Old Fender Bassman 100 speaker outputs - how do they work?

[moishelichtfuss]

I'm simply starting this, because I found loads of contradictory statements about the "SPEAKER" and the "EXT. SPKR" jacks on the Bassman 100.

So far I've always used my Bassman with two 2x10 cabs, the first cab connected to the Bassman's SPEAKER jack and the second cab connected to the first cab's output. The amp needs a load of 4 ohms, so I guess that's okay that way?

But then what'S the EXT. SPKR jack for and what can you connect to it or how does it influence the load?

On another note: I read somewhere that if you run the Bassmann 100 at 50 watts by removing two of the power tubes the speaker load has to be 8 ohms. Is that correct?

Thanks from a guy who can't find a manual for this amp anywhere.

 

[agedhorse]

The Bassman 100 uses a single 4 ohm output transformer tap, the load should be in the range of 4 ohms. The external speaker jack is in parallel with the internal speaker jack, the reason they have the second jack marked ext. is because the main jack incorporates the back EMF clamping circuit that's switched out when a plug is inserted.

The reason why you will need to use an 8 ohm load when removing the outside pair of tubes is because each tube pair reduces the effective source impedance, and since the transformer matches the source impedance to the load, doubling the source impedance by removing 1/2 of the power tubes doubles the necessary load so that the matching ratio remains the same. The load impedance is reflected back across the transformer to the source. If you were to design a 50 watt version of the amp, the primary impedance of the transformer would be need to be doubled to drive a 4 ohm load.

 

[Chris 'Wighat' Jordan]

So far I've always used my Bassman with two 2x10 cabs, the first cab connected to the Bassman's SPEAKER jack and the second cab connected to the first cab's output. The amp needs a load of 4 ohms, so I guess that's okay that way?

Assuming your cabs are both 8 ohms.

 

[BassmanPaul]

A further point if I may, plugging your speaker cable into the Extension jack with nothing in the Speaker jack will not work properly. In most Fender tube amps the Speaker jack shorts the output to protect the tubes from an open load. This is not good for the amp. With a load on the Speaker jack the Extension jack is in parallel.

 

[markjazzbassist]

4 ohm cab plug into INT Speaker OR 1 8 ohm cab into Int Speaker, 1 8 ohm cab into Ext Speaker.

Or one 8 ohm cab into Int Speaker, those trannies are well known for taking impedance mismatches well, but you won't get full power/volume like a 4 ohm or 2 8 ohms will.

 

[moishelichtfuss]

Assuming your cabs are both 8 ohms.

Yes. I forgot to mention that.

Thanks to all of you. So it works kind of like I thought.

So is there any difference between daisy-chaining two 8 ohm cabs to the main output and connecting an 8 ohm cab to SPEAKER and EXT. SPKR each? Probably not.

 

[agedhorse]

Yes. I forgot to mention that.

Thanks to all of you. So it works kind of like I thought.

So is there any difference between daisy-chaining two 8 ohm cabs to the main output and connecting an 8 ohm cab to SPEAKER and EXT. SPKR each? Probably not.

No, either way will work the same.

 

[moishelichtfuss]

No, either way will work the same.

Got it. Thank you again!

 

[hennessybass]

I always thought that was a weird design. I assume that fender kind of marketed and sold the Bassman heads as a piggyback combo along with a single 4 ohm cab (a 4x12 or 2x15?). So if you wanted to add another cab, it would make sense to me that you would add another 4 ohm fender cab to match the one you already had. Like the SVT... 4 or 2 ohms,, to go along with one or two of their 8x10 cabs. Just seems really strange to me.

Maybe it was cheaper / easier to manufacture with a transformer that had just the one tap?

 

[BassmanPaul]

It was more likely that Leo didn’t really consider correct matching of the load as that important. Tube amps handle a lower impedance gracefully but for best performance matching the load to the tap is best.

 

[hennessybass]

It was more likely that Leo didn’t really consider correct matching of the load as that important. Tube amps handle a lower impedance gracefully but for best performance matching the load to the tap is best.

Makes sense. I've always heard that Fenders are very forgiving when it comes to impedance matching.

 

[Jazzy Jeff]

The Bassman 100 uses a single 4 ohm output transformer tap, the load should be in the range of 4 ohms. The external speaker jack is in parallel with the internal speaker jack, the reason they have the second jack marked ext. is because the main jack incorporates the back EMF clamping circuit that's switched out when a plug is inserted.

The reason why you will need to use an 8 ohm load when removing the outside pair of tubes is because each tube pair reduces the effective source impedance, and since the transformer matches the source impedance to the load, doubling the source impedance by removing 1/2 of the power tubes doubles the necessary load so that the matching ratio remains the same. The load impedance is reflected back across the transformer to the source. If you were to design a 50 watt version of the amp, the primary impedance of the transformer would be need to be doubled to drive a 4 ohm load.

I know it is an old thread but...
I have pics of two Bassman 100 heads and on the back panel one says Total load 8 ohms and on another amp it says Total load 4 ohms. Both are labelled under the two speaker out jacks. Can someone tell me why this is the case and stop me from going mad?

 

[agedhorse]

I know it is an old thread but...
I have pics of two Bassman 100 heads and on the back panel one says Total load 8 ohms and on another amp it says Total load 4 ohms. Both are labelled under the two speaker out jacks. Can someone tell me why this is the case and stop me from going mad?

Was one amp originally a combo and the other a head?

Which version of Bassman 100?

 

[Jazzy Jeff]

Was one amp originally a combo and the other a head?

Which version of Bassman 100?

Both were heads, from the 72 to 77 Silverface model. Is that enough?

 

[Wasnex]

Both were heads, from the 72 to 77 Silverface model. Is that enough?

Are you sure they are both 100s. AFAIK Bassman 100 and the Super Bassman had 4 ohm OTs. You can check the code on the transformer to confirm.

The Bassman 135 had two output taps. If you use the Speaker out the output is set for 4 ohms . If you use both the Speaker out and the EXT, the output is set to 8 ohms and the two jacks are connected in series.

 

[Jazzy Jeff]

Model number: CFA 7100

 

[Jazzy Jeff]

Are you sure they are both 100s. AFAIK Bassman 100 and the Super Bassman had 4 ohm OTs. You can check the code on the transformer to confirm.

The Bassman 135 had two output taps. If you use the Speaker out the output is set for 4 ohms . If you use both the Speaker out and the EXT, the output is set to 8 ohms and the two jacks are connected in series.

Yep, sure. Just can't be bothered posting the two pics.

 

[Jazzy Jeff]

Are you sure they are both 100s. AFAIK Bassman 100 and the Super Bassman had 4 ohm OTs. You can check the code on the transformer to confirm.

The Bassman 135 had two output taps. If you use the Speaker out the output is set for 4 ohms . If you use both the Speaker out and the EXT, the output is set to 8 ohms and the two jacks are connected in series.

Are you sure the jacks are in SERIES? Do any of the two outputs have the jack that can cancel the jack beside it when a lead is plugged in? Savvy? See the following pics for model number.

 

[Jazzy Jeff]

So here are the two amps in question.

 

[beans-on-toast]

Sometimes panel mistakes happen. Some reproduction panel set are available, I’ve used them for builds/restorations. I would want to examine the output transformer part number and spec sheet to determine what tap is used and how it is wired in the amp. You never know what you are getting with a vintage amp and what mods were performed. Don’t assume anything, an attention to detail is important.