Fender Bassman 135 speaker connections (ohm matching)

[lastnerve]

Below is a quote from an article I recently read about the Bassman 135 speaker connections and ohm matching:

1) The Bassman 135 is designed for speaker cabs of 4 ohm minimum
impedance, for both the main cab and any extension cab.

2) The Bassman 135 and the similar 135 watt Twin Reverb amps are
unique among Fender amps, in that the jacks for the main and extension
speakers are wired so that when both jacks are in use the cabs are
connected in SERIES rather than parallel, and plugging in an extension
cab also switches the speakers to a different tap on the output
transformer (switched from the 4 ohm "center" tap to the "whole" 8 ohm
secondary winding).

Has anyone heard about this before? It's news to me.

I just read the sticky on "AMP FAQS" about impedance and ohms. So the way I understand it, if I run an 8 ohm load on my 4 ohm amp I'm only going to getting about half the power of the amp to the cabs.

If I connect one 4 ohm cab to the MAIN 4 ohm speaker output, and then connect the second 4 ohm cabinet in parallel, then I end up with a 2 ohm load (which could overdrive the amp, I think).

So the only way that I can see to get a 4 ohm load is to connect one 8 ohm cab to the MAIN 4 ohm speaker output, and then connect the second 8 ohm cabinet in parallel.

But I want to use my 4 ohm cabs. I have two 4 ohm cabs (2x12 and 4x10), and two 8 ohm cabs (1x15 each).

How would I get a 4 ohm load with two 4 ohm cabs?

The AMP FAQs seem to indicate that it is safe to run a 2 ohm load on a 4 ohm output (for a tube amp). Has anyone done this?

[will33]

Originally Posted by lastnerve

Below is a quote from an article I recently read about the Bassman 135 speaker connections and ohm matching:



I'm no expert on amps and cabs, but the way I understand ohmage and wattage is that if I run an 8 ohm load on my 4 ohm amp I'm only going to getting about half the power of the amp to the cabs.

If I connect one 4 ohm cab to the MAIN 4 ohm speaker output, and then connect the second 4 ohm cabinet in parallel, then I end up with a 2 ohm load (which could overdrive the amp, I think).

So the only way that I can see to get a 4 ohm load is to connect one 8 ohm cab to the MAIN 4 ohm speaker output, and then connect the second 8 ohm cabinet in parallel.

But I want to use my 4 ohm cabs. I have two 4 ohm cabs (2x12 and 4x10), and two 8 ohm cabs (1x15 each).

So my questions are:

How would I get a 4 ohm load with two 4 ohm cabs?

What would be the load resulting from connecting a 4 ohm cab and an 8 ohm cab in parallel?

Tube amps don't work that way, you get pretty much the same power from all the different impedances it's designed to run. They're also opposite of SS in that the danger comes in running an impedance that's higher than what they want rather than lower, although those old Fenders were all pretty robust.

Plug one 4ohm cab in the Main jack, one in the ext. jack and play.

If you use your pair of 8ohm 15's, daisy chain them together and plug the one speaker cable in the main speaker jack, resulting in a 4ohm load for the amp.

You'll get the same amp power in each case.

[hennessybass]

So when you hook up 4ohms to speaker, and 4 ohms to ext speaker, you are actually running an 8ohm rig? Am I understanding this correctly?

Anyone ever heard of using a 1/4" plug with nothing attached to get the 8ohm tap... like plug an empty 1/4" plug into the speaker out, and then an 8 ohm cab into the Ext speaker out to run a single 8 ohm cab? I think there is some amp out there that does this. Maybe the Ampeg b25b?

[will33]

Originally Posted by hennessybass

So when you hook up 4ohms to speaker, and 4 ohms to ext speaker, you are actually running an 8ohm rig? Am I understanding this correctly?

Anyone ever heard of using a 1/4" plug with nothing attached to get the 8ohm tap... like plug an empty 1/4" plug into the speaker out, and then an 8 ohm cab into the Ext speaker out to run a single 8 ohm cab? I think there is some amp out there that does this. Maybe the Ampeg b25b?

Yes, you end up with an 8ohm stack with this particular amp.

In mine, it plays fine plugging an 8ohm cab in the ext. jack with nothing in the main jack, although I haven't figured out which tap it's actually connected to that way. Plugging the 8ohm cab in the main jack and an empty plug in the ext. jack should certainly switch it to the 8ohm tap. This is one of a very few amps with series wired switching jacks like this instead of a switch or multiple jacks for the different taps.

[will33]

For the OP, or whoever may not know......NEVER turn on any tube amp without a speaker connected, you can destroy it. ALWAYS hook up the speakers first, THEN turn the amp on. It's another difference between tube and solid-state. Some have shorting jacks as a fail-safe measure, some don't, so just don't do it with any of them.

Search the phrase "voltage drive vs. current drive" for an explanation of how impedance works with tube power vs. SS.

[hennessybass]

Originally Posted by will33

In mine, it plays fine plugging an 8ohm cab in the ext. jack with nothing in the main jack, although I haven't figured out which tap it's actually connected to that way. Plugging the 8ohm cab in the main jack and an empty plug in the ext. jack should certainly switch it to the 8ohm tap. This is one of a very few amps with series wired switching jacks like this instead of a switch or multiple jacks for the different taps.

I think when I have accedently pluged my stack into the ext speaker jack, with nothing pluged into the sepaker jack, I get no sound!

I'm about to do some testing I think

The ampeg b15 has a series wired switching jack i believe. It runs an 8 ohm speaker, there is a "ext speaker" jack that is also 8 ohms, plug em both in, and the amp runs off a 16ohm tap.

[will33]

Originally Posted by hennessybass

I think when I have accedently pluged my stack into the ext speaker jack, with nothing pluged into the sepaker jack, I get no sound!

I'm about to do some testing I think

The ampeg b15 has a series wired switching jack i believe. It runs an 8 ohm speaker, there is a "ext speaker" jack that is also 8 ohms, plug em both in, and the amp runs off a 16ohm tap.

Wierd, mine works with a speaker in the ext. jack and an empty main jack. I have to open it up to fix some other stuff, can dig deeper but that won't be for a couple months at least, probably longer....too much going on right now.

[will33]

Originally Posted by hennessybass

I think when I have accedently pluged my stack into the ext speaker jack, with nothing pluged into the sepaker jack, I get no sound!

I'm about to do some testing I think

The ampeg b15 has a series wired switching jack i believe. It runs an 8 ohm speaker, there is a "ext speaker" jack that is also 8 ohms, plug em both in, and the amp runs off a 16ohm tap.

I never knew that about the B15 either. Played a couple, heard a few more, never owned one, as much as I'd like to.

[hennessybass]

OK,, testing done.

speaker in the ext speaker jack with empty speaker jack = no sound.

speaker in the ext speaker jack with a dummy 1/4" in the speaker jack = sound!

So if that set up (the dummy plug in the speaker jack) actually switches the amp to an 8 ohm load, that's awesome! Really changes up the versatility of the amp for me. I have 2 1x15 cabs - 8 ohms each. This means using my 2x15 stack, or a 1x15 without having to worry about running with a mismatched impedance.

[lastnerve]

Thanks to all for the help.

Tube amps don't work that way, you get pretty much the same power from all the different impedances it's designed to run.

So that seems to mean that the amp is going to provide it's full power to a single 8 ohm or 4 ohm cab run from the MAIN (4 ohm) output?

[hennessybass]

Originally Posted by lastnerve

Thanks to all for the help.



So that seems to mean that the amp is going to provide it's full power to a single 8 ohm or 4 ohm cab run from the MAIN (4 ohm) output?

Nope... It will provide full power to a 4ohm cab run out of the main output. It will provide less to a 8ohm cab run out of the main output.

[lastnerve]

Yeah that's what I've always heard.

So I guess will33 meant that I'd get the full power of the amp if I connected one 4 ohm cab to each of the MAIN and EXT outputs. Which would mean that somehow the tap switch to an 8 ohm load would still allow full power.

[hennessybass]

Originally Posted by lastnerve

Yeah that's what I've always heard.

So I guess will33 meant that I'd get the full power of the amp if I connected one 4 ohm cab to each of the MAIN and EXT outputs. Which would mean that somehow the tap switch to an 8 ohm load would still allow full power.

Yep.

And, I think after some testing (but don't quote me on this), you can also get full power using one 8ohm cab by plugging a dummy plug into the speaker, and an 8ohm cab in the the ext. It tricks the amp into switching to its 8ohm tap... but that might not apply to you since you have 2 4ohm cabs.

[will33]

Originally Posted by lastnerve

Yeah that's what I've always heard.

So I guess will33 meant that I'd get the full power of the amp if I connected one 4 ohm cab to each of the MAIN and EXT outputs. Which would mean that somehow the tap switch to an 8 ohm load would still allow full power.

If you're running a single 8ohm cab, plug it in the main jack and a dummy plug in the ext. jack

If running two 8ohm cabs, run one cable from the main jack to the 1st cab, and a second cable from the 1st cab to the second cab. (only one cable in the back of the amp in this case.)

If running one 4 ohm cab, plug it in the main jack.

If running two 4 ohm cabs, plug one in the main jack and the other in the ext. jack (don't chain the 2 cabs together in this case).


Those configurations will give the amp the correct impedance and the cabs the 135 watts in every case.

While you're at it, try plugging your bass into one input of the bass channel and run a patch cord from the second input on the bass channel to one of the inputs on the normal channel. That'll give you more gain/thicker sound to play with....another tonal option. It also gives you double controls of everything.

[will33]

I think the switching jack must be broken in mine. Everybody else says theirs doesn't make any sound when they plug a single cab in the ext. jack with an empty main jack either.

[will33]

Originally Posted by lastnerve

Yeah that's what I've always heard.

So I guess will33 meant that I'd get the full power of the amp if I connected one 4 ohm cab to each of the MAIN and EXT outputs. Which would mean that somehow the tap switch to an 8 ohm load would still allow full power.

Yes, that's correct. Tube amps don't do the "lower ohms = more power" thing SS amps do. When hooked up properly, for all practical purposes, they put their full power into all the different impedances they're designed to run at, in this case, 4 and 8. Some other models also do 2 or 16.

[lastnerve]

Thanks Will.

Originally posted by will33:

If you're running a single 8ohm cab, plug it in the main jack and a dummy plug in the ext. jack

If running two 8ohm cabs, run one cable from the main jack to the 1st cab, and a second cable from the 1st cab to the second cab. (only one cable in the back of the amp in this case.)

If running one 4 ohm cab, plug it in the main jack.

If running two 4 ohm cabs, plug one in the main jack and the other in the ext. jack (don't chain the 2 cabs together in this case).

I should print your connection advice and tape it to my amp.

Should a dummy plug simply be a phone jack with no connections?

While you're at it, try plugging your bass into one input of the bass channel and run a patch cord from the second input on the bass channel to one of the inputs on the normal channel.

Yeah I’ve started doing that recently and it really does add to the variety of sounds available. But, I also use a graphic EQ on my input. I don’t know if it’s the best idea, but I’ve been setting treble, mid, bass to 10, DEEP and BRIGHT on, and then using the graphic EQ to define the tone. The graphic EQ gives me somewhat better control of my sound, but I still have a good bit of experimenting to do with the amp’s EQ.

I’ve also been running the Master volume at 9 (I’m not sure how safe it is to run it at 10), and setting the preamp volumes to 2 or 3 (to minimize preamp distortion).

[will33]

Ya, use just the plug off an old bad guitar cord or something.

There's really no reason for anyone to be poking around the back of your amp while you're playing but may want to tape up the exposed part of the dummy plug so nobody accidently gets shocked, there will be speaker level juice on it.

[will33]

Originally Posted by lastnerve

Thanks Will.



I should print your connection advice and tape it to my amp.

Should a dummy plug simply be a phone jack with no connections?



Yeah I’ve started doing that recently and it really does add to the variety of sounds available. But, I also use a graphic EQ on my input. I don’t know if it’s the best idea, but I’ve been setting treble, mid, bass to 10, DEEP and BRIGHT on, and then using the graphic EQ to define the tone. The graphic EQ gives me somewhat better control of my sound, but I still have a good bit of experimenting to do with the amp’s EQ.

I’ve also been running the Master volume at 9 (I’m not sure how safe it is to run it at 10), and setting the preamp volumes to 2 or 3 (to minimize preamp distortion).

I've read different stuff on the DEEP switch. Some say it's a 10db boost at 50hz, others say it's actually a mid scoop that makes it sound deeper. Either way could get you buried in a band mix. Just so ya know...of course, do whatever sounds best.

[lastnerve]

Originally posted by will33:
I've read different stuff on the DEEP switch. Some say it's a 10db boost at 50hz, others say it's actually a mid scoop that makes it sound deeper.

Interesting! I’ve never heard that it could be a mid range scoop. It sounds to me more like the 10db boost though.
Because of this thread my amp just keeps getting more interesting (to me anyway).

From “AMPS FAQ” sticky:
Generally it’s safe to be up to 100% off on the load hooked up to a tube amplifier from where it’s selected. For a tube amp set for a 4 ohm load that would mean that you could run it into anywhere from an 8 ohm load to a 2 ohm load and still be reasonably safe.

Any experience running a 2 ohm load from a 4 ohm output? Would the amp actually be able to generate the additional power (135 x 2 = 270 watts) for a 2 ohm load? I’ve also heard that the sound quality deteriorates with a 2 ohm load.