to second cab. That gives a 4ohm load to the 4 ohm tap.

 

[Steve Dallman]

I just looked at a schematic. With a 4 ohm load plugged into the main output, the speaker connects to a 4 ohm tap. When you plug a second 4 ohm cabinet in, the jacks are wired in SERIES and that series pair goes to an 8 ohm tap. There is no 2 ohm tap, or separate 4 ohm taps.

Two 4 ohm cabinets will be put in series for 8 ohms, and that series pair will go to the 8 ohm tap. So you are good to go.

If you run two 8 ohm speakers, plug one into the main speaker jack, and plug the second cabinet into a second jack on the speaker, assuming there is a pair of paralleled jacks on the speaker cabinet. If there is only one jack per cabinet, then you will need a paralleled "Y" speaker cable to plug into the speakers. You will not use the ext speaker jack in this application.

 

[ThisBass]

Jack 1 = Speaker Jack 4 Ohm
Jack 2 = Ext. Speaker Jack 4 Ohm


The following combinations are possible

Jack 1: 4 Ohm Load
Jack 2: no Load

or

Jack 1: 4 Ohm Load
Jack 2: 4 Ohm Load

or

Jack 1: no Load
Jack 2: 8 Ohm Load

 

[will33]

You can also run a single 8ohm cab by plugging the cab into the ext. speaker out, and a dummy plug (just a plug not hooked up to anything) in the main speaker out. That trips the switch and puts your cab on the 8ohm tap, though I ran 8ohm cabs on the 4ohm tap for a long times with no trouble. Not something you want to do with just any tube amp, bit those fenders are pretty robust. Will sound a little better though running the cab on the matched/proper tap.


I still keep my 135. Not everybodys favorite amp, but I like them.

 

[takeout]

AH. Nicely done, gents - this is the kind of detail I needed. WAY more flexible than I initially understood.

 

[ThisBass]

You can also run a single 8ohm cab by plugging the cab into the ext. speaker out, and a dummy plug (just a plug not hooked up to anything) in the main speaker out. That trips the switch and puts your cab on the 8ohm tap, though I ran 8ohm cabs on the 4ohm tap for a long times with no trouble. Not something you want to do with just any tube amp, bit those fenders are pretty robust. Will sound a little better though running the cab on the matched/proper tap.


I still keep my 135. Not everybodys favorite amp, but I like them.

The dummy plug must be shortened.
By the other hand the speaker Jack shortens by itself when nothing is plugged in.

 

[takeout]

The dummy plug must be shortened.
By the other hand the speaker Jack shortens by itself when nothing is plugged in.

So, solder a jumper from the Tip lug to the Sleeve lug, close it up, done?

 

[Jazzy Jeff]

I just looked at a schematic. With a 4 ohm load plugged into the main output, the speaker connects to a 4 ohm tap. When you plug a second 4 ohm cabinet in, the jacks are wired in SERIES and that series pair goes to an 8 ohm tap. There is no 2 ohm tap, or separate 4 ohm taps.

Two 4 ohm cabinets will be put in series for 8 ohms, and that series pair will go to the 8 ohm tap. So you are good to go.

If you run two 8 ohm speakers, plug one into the main speaker jack, and plug the second cabinet into a second jack on the speaker, assuming there is a pair of paralleled jacks on the speaker cabinet. If there is only one jack per cabinet, then you will need a paralleled "Y" speaker cable to plug into the speakers. You will not use the ext speaker jack in this application.

Steve. I know it is an old thread but could you confirm you could then have a pair of 8 ohm cabs in parallel plugged into the EXT jack making 4 ohms and have the same setup as described in your last paragraph above plugged in too?
Kind regards, J

 

[Wasnex]

Steve. I know it is an old thread but could you confirm you could then have a pair of 8 ohm cabs in parallel plugged into the EXT jack making 4 ohms and have the same setup as described in your last paragraph above plugged in too?
Kind regards, J

That is not correct.

To run a 4 ohm load, daisy chain both 8 ohm cabs from the Spkr jack. Do not insert a plug or dummy into the EXT jack.

Green Yellow is the 4 ohm tap. The plug lifts the ground on the Spkr jack. The signal passes through the contacts in the EXT jack down to the Spkr jack. Red goes to tip (+) and blue goes to sleeve (-).

 

[KenB]

Regarding the output jacks on the Bassman 135...

The External Speaker jack is a high current switching jack. If you look at the jack you'll see a set of heavy-duty contacts that are normally closed when there is no plug inserted. When you insert a plug the contacts open. These contacts are shown on the schematic as the two little rectangles on the ground sleeve.

Fender uses this type of jack for the External Speaker output because, when there is no external speaker attached, the contact must always be closed.

That contact needs to be "beefy" and reliable enough to pass the amp's full output wattage to the speaker day after day!

I'm attaching a photo of the speaker output jacks in my Bassman 135 amp. You can see that both output jacks have contacts that are closed when no plug is inserted.

The larger EXT. SPEAKER jack has the heavy-duty contacts (circled in green), and the smaller NORMAL SPEAKER jack is a normal shorting jack (contact circled in red).

NOW - a word of caution!!

As has been mentioned, it is perfectly OK to run an 8 ohm load from the 135 by plugging the cab into the External Speaker jack only.

There might be some CONFUSION about the need to insert a "dummy plug" into the Normal Speaker jack when using the External jack alone.

The plug must be a SHORTING plug (a plug that has the tip contact soldered directly to the sleeve contact).

HERE'S WHY YOU SHOULD USE THAT SHORTING PLUG:

The Normal Speaker jack has the same kind of tiny shorting contact as the input jacks on the amp.

The reason the "normal" speaker jack has that contact is simply to short out the output of the amp if you inadvertently operate the amp without a speaker. Shorting the output is a safety measure to protect the output transformer.

But that type of tiny contact is nowhere NEAR heavy-duty enough to reliably pass the full output of the amp to a speaker!

Inserting a SHORTING plug into the Normal Speaker jack ensures that there is a RELIABLE ground connection for the 8 ohm transformer winding.

You don't want that tiny contact to fail while in use!