Helix and (real) Marshall 1960A 4x12 Cab … thinking how to …

[Optimist]

Hi everybody,

I was thinking about how to connect my helix with my old 1960A cabinet, just for fun.

I would be like this, right?: Helix (with any amp simulation ON, FX etc., but any cab sim OFF) > ??? > 1960A.

The »???« would be what? Kind of a power amp of course.

I have a spare 2 x 100W power amp from an old PA somewhere in the basement (no idea about the ohm settings).

Would this work? Or would I blow something?

 

Thanks in advance

Opti/Alex

[soblivion]

It would most likely work.

 

It's really going to depend on the amp. I have a 1960a in storage, and if I recall correctly it can be run in stereo at 8 ohms. It also can be run at 4 or 16 ohms.

 

This gives you alot of options.

[HonestOpinion]

Hi everybody,

I was thinking about how to connect my helix with my old 1960A cabinet, just for fun.

I would be like this, right?: Helix (with any amp simulation ON, FX etc., but any cab sim OFF) > ??? > 1960A.

The »???« would be what? Kind of a power amp of course.

I have a spare 2 x 100W power amp from an old PA somewhere in the basement (no idea about the ohm settings).

Would this work? Or would I blow something?

 

Thanks in advance

Opti/Alex

 

I would start with the volume way down on the power amp as it was designed for PA not a guitar cab. Make sure the power amp is not in "bridged" mode as this would probably be more power than you want going into the cab. If you can find it online or better yet know how to test with a multimeter try to figure out the ohm rating for the output(s) of your amp and and the input(s) of your 1960A cab. You might want to keep in mind that if you do have to go with an impedance mismatch it is always safer for the cab and especially the power amp to have the higher ohm setting be on the cab, NOT the power amp. For example 8ohm output from the power amp into 16ohms on the cab is preferable to doing the reverse. The higher resistance on the cab keeps the power amp from having to work too hard and potentially burn something. It will also be easier on the cab. With that said, running too extreme an ohm mismatch in either direction can damage the amp or speaker. Also, some equipment is more forgiving than others about impedance mismatches, especially if the ohm ratings are close to each other, regardless of whether it is the amp or the cab that has the higher ohm rating. 

 

You may also find that you can get away with using some of the Helix cab sims even though you are already going through a Marshall cab. Not a hard and fast rule there.

[Optimist]

Thanks guys, I’ll have a try and will report what happened.

Anyway I also still have the JCM800 head (2210, 100W)) … perhaps this will be another option by going thru the FX return (bypassing the preamp).

I just was wondering how to feed the 1960A without it (since the 2210 is an old lady).

Thanks anyway,

Opti/Alex

[brue58ski]

I would start with the volume way down on the power amp as it was designed for PA not a guitar cab. Make sure the power amp is not in "bridged" mode as this would probably be more power than you want going into the cab. If you can find it online or better yet know how to test with a multimeter try to figure out the ohm rating for the output(s) of your amp and and the input(s) of your 1960A cab. You might want to keep in mind that if you do have to go with an impedance mismatch it is always safer for the cab and especially the power amp to have the higher ohm setting be on the cab, NOT the power amp. For example 8ohm output from the power amp into 16ohms on the cab is preferable to doing the reverse. The higher resistance on the cab keeps the power amp from having to work too hard and potentially burn something. It will also be easier on the cab. With that said, running too extreme an ohm mismatch in either direction can damage the amp or speaker. Also, some equipment is more forgiving than others about impedance mismatches, especially if the ohm ratings are close to each other, regardless of whether it is the amp or the cab that has the higher ohm rating. 

 

You may also find that you can get away with using some of the Helix cab sims even though you are already going through a Marshall cab. Not a hard and fast rule there.

 

This is HUGE. I learned this one the hard way. Only popped a fuse but it could have been worse.

[cruisinon2]

The higher resistance on the cab keeps the power amp from having to work too hard and potentially burn something. It will also be easier on the cab.

