Do Quads wear out "all at once"?

[godofwealth]

When using Audio Reseach tubes I always used the ESL63 with the 8 ohm taps. They are around 4 ohm only in the last octave where in general instrumental music has low power. VTL and conrad johnson were internally wired to around 5 ohm, so I had no choice!

I know some reviewers suggest the 4 ohm taps.

The 57s are a more challenging load than the later 63 based Quads. Quad’s instruction manual specifically forbade use of the 57s with anything but the Quad IIs, or the Quad 303. Legend has it many an otherwise good amplifier in the UK melted when faced with the 57s low 2 ohm impedance in the high treble.



This impedance plot however does not give the whole picture. The reactivity of the impedance makes a huge difference. Quads can be highly reactive (i.e., capacitive). In fact, even above 20 KHz, they can represent rather nasty loads, as an old Audio review from the 1980s showed. Needless to say, you need a power amplifier that is stable into low impedance loads. Quad amplifiers were highly stable, and you could even use the Quad 405 as long as it had the limiting switches installed in It for the 57s. The plot below gives the impedance for the 2912, which is not as nasty as the older 57s.



Of course, no Quad is as nasty a load as Martin Logan stats are, which can go below 1 ohm in the high treble! The plot for the Martin Logan CLS shows an impedance of 1.6 ohms in the high treble.

 

[Dogberry]

@microstrip Thanks. No great benefit from risking an experiment, then.

@godofwealth I see why the Quad 306 amps I have sport a button on the back to reset them after they come a cropper driving Quad ESLs!

 

[R Johnson]

My Quad 57 pair blew up a Dynaco Stereo 120 in about 1970.
I then happily used the Quad 303 for years with the 57. In 1999, the 57 pair found a new home with a friend who then had quad Quads.

 

[AMR / iFi audio]

The 2905 speakers are said by Quad to have an impedance variation of 4-20?, and nominal is 8?. I've wondered about connecting them to the 4? speaker outputs instead of the 8. Any advantage anyone knows of, or potential harm?

If they are indeed down to 4 Ohms it's gonna be okay. Higher load Impedance doesn't hurt the amplifier as long as it has a voltage headroom (and tubes have a lot of that). If it dips below 4Ohm tough, it will increase distortion first, then it can increase the wear on the tube. In extreme cases that can overload the amp and create a failure.

 

[Dogberry]

I'll stick with the 8? outputs. Have to look after my Sophia Electric KT88s!

 

[Mendel]

Thanks for the photos and story of your Quad repair! I do hope you end up with at least 10 years of good service. I agree that once you get a taste of Electrostatic sound nothing else will quite do. As a back up at least, perhaps you should consider a pair of the most durable electrostatics ever built. Of course I am referring to Acoustats. I don’t know exactly why they are so tough, but the panels seem to be nearly indestructible. The only problem they ever seem to have is the electrical interfaces lose bias current over a long period of time, but they are easy and relatively cheap to rebuild. Very worthwhile as well, as the interfaces used cheap parts that let down the quality of the panels.
Even though the company is long gone, there are thousands of working pairs still around. They can be bought for a song (often less than $1K) and when the interfaces are rebuilt (also under $1K) they sound very impressive indeed. The cost is a joke for the performance you end up with, and Acoustats can do bass ! The more panels per side, the higher the efficiency and the more bass you get. I have had mine for over a decade and they were probably 15 to 20 years old when I got them. Never a problem.
Sorry to highjack the thread, Quads are great also!
Cheers