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Old 28th October 2010, 07:52 AM   #1
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Join Date: Jul 2002
Location: arizona
Default I think my newly repaired tube amp....(no, this isn't O.T.)

....just had a bad case of indigestion. Allow me to back up a bit:

I've got a little (well, maybe not so little ) pseudo surround (a la a variation on the Hafler Circuit) set up for my computer system. I've had an Adcom 100W GFA 545II running the mains and the very very padded down surrounds. (Just running resistors in series with the surrounds.) I run the surround set up to enhance the front soundstage, not to hear sounds coming from the back. Anyway, I've had a tube amp sitting on my kitