Bassman 135 questions

[toobalicious]

Also, if you mismatch impedance upwards in a tube amp, you get MORE than rated power, but at the expense of stability.

also this is against my understanding, which is that the mismatch up increases voltage, but due to the increased resistance, *less* power actually hits the voice coil.
from "london power"

A nominal 8-ohm load may reflect 4k to the plates of the output tubes with a given transformer. The amp might be designed to produce its maximum power into this load, with a designed frequency response. This is the "power bandwidth". If we change the load to 16-ohms, the reflected load doubles and the frequency response shifts upward. We lose bass but have a brighter sound, and also lose power. If we change to a 4-ohm load, the reflected impedance drops to 2k, into which the tubes produce less power, and the bandwidth is again narrowed.

The reason for the confusion, I believe, is that people think tubes will try to behave the same way transistors do. Into half the load impedance, a transistor will try to deliver twice as much current. The device may overheat and destroy itself in the process. Tubes, however, simply don't behave like transistors.

 

[Psycho Bass Guy]

If you'll notice, Kevin O'Connor never says why this is supposed to be the case, just that according to him, it is. Simple physics: when the primary voltage increases from the lower secondary current draw, so does the the secondary voltage since it is easier to produce voltage across a higher resistance than current. Higher output voltage = more power (until the tubes or the OT arc over )

 

[willie1968]

I see two jacks and they are both 4 ohm (not one at 4 and one at 8) When you plug a second speaker of 4 ohms into the extension jack then the impedance would be 2 ohms. I have never heard of plugging a speaker into the extension jack without a plug inserted in the main jack first. Where are you seeing 8 ohms in your schematic?

What bassman do you have? This is one of the main differences of the "super bassman" and the other bassmen.
The bassman 135 has an ultra linear output transformer with two taps. One for 4 Ohm one for 8 Ohm.
The output jacks are some special versions with swtiches.
If you plugin only one cab into the speaker output, the 4 Ohm tap is used.
If you plugin only one cab into the ext. speaker, the 8 Ohm tap is used.
If you use both jacks, the cabs will be wired in series and the 8 Ohm tap is used.
Attached you can see the output stages of a bassman 100 and a bassman 135.

 

[insane bass]

I have a Fender Bassman 135, not super, it was manufactured in the later weeks of 1970 (if that helps). But i checked the back and they both said 4 ohms. I plugged it in anyways and i got alot of muffled sound and not much louder at all. Probably a case of a crappy cab im thinking, not to good.
Gallien Krueger Backline 4x10BLX-II.
Probably not getting much good sound.
Could ALSO be that the tubes are old and need to be replaced.
(just my thoughts i dont know if im right though, correct me if im wrong which i probably am .)

 

[willie1968]

That's korrect, both jacks say min. 4 Ohm load. Thats how fender try simplify there cuircut. Muffeld sound could be from everywhere. Speakers, bad tubes in pre or poweramp, bad cabs, especially the cathods elkos in the preamp, and many more causes. Without checking the amp and the cab, no one could say, whats wrong.

 

[insane bass]

Yeah ill just change out the tubes once i get the money haha. But who knows what it is, but in the 4 ohm socket although its not muffled at all.

 

[chicago_mike]

If I remember correctly the output tranny is ultralinear on a 135 yes?

 

[will33]

Can't find my emails from fender customer service or I'd paste them here. I had the same question about ohm loads and the tap. They basically said it's fine to run either 8 or 4ohms, doesn't have to be 4, just don't go to 2. I don't know about 16, I don't have any 16ohm cabs. Just FYI, I've been running it for bass with a single 8ohm 212 and it keeps up playing old R&B with a sane drummer and a little guitar combo like a Blues Jr. or something. This is about as much as I've been able to get from it and stay "warm but relatively clean". I'm not a big fan of the overdrive sound on these for bass. I'm re-doing this 212 in a better cab and using it in other areas, the bassman is getting a pair of 8ohm 15's. That makes the 4ohm load with a little more cone surface. The higher SPL drivers you can get for it the better as it's low power for bass. I've been using it for guitar with a pair of 8ohm cabs also but this is an incredibly loud rig. The future calls for a single 4ohm 212 for this application. The speaker outs on mine are both labeled 4ohm but I can run from the ext. out without plugging into the main out. I run with the channels jumped but change preamp tubes depending on the application. For guitar it gets 2 - 12ax7's, for bass either 2 - 12at7's or one 12ay7 and one 12at7. That seems to get more clean volume, if you want overdrive, leave the ax7's in there.


The emails I got from fender were rather vague, had to re-ask a couple questions, maybe it was some guy who only knew about the new line of gear or something. I'd be listening to PsychoBassGuy regarding this. I'll also be following this thread just to make sure I'm doing it right.

 

[will33]

They're not hooked up in series. There is a switching jack in the extension like the SVT.

This makes me wonder if mine's not working properly or if it was modded somewhere along the way. I can use my ext jack WITHOUT anything plugged in the main speaker jack.