Any pros/cons about an Ampeg SVT-CL, 2010 model?

There's no "on the fence" about it - tube amps act the opposite of SS or hybrids in regards to output loads...

Just like you can "get away with" running some SS/hybrid amps below their impedence ratings, you can also "get away with" running some tube amps above their impedence ratings - BUT, just because you might get away with doing something, doesn't mean it's very smart to do so...

I would also say that doing so pretty much releases the seller from any obligation of taking back the amp, should you develop a problem - something to think about... I have never had any troubles with any of my amps, over 30+ years of gigging - but then, I don't go against manufacturer's recommendations... I have to wonder if many of the amps that have crapped out have been operated outside of their recommendations???


- georgetrings

 

For a SS amp this is true, they're direct coupled and going too low causes higher current flow and overheating.

With a tube amp, or any amp with an OT (there are some SS amps with an OT) you want to have the impedance matched as best as you can. If you mismatch there are two scenarios:

If your speaker impedance is lower than your OT impedance setting you have more current running through the system, but the tubes themselves can only run so low and while they may get hotter than usual it shouldn't cause an issue.

If your speaker impedance is higher than your OT impedance setting you can get magnetic leakage which can induce voltage spikes. The voltage spikes can be across the tubes, tube sockets, or the OT windings, the last of which can leave you with a shorted OT.

Either way gets you less power from the amp than an ideal matched load. A robust and well designed OT should withstand a mismatch 100% in either direction (that is set on 4 ohms it should be okay with 2 ohms and 8 ohms), but generally it's just better not to tempt fate if you don't have to.

And this is all ignoring that speakers aren't a static load, the impedance changes with frequency, but that's a whole other issue to add to the confusion.

 

Smart move - this way, should you develop a problem later(heaven forbid) - atleast you'll know that you didn't cause it...


- georgestrings

 

No problem running an SVT with an 8-ohm cabinet at full power. I do it all the time.

 

He gave you bad advice - don't listen to it... See what the owners manual has to say about it...


- georgestrings

 

Cabinet type does not matter -- as long as it is a reactive load (speaker/inductor). The only time you'll get into danger running the amp into a 100% upward mismatch is if you're operating into a purely resistive load, like an attenuator.

It's amazing how somebody can say something false on the internet, and then the mis-statement is believed to the point that it becomes sticky and becomes accepted internet lore. The claims that an SVT cannot operate into a 8-ohm load is an example of people misinterpreting data that is placed before them because they don't understand enough facts to draw the right conclusion.

As someone who designs tube amps, I can tell you that there is nothing wrong with operating an SVT into an 8-ohm load. If anyone is in disagreement, I'd like for them to elucidate the specific reasons that they believe doing that is dangerous, and explain from an electronics standpoint exactly what adverse effects will take place in the amp if it is operated into an 8-ohm load, and the mechanism by which the amp will be damaged.

 

It was already explained in post #23 - You claim you "design tube amps" - any valid credentials, or are we just supposed to take your word for it???


- georgestrings

 

I can't, so I have to trust the words of those who know more than me and base it on my own experiences. I've run SVT's into 8 ohm loads many times, including the first night I got my Heritage rig because I misunderstood how those fancy new splitter jacks on the 810e worked and never had a problem. However, I'm not running them at very high volumes. The word I get from most techs on here is it's fine at normal volumes, but when you start pushing the head into the red, it causes extra wear on the output tranny and power tubes, shortening their lives.

Interested in any pertinent comments from those in the know and not dunces like me

 

Take a look here:

http://www.talkbass.com/forum/f15/ampeg-svt-ohm-question-250822/

Post #33, in particular...


- georgestrings

 

The better post from that thread would be #39 from Jerrold Tiers one of the design engineers at SLM/Ampeg. Much more technical answer from someone with real experience with these amps.


http://www.talkbass.com/forum/f15/ampeg-svt-ohm-question-250822/index2.html#post11975232

 

Dino at Ampeg made a video on amp and speaker impedance. This is Ampeg's policy. Check it out here on AmpegTV.

 

This is a case where people who are capable of solving the problem on their own are able to come up with their own answers, while people who aren't capable of solving the problem on their own have to quote someone else's expert opinion.

> Post #33, in particular...

Interesting that you're quoting someone's post, when that someone was quoting R.G. Keen's tube amp FAQ. I'm one of the contributors at the now-defunct site where that FAQ was originally assembled as a collaborative effort (Ampage), and R.G. and I have collaborated on a several circuit projects.

It's unfortunate that people who aren't all that familiar with tube amp circuits will blindly quote someone else's work, without appreciating when the statements made in those works are applicable and when they are not.

Referencing the quote in Post # 33:

Part of the problem that leads to the misconceptions we're talking about is that end users typically aren't capable of determining how high a load equals "too high" a load; the result is that because they're afraid of something that they don't understand, they choose to err on the side of safety, and avoid any load that is above the factory recommendations. Although that's certainly the safe way to make a decision if you don't know how to determine the right answer through engineering, it leads you to make unsubstantiated warnings about operating into 8-ohms, like those that we're hearing in this thread.

What's important in a circumstance like this is knowing what "too high a load" really means. The SVT does not employ a "poorly designed (high leakage, poor coupling, not well insulated or potted) transformer," like R.G.'s paper warns about. The result is that a 2:1 mismatch to the upside is not equivalent to operating in a marginally safe operating area for an SVT output transformer or a sextet of 6550.