Huh? Providing more resistance than the amp is expecting results in a loss of power and volume. The amp has to do more work to achieve a given volume, not less...because it's now effectively pushing up-hill. You probably won't damage anything, however...I ran an 8 ohm power amp into a 16 ohm 4x12 for years without issue, there's just not as much headroom as you'd have from that same amp running into a cab with matched impedance.

[spikey]

I ran an 8 ohm power amp into a 16 ohm 4x12 for years without issue,

 

 

And that's the way its supposed to work.  Like Cru stated, As long as the impedance of the amp is less than the Cabinet (and your cab/speaker can handle the wattage) you won't hurt the amp, but a closer match gives you better efficiency/output.

[brue58ski]

Huh? Providing more resistance than the amp is expecting results in a loss of power and volume. The amp has to do more work to achieve a given volume, not less...because it's now effectively pushing up-hill. You probably won't damage anything, however...I ran an 8 ohm power amp into a 16 ohm 4x12 for years without issue, there's just not as much headroom as you'd have from that same amp running into a cab with matched impedance.

 

Yeah, that's what I thought. Then I burned up an amp. The water/pipe analogy is the best for this.

 

You have a pump pumping water into a pipe. This pump regulates however many RPMs it needs to keep the pipe flowing at it's capacity (the pipe's capacity) by itself. The less water a pipe allows to flow, the less RPMs the pump has to do to keep it flowing as efficiently as that pipe allows.. If the pipe is small (i.e. more resistance) the pump doesn't have to work at as high an RPM to pump the water into that pipe since the pipe doesn't allow as much water. An amp is the same, the higher resistance doesn't allow as much power to go to the speaker so you get less volume. The amp then doen't work as hard because it's not being allowed to create as much power. As the pipe gets bigger (i.e. less resistance) it has to pump at higher RPMs to keep the water flowing at the most efficient flow rate. The pipe could get so big that the pump would burn itself out trying to keep the pipe flowing. This is an illustration of  what your amp tries to do. I hope this makes sense. It took me awhile to "get it" even with this analogy.

[HonestOpinion]

Huh? Providing more resistance than the amp is expecting results in a loss of power and volume. The amp has to do more work to achieve a given volume, not less...because it's now effectively pushing up-hill. You probably won't damage anything, however...I ran an 8 ohm power amp into a 16 ohm 4x12 for years without issue, there's just not as much headroom as you'd have from that same amp running into a cab with matched impedance.

 

You are right about there being a potential difference in the sound when running into a cab with a higher impedance than the rated impedance output on the amp, but this generally does not damage the amp unless the impedance is grossly mismatched. It would seem on the face of it that if the cab has too low an impedance rating than the amp would need to push less hard and do less work but that is not the way it works.  Even though it is not intuitive, generally running into too low an impedance is what is more dangerous as it actually makes an amp work too hard and is more likely to cause an amp failure. When the cab does not have a high enough resistance(ohms) the amp just keeps pushing and that is when you can have something burn up. The scenario you ran for years (an 8ohm amp into a 16ohm cab) is perfectly fine with most amps, the mismatch is in the right "direction", and is generally safer than for instance running a 16 ohm output on an amp into a 4ohm cab(s). As stated though, some equipment is more tolerant than others to mismatches and will result in zero damage to speaker or amp . Maybe an electrical engineer on the forum could explain this better but when an amp does not see enough resistance in the speaker cab it keeps working harder rather than less and is more prone to damage. Goes against common sense but there it is.

[HonestOpinion]

Yeah, that's what I thought. Then I burned up an amp. The water/pipe analogy is the best for this.

 

You have a pump pumping water into a pipe. This pump regulates however many RPMs it needs to keep the pipe flowing at it's capacity (the pipe's capacity) by itself. The less water a pipe allows to flow, the less RPMs the pump has to do to keep it flowing as efficiently as that pipe allows.. If the pipe is small (i.e. more resistance) the pump doesn't have to work at as high an RPM to pump the water into that pipe since the pipe doesn't allow as much water. An amp is the same, the higher resistance doesn't allow as much power to go to the speaker so you get less volume. The amp then doen't work as hard because it's not being allowed to create as much power. As the pipe gets bigger (i.e. less resistance) it has to pump at higher RPMs to keep the water flowing at the most efficient flow rate. The pipe could get so big that the pump would burn itself out trying to keep the pipe flowing. This is an illustration of  what your amp tries to do. I hope this makes sense. It took me awhile to "get it" even with this analogy.

 

Great analogy, this helped me wrap my head around why ohm mismatch rules seem to work so counter intuitively.

[cruisinon2]

Perhaps I'm mistaken, but I think where were differ is the definition of what "working harder" means as far as the amp is concerned...

 

I get the water analogy, and I understand that you can use a cabinet with a higher impedance rating than the amp without damaging anything... as I said, I did exactly that for many years. However, practically speaking, if one were to take an amp with 8 ohm outputs and plug it into an 8 ohm cab, followed by a 16 ohm cab, you'll find it necessary to crank it up more with the higher impedance cabinet to achieve the same volume that you had when everything matched. So just for giggles, if I have to turn the amp up to "7" with a 16 ohm load in order to achieve the same volume that I previously had at "5" with an 8 ohm load, then it would seem to me that the amp is working harder to drive the higher impedance cabinet, no?

[HonestOpinion]

I think where were differ is the definition of what "working harder" means...

 

I get the water analogy, and I understand that you can use a cabinet with a higher impedance rating than the amp without damaging anything... as I said, I did exactly that for many years. However, practically speaking, if one were to take an amp with 8 ohm outputs and plug it into an 8 ohm cab, followed by a 16 ohm cab, you'll find it necessary to crank it up more with the higher impedance cabinet to achieve the same volume that you had when everything matched. If I'm turning it up to "7" with a 16 ohm load in order to achieve the same volume that I previously had at "5" with an 8 ohm load, then it would seem to me that the amp is working harder to drive the higher impedance cabinet, no?

You make a good point and this is why the mismatch rules appear to violate common sense until you apply the water&pipe analogy. Perhaps it is a matter of degree and "how much harder" the amp is working. Too low an impedance on a cab can be kind of like setting up your cab for a concert on a cliff side. You go to move your cab two feet expecting it to weigh 50lbs and it only weighs 3lbs. Both you=amp and the cab go over the cliff from too strong a push.

[spikey]

Common sense would seem to dictate it works this way but it is actually the reverse. If you have to go with a mismatch it is generally safer to run a lower amp ohm rating into a higher impedance on the cab. Brue58ski explains it perfectly above.

 

 

 

For the record, that's what I was saying too... Please re-read it.

 

I see it this way (and many are saying the same things  :P ) . The way it works in reverse (the amp's tube outputs seeing a short, rather than the correct load) is much worse on the amps output section, believe me. A short to an amp can be a cabs 4-ohm load when the amp is looking for an 8-ohm load. The overcurrent thru the amp's output (when the amps output is mismatched in less a load than it needs) is when you eventually blow up the output section on your tube amp. OTOH, If you're running a 16-ohm cab and the amp is looking for a 4-ohm load the only thing that should happen (as long as the amp wasn't about to blow up in the 1st place and the 4x12 cab is working properly) is that it will not be as loud as it would be if the amps output impedance is matched with its cab. Less power transfer= less noise/output.  In other words, the power from the amp is being used trying to push wattage thru too big an impedance (resistance and reactance to current flow) for the amp and not utilizing the speaker (load) properly.

[jbuhajla]

Over current is the killer of the amp. In basic ohms law sense, the less resistance (impedance) the higher the current. If the power amp is expecting an 8 ohm load, but you are running a 4 ohm speaker cabinet, your current doubles. It is like trying to run a bunch of hair dryers from one circuit in your house. Your breaker will trip. If your speaker cabinet is not fuse protected, something is going to smoke and not work any more. 

 

(disclaimer- the above is a basic description of ohms law in DC terms not AC. You electronics nerds out there, don't get your undies in a twist)

[cruisinon2]

For the record, that's what I was saying too... Please re-read it.

 

Everybody is saying the same thing...its just become a semantics and nifty analogy festival. ;)

[HonestOpinion]

Everybody is saying the same thing...its just become a semantics and nifty analogy festival. ;)

 

A "nifty analogy festival" is like a Renaissance Fair without the medieval costumes, or giant turkey legs, but with the jousting.  Learned that one at the "Simile Tent" workshop at the festival  ;)

[Optimist]

Hey guys,

great discussion here, thanks for all the info!

Anyway I made some mistakes in my first posting.

The cab is NOT a 1960A. It’s a »JCM800 Lead 1960«, 300 watts, mono and 16 ohm.

The power amp is »t.amp TA1400 MKII« with 2 x 700 watts on 4 ohm (the internet says it can also do 2 x 450 watts on 8 ohm, but I can’t seem to find a switch for that *).

Here’s a pic of its backside:

 

So again the question:

Will this work in any way?

If yes, I would need a cable like this: https://www.thomann.de/gb/pro_snake_bassamp_speakercable_15_m.htm … correct?

What would the switch be set to? »BRIDGED«, »PARALLEL« or »STEREO«? *) Maybe this is also the switch for 700/450 watts?

 

Thanks in advance,

Opti/Alex

[cruisinon2]

Hey guys,

great discussion here, thanks for all the info!

Anyway I made some mistakes in my first posting.

The cab is NOT a 1960A. It’s a »JCM800 Lead 1960«, 300 watts, mono and 16 ohm.

The power amp is »t.amp TA1400 MKII« with 2 x 700 watts on 4 ohm (the internet says it can also do 2 x 450 watts on 8 ohm, but I can’t seem to find a switch for that *).

Here’s a pic of its backside:

 

So again the question:

Will this work in any way?

If yes, I would need a cable like this: https://www.thomann.de/gb/pro_snake_bassamp_speakercable_15_m.htm … correct?

What would the switch be set to? »BRIDGED«, »PARALLEL« or »STEREO«? *) Maybe this is also the switch for 700/450 watts?

 

Thanks in advance,

Opti/Alex

Is that a two or three position switch? If it's 3, I've never seen that before. Usually it's just "bridged mono" vs. "stereo", bridged mono sending the power from both channels to a single output (usually channel A), and stereo with both channels and outputs functioning independently. If "stereo", "parallel" and "bridged" are all separate options, I don't know what to make of it...seems odd.

 

I'm assuming from the specs I looked up that in this case:

Bridged = 2×700W @ 4 ohms, and

Parallel = 2×450W @ 8 ohms.

 

That's all that's listed...

 

There's no impedance selector switch, so you're stuck with it as is. They're only providing power ratings for 4 and 8 ohm loads...so if you're using a 16 ohm cab, you can expect to get considerably less power than the claimed 450W @ 8 ohms, but what the power output will actually be with a 16 ohm load, I don't know...and they're not saying. Will it work? Yes, sound will come out. Will you damage anything? Depends on what the amp is actually pushing out @ 16 ohms. It's a 300W cab, so as long as you're not exceeding that, it's fine... but unfortunately that's the unknown in the equation. Matching impedance is always easier...then there's no guesswork.

[HonestOpinion]

Hey guys,

great discussion here, thanks for all the info!

Anyway I made some mistakes in my first posting.

The cab is NOT a 1960A. It’s a »JCM800 Lead 1960«, 300 watts, mono and 16 ohm.

The power amp is »t.amp TA1400 MKII« with 2 x 700 watts on 4 ohm (the internet says it can also do 2 x 450 watts on 8 ohm, but I can’t seem to find a switch for that *).

Here’s a pic of its backside:

 

So again the question:

Will this work in any way?

If yes, I would need a cable like this: https://www.thomann.de/gb/pro_snake_bassamp_speakercable_15_m.htm … correct?

What would the switch be set to? »BRIDGED«, »PARALLEL« or »STEREO«? *) Maybe this is also the switch for 700/450 watts?

