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:
If you do too high a load, the power tubes still limit what they put out, but a second order effect becomes important...
For a poorly designed (high leakage, poor coupling, not well insulated or potted) transformer, 2:1 may well be marginal.
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.
There is magnetic leakage from primary to secondary and between both half-primaries to each other. When the current in the primary is driven to be discontinuous, you get inductive kickback from the leakage inductances in the form of a voltage spike.
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.
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.