What I find most interesting about the Internet Lore is that people will talk all day long about how well Fender amps that have only a single tap on their output transformer will tolerate a 100% mismatch to the upside or the low side without having any problems. Yet the same Internet Lore says that you can't do the same amount of mismatch to the upside with an SVT because it will magically blow up if you do that. That's not at all the case. If it were the case, we'd hear reports of people blowing up their SVT every time that they had the impedance selector switch improperly set on the back panel. But that doesn't happen. In fact, I'm familiar with people who have gigged all night with an SVT at full volume, with the Z-selector switch mistakenly set to 2-ohms while driving a single 4-ohm fridge cabinet. Do the math -- that's exactly the same as driving an 8-ohm load with the impedance selector set to the 4-ohm setting.

If doing this is so horrible, then how come we aren't always hearing about people blowing up their SVT just because the impedance selector was mis-configured? The answer is obvious -- the amplifier has a robust design, and it's not going to fail from something trivial like a minor secondary load mismatch, just because somebody had a back panel slider switch pushed the wrong way.

In order for the horrible effects that you guys are imagining to occur, the current on the primary side of the transformer has to be driven into discontinuity. Do you have any idea what sort of impedance mismatch is necessary and sufficient for that to occur?

A 2:1 mismatch in the optimal Z-ratio is insufficient to cause inductive kickback effects that are sufficient to punch through a transformer's insulation. Transformer insulation is rated for well over a kilovolt. With a 2:1 mismatch, you're still operating within the SOA (safe operating area) for the tubes and the transformer.

The result of a 2:1 mismatch is that the impedance mismatch will cause your operating point to be displaced along the tubes' impedance characteristic; this will cause you to shift to a new operating point with respect to distortion, and a new operating point with respect to power output. What is important to remember is that as long as you're operating within the published specifications for the 6550 output tube, then you remain within the SOA for the amplifier, regardless of the degree of mismatch. You may gain or lose some power output by being displaced along the impedance characteristic, but as long as the operating point remains in the SOA for the tube you will not burn the OT; there is an element of protection that is being provided by the reactive speaker load -- it presents a sufficiently inductive / reactive load to absorb any possible flyback voltage peaks that are caused by current surges that are generated at the higher impedance.

This is a case where people like to quote a paper that says "too high an impedance is bad", though they actually have no idea what threshold number amounts to being "too high" and what sort of number is not "too high."


So why is it that people have an irrational fear of operating an SVT into an 8-ohm load? In part, I think that this is because people have been misinterpreting for decades why there isn't an 8-ohm tap on the back of an SVT.

A review of the history of the SVT is required to put this all into perspective:

The original SVT was released with an output transformer that had an 1850-ohm primary specification and a 4-ohm and 2-ohm secondary specification. Unlike other amps of the day, it had no 8-ohm secondary tap because there were no 8-ohm speakers made in the late 1960s that were capable of handling the SVT's 300W RMS / 600W peak power output.

To create a speaker array that could handle the SVT's power, Ampeg had to pair up an array of 16 low-power 10-inch speakers, each having a 32-ohm nominal impedance. When connecting 8 of these 32-ohm speakers in parallel the result was a 4-ohm cabinet. Connecting 2 of these 4-ohm cabinets in parallel yielded a 2-ohm load. That's why the SVT was chosen to have a 2-ohm tap, a 4-ohm tap, and no 8-ohm tap.

Does anyone remember back in the day when Ampeg first released the SVT? The speaker cabinets came with a lifetime warranty, but if and only if you ran two of them with an SVT head. If you ran an SVT head with only one cabinet then Ampeg wouldn't honor the speaker cabinet's lifetime warranty, because 8 speakers couldn't adequately handle the SVT's power output. The reason then, that the SVT only had 4-ohm and 2-ohm taps on the secondary was because those were the only loads that were capable of handling the amp's 300W RMS output in 1969. There weren't any 8-ohm speakers that were capable of handling 300W, so Ampeg didn't bother with an 8-ohm tap.

Somehow people noticed the absence of an 8-ohm tap on the back of the transformer and mistakenly assumed that the amp wouldn't operate with stability or reliability into an 8-ohm load. That's an error in logic.

 

I just noticed that the quoted text referenced in "quote 33" in that other thread left out some important text. The complete transformer mismatch section of R.G. Keen's article is quoted below, with the suspiciously omitted text highlighted in bold. The omission of the highlighted text is significant:

source: http://www.geofex.com/tubeampfaq/TUBEFAQ.htm

 

Well I'll give you a more concrete reason not to use mismatched loads...it doesn't sound as good It always sounds slightly tighter and slightly more compressed to my ears.

 

While anecdotal evidence has indicated that an SVT can be operated at 8Ω there are some other considerations.

In general terms, some amps can tolerate an impedance mismatch more than others. Let's reiterate that this can't be done with all amps. It depends on the design. As noted above there are good designs but not all are.

I wanted to make the point that the health of the amp is important. For example, a badly matched set of output tubes can send an amp into oscillation that can cause voltage spikes and flash over. Pushing an amp that has an issue, coupled with an impedance mismatch can lead to serious problems. I suppose, one of the reasons why manufacturers recommend being conservative and operating the amp as directed.