Music Man HD-130 amp and B-15 cab - Series wiring question added!

[Psycho Bass Guy]

In my glorious ignorance, I have to say, I don't recall ever seeing series jacks on the back of an amp.  They must have had a good reason, which we'll probably never know.

It was a way to use a higher output (read cheaper) impedance transformer.

[drbassman]

I knew the answer would be over my head!!!! 

[Granny Gremlin]

Oh, I see.  Even better I think.

That only applies if the the load (all cabs together) matches the output impedance setting (transformer tap used) on the amp.  If you ran those 2 8 ohm cabs in series with the amp set to 8 ohms, then it would be putting out half it's rated power due to that mismatch.

It was a way to use a higher output (read cheaper) impedance transformer.

Honest question: how does parallel vs series jacks affect that at all (given identical output impedance options, output wattage, and load)

[Psycho Bass Guy]

That only applies if the the load (all cabs together) matches the output impedance setting (transformer tap used) on the amp.  If you ran those 2 8 ohm cabs in series with the amp set to 8 ohms, then it would be putting out half it's rated power due to that mismatch.

Only the jacks on the amp itself are series-connected. Cab-to-cab connections are still parallel. If you put a 16 ohm load across the 8 ohm tap, the net result would NOT be less power, but more, at the risk of power tube instability, which in those amps is already skirting Class B at around 700 volts on the tube plates; the design gives more power than a conventional amp, but requires very high quality output tubes.

Honest question: how does parallel vs series jacks affect that at all (given identical output impedance options, output wattage, and load)

The lower the secondary load, the more wire is physically required in the transformer to induce a current at that impedance. Putting the output jacks in series effectively limits the amount of current the transformer will be asked to supply.

[Granny Gremlin]

Only the jacks on the amp itself are series-connected. Cab-to-cab connections are still parallel.

Yes, which is why, in my original post in this thread, I recommend the exact same solution you later did.

If you put a 16 ohm load across the 8 ohm tap, the net result would NOT be less power, but more

You wanna double check that? Am I missing something? I've been working off of the (corroborated)  assumption that increasing the load (without changing the output transformer tap) lowers output wattage and vice versa.

To use a terrible example (SS amp not tube, no tranny but also no selectable output Z) because this sort of spec is somewhat irrelevant for Tube amps with selectable output taps:

The lower the secondary load, the more wire is physically required in the transformer to induce a current at that impedance. Putting the output jacks in series effectively limits the amount of current the transformer will be asked to supply.

But current is a function of load and output voltage, which are (or can be, given the assumption I requested) constant whether or not the jacks are series or parallel (unless you mean that by making them series you somewhat impede, though not eliminate, the user's ability to attach a lower load... which seems moot anyway because either the amp handles a 4 ohm load or it doesn't). Are you saying that this is just a way they don't have to worry about someone trying to attach a 2 Ohm load (either by accident or because they think 'close enough')  and therefore the tranny doesn't have to be as beefy?  Becasue that I understand, but is, IMHO, a stupid business decision (especially given the fact that Leo never really used very good output trannies to begin with; that was part of the original Fender sound - easy saturation).



.... yeah, I'm one of those knows-enough-to-be-annoying people

[Pilgrim]

In my glorious ignorance, I have to say, I don't recall ever seeing series jacks on the back of an amp.  They must have had a good reason, which we'll probably never know.

That sounds like a German-made product. They always design "correctly", no matter how obscure their good reason is to the rest of the world. Having worked on Audis and BMWs, I have learned something about this mindset. You don't question it, you just nod and enjoy it.

[Psycho Bass Guy]

Yes, which is why, in my original post in this thread, I recommend the exact same solution you later did.

I generally ignore the "advice" you give until you begin arguing with me. I have long since abandoned my mission to "educate" the public at large about amps as it has been made abundantly clear to me that I often do more harm than good. If people want to follow the Pied Piper; I'm not going to stop them.

You wanna double check that? Am I missing something? I've been working off of the (corroborated)  assumption that increasing the load (without changing the output transformer tap) lowers output wattage and vice versa.

I have tried to explain the issue of tube power and secondary impedance to you too many times already. I have no interest in wasting any more time attempting to do so. Use the search function. Look up terms like reflected impedance.

But current is a function of load and output voltage, which are (or can be, given the assumption I requested) constant whether or not the jacks are series or parallel (unless you mean that by making them series you somewhat impede, though not eliminate, the user's ability to attach a lower load... which seems moot anyway because either the amp handles a 4 ohm load or it doesn't).

The minimum load for the HD-130 is 4 ohms. Most similar bass amps of the era went down to 2 (solid state as well).

Are you saying that this is just a way they don't have to worry about someone trying to attach a 2 Ohm load (either by accident or because they think 'close enough')  and therefore the tranny doesn't have to be as beefy?

The OT has to be able to handle the operational voltages of the tubes without arcing, which requires serious lamination for audio frequencies at the voltages Music Man amps employ. If it were to be made 2 ohm capable, it would be near the size and weight of an SVT OT, which for the power level, doesn't make sense and would more than double the cost of the OT.

Becasue that I understand, but is, IMHO, a stupid business decision (especially given the fact that Leo never really used very good output trannies to begin with; that was part of the original Fender sound - easy saturation).

That's another bullshit myth that has gained traction from continuous quoting. Fender transformers were quite capable. They were, after all simply adapted RCA PA/HiFi designs with integrated preamps.

.... yeah, I'm one of those knows-enough-to-be-annoying people

Revelling in it is not endearing you to me. "Arguing" questions instead of asking them doesn't impress me. It's annoying and it confuses others.

[Pilgrim]

Guys, can you both please settle for giving us the practical info and pass on arguing about how many angels can dance on the head of the pin?

[Psycho Bass Guy]

Guys, can you both please settle for giving us the practical info and pass on arguing about how many angels can dance on the head of the pin?

This is exactly what I am talking about. GG HAS NO IDEA WHAT HE'S TALKING ABOUT, but because he uses lots of technical terms and fancy words, it ends up looking like he does and thus, spawns comments like this. CG has very little practical info to give. I am perfectly content to allow anyone to hold any opinion they like; facts, however, are not subjective. If you want me to keep the amp forum running and free of the detritus that infests most other bass boards, that means that factual standards must be respected. If that's too much to ask, find someone else.

[drbassman]

OK, all of my amp questions have been answered.  We can move on now.  Thanks!