 

Thanks in advance,

Opti/Alex

That does look like the right cable from Thomann (NL4 --> 1/4). But I would add this caveat, apparently although NL2 jacks have a standard wiring scheme, NL4 do not. Here is a warning from one site regarding the use of NL4 jacks. "If either your amplifier or speaker cabinet are equipped with NL4 type Speakon jacks, it's naive to assume the wiring scheme. To construct a proper cable, the panel jacks must be positively identified as being NL4 type and the pin-out configuration of both the amplifier NL4 jacks and speaker NL4 jacks must be known.". See this link: http://www.audiopile.net/Technical_Library/speakon_explained.htm

The manual for your amp does have the pin assignment diagram for their NL4 connectors. You might want to provide that to Thomann and verify that their cable wiring matches that and your Marshall. https://images.thomann.de/pics/prod/c_156554ff_en_online.pdf

I don't think there is an external switch on this amp to change the ohms at the output.

Cruison2 is right about it being difficult to predict the wattage output into a 16ohm input because not only is the 16ohm rating not included but you would expect it to be roughly half of the wattage into an 8ohm input. The spec listed in the manual has 700watts into 4ohms and 450watts into 8 ohms which seems a little strange. I would have expected to see the 700watts rating roughly halved to 350watts at 8ohms. I defer to the experts out there to explain why this amp's output wattage may be rated this way.

 

Here is the blurb from the manual regarding parallel, stereo, or bridged operation:

STEREO | PARALLEL | BRIDGED switch Switch to select the operating modes ‘STEREO’ (both channels operate independently of each other), ‘PARALLEL’ (the inputs of both channel are interconnected) and ‘BRIDGED’ (both channels are interconnected to form one channel with double output power)

In your case where you are just hooking up the Helix you could select either the "Stereo" or "Parallel" mode ( Helix --> Chan. A input, Chan. A output --> Marshall cab). It probably makes more sense to select "Stereo" even though you are not running out to two cabs but either setting will probably work given your intended use. Just DON'T use "Bridged" mode.

The "Parallel" mode in essence acts almost like a simple onboard mixer/combiner for the power amp. If you wanted to plug in both the Helix and an Ax8 for example you could use the Chan. A input for Helix, Chan. B input for the Ax8, select parallel mode, and the output from both the Line6 and the Fractal would be combined and appear at both the Chan A and Chan. B outputs giving you the option to select either channel's output for connection to your Marshall cab.

The main difference on the parallel versus stereo setting is that in "Stereo" each channel will act independently. So anything plugged into Channel A will be output only to the A output and anything into the Channel B input will be output only to Channel B. If you use the amp in "Parallel" mode anything plugged into the Channel A or Channel B inputs will be combined and output to both the Chan. A and the Chan. B outputs.

Last but not least just wanted to reiterate I would have the volume for both channels way down low before trying this out. Good luck!

[CraigGT]

Speakon plugs generally use grub screw and allen key terminals so correcting the pinouts shouldn't be much of a problem if they're wrong.

 

Craig

[Optimist]

Dear HonestOpinion and all,

this was very helpful, thank you very much!

I’ll order the cable and will report what happened.

Thanky again,

Opti/Alex

[Rloewenherz]

Sorry to jump back to the earlier discussion about impedance mismatches, but i feel i should add my .2 on this.
 
First, i would highly recommend not to use the water/pump analogy on amplifiers, since the reality is much more complex. It is only a good tool for passive circuits.
I will focus on the example of vacuum tube amplifiers since they are still very common in guitar related applications, but pretty much all of it translates to solid-state amps as well.
 
Most power amps (pretty much all tube and many if not most transistor based ones) have an output transformer.
A mismatched load impedance will not only affect the current through the power stage components, but also the voltage across the Transformer primary widnings.
Especially in the case of a vacuum tube amplifier, this is a much more immediate concern than the current draw.
The thermal time constants of the tubes, transformer and all the passive components are very high.
So even if you operate them in overcurrent for quite some time they wont get damaged that easily. An output tube will actually start glowing red (redplating) for some time before it dies. To cook an output transformer you really need to push it. You will usually smell something is off before the insulation melts.
 
The over-voltage faults are not so forgiving however. The a higher load impedance (than matched) will increase the voltage swing across the transformer primary significantly.
(In an idealized case of an ideal pentode, transformer and infinite impedance the votlage swing would become infinite for any non zero gate signal. And yes, the voltage can exceed the supply voltage because of the inductive behaviour of the transformer)
Over-voltage on the transformer will cause arcing between the infividual windings that will damage the insulation, resulting in shorted paths in the windings which if bad enough will ruin your output transformer. It can also cause arcing between output tube grids in case of a vacuum tube amplifier. I usually worry about the output transformer first, since it is the most expensive component in the power stage.
 
That said, semiconductor devices like MOSFETs or BJTs are usually more susceptible to over-current than vacuum tubes (due to shorter thermal time constants and low maximum junction temperature), therfore it is more critical to match the impedance in a solid state amplifier, than it is in a vacuum tube one.
 
Besides the danger of faults, a mismatched impedance will also alter the sound characteristics of your amplifier. For example, it can cause earlier and asymmetric clipping.
 
Sorry if this got long and technical, the tldr is this:

• Match your load impedance!
  

-> If you cant:

• Vacuum tube amplifier: use a lower load impedance (within reason!) and be careful with the volme settings, monitor the output tubes for redplating and keep your nose sharp for the smell of something cooking. (its usually safe though)
• Solid state amplifier: only use mismatched impedance if you like living on the edge!
  

Hope this is helpful to someone. :D
 
Cheers,
Rolf
 
PS: typed this in a bit of a rush at work, may edit some mistakes later. :rolleyes:

[brue58ski]

Sorry to jump back to the earlier discussion about impedance mismatches, but i feel i should add my .2 on this.

 

First, i would highly recommend not to use the water/pump analogy on amplifiers, since the reality is much more complex. It is only a good tool for passive circuits.

 

 

I would agree. I meant to point out that it really only represents one aspect of a more complex process (can you say reactance). It was only a quick and easy way to explaing why the impedance mismatching works the opposite of the way it seems like it should. Having said that, I have often mismatched a lower ohm guitar amp to a higher ohm cab and have never had any issues other than a loss of power. But, having said THAT, matching impedances is the way to go and running mismatched impedances is something you do when you have no choice and will correct soon.

[HonestOpinion]

Some great and scholarly comments on this thread and I am not surprised that there is so much debate as there are a lot of conflicting opinions out on the web regarding this subject. Wanted to point out that the OP's amp is solid state. The advice that states matching the impedance correctly to the speaker is the safest way to go is doubtless correct. Almost everything I have read and my own personal practice is that if you do have to go with an impedance mismatch, particularly with a solid state amp, a higher impedance on the speaker is the safer way to go. Can Mr. Tessla please weigh in already and no I am not referring to Elon Musk.  :)

Regarding tube amps, after reading the posts here and elsewhere I find myself actually less sure than I have always been that a higher impedance is safer when you have to setup a tube amp and speaker with an impedance mismatch. I would say the preponderance of web posts and articles on this topic indicate that in situations where an impedance mismatch is the only option, a higher impedance on the speaker (relatively close to the rated output on the amplifier) is better on both tube and solid state amps. However, there are also a fair number of well written posts by seemingly knowledgeable sources that seem to indicate that in the case of tube amps, a lower impedance on the speaker is better.  I still don't feel as if I have located a totally definitive answer on this subject.

Another good post on this subject from TDPRI.com and this one seems to indicate that a lower impedance on a mismatch is preferable if using a tube amp and a higher impedance on the speaker is better when using a solid state amp. Sheesh, I was way less confused before I "educated" myself on this subject.: http://www.tdpri.com/threads/speaker-impedance-mismatch.197323/#post-2286808

[spikey]

I'm curious... Given that a short causes higher current flow and resistance causes less, I would like to ask why it is that the primary side of an amps output transformer can have higher voltage swings (and thus more current fluctuation) when using a higher impedance load. Because of the higher output load (Example= 8 ohm amp hooked to a 16-ohm cab), wouldn't you then draw less current thru the "secondary" of the same output transformer? Please explain.

[Rloewenherz]

... I would like to ask why it is that the primary side of an amps output transformer can have higher voltage swings (and thus more current fluctuation) when using a higher impedance load. Because of the higher output load (Example= 8 ohm amp hooked to a 16-ohm cab), wouldn't you then draw less current thru the "secondary" of the same output transformer? Please explain.

You are right there. a higher load impedance will result in less current flow for a given load voltage. This is however precisely what causes the higher voltage swing, as an output stage behaves somewhat like a current source, so it "tries to push the current into the load by applying more voltage".

 

I can try to explain it better, but it will get quite long again.

First a little preface: i am tallking about classic vacuum-tube amplifiers in Class A or AB Operation. Most of it translates to similarly operated solid state amplifiers.But many modern solid-state power amps are so called Class-D amplifiers and do not contain an Output transformers but only a low-pass filter. Also, pure Class-B amplifiers (rare in guitar applications) are not as susceptible to transformer overvoltages.

 

The load impedance will be reflected (say "translated") by the output transformer. For a common vacuum-tube power-stage the say 8 Ohm load impedance gets reflected to a few thousand Ohms (depending on the winding ratio of the OT) at the power tubes. This reflected load impedance is what the output tube see and it defines the slope of the so called load line. A higher impedance means bigger voltage change for a given change in current and vice versa. Since we are dealing with a reactive load,  you can have a dc bias current without any significant voltage drop over the transformer. So your output stage is sitting at the full supply voltage already when its idle. If you now put a signal into it, the voltage at the primary side will swing around this operating point. So with a higher load impedance the voltage will swing to much higher levels during the negative half swing of the input signal.

 

For anyone interested in learning about design of vacuum tube amplifiers i can highly recommend the website of the brilliant merlin blencowe : http://www.valvewizard.co.uk/index.html

You can find a bit more in depth explanation on this subject on the "Single Ended" page. Though if you are new to vacuum tubes maybe check out the triode and Pentode section first.

 

 

BTW: OPs power-amp is actually a Class H one, which is basically a Class B amplifier with variable levels of supply voltage.  

 

 

[Rloewenherz]

[...]

Regarding tube amps, after reading the posts here and elsewhere I find myself actually less sure than I have always been that a higher impedance is safer when you have to setup a tube amp and speaker with an impedance mismatch. I would say the preponderance of web posts and articles on this topic indicate that in situations where an impedance mismatch is the only option, a higher impedance on the speaker (relatively close to the rated output on the amplifier) is better on both tube and solid state amps. However, there are also a fair number of well written posts by seemingly knowledgeable sources that seem to indicate that in the case of tube amps, a lower impedance on the speaker is better.  I still don't feel as if I have located a totally definitive answer on this subject.

 

Another good post on this subject from TDPRI.com and this one seems to indicate that a lower impedance on a mismatch is preferable if using a tube amp and a higher impedance on the speaker is better when using a solid state amp. Sheesh, I was way less confused before I "educated" myself on this subject.: http://www.tdpri.com/threads/speaker-impedance-mismatch.197323/#post-2286808

 

 

For (regular Class A or AB) tube amps there is a definitive answer: If you have to mismatch, go for a lower load impedance.

Reasons as in my earlier post in this thread.

 

Solid state amps are not so simple, their tolerances vary widely depending on components and topology used. If i had to lean one way, i would agree with the statement that a higher load impedance would be preferable here. Though i would not mismatch my own power amps unless i knew exactly what topology and components were used.

The reasoning would be that many semiconductor devices used in audio amplifier are indeed less resistant to overcurrents than vacuum tubes and will most likely be used in topologies without output transformers (significantly reducing the potential for over-voltages) or with multilevel/adaptive supply-voltages (which should add some safety margin for over-voltages).

 

PS: sorry for double post, messed up quotes.

[spikey]

You are right there. a higher load impedance will result in less current flow for a given load voltage. This is however precisely what causes the higher voltage swing, as an output stage behaves somewhat like a current source, so it "tries to push the current into the load by applying more voltage".

 

I can try to explain it better, but it will get quite long again.

 

If i had to lean one way, i would agree with the statement that a higher load impedance would be preferable here.

 

 

Thanks for your input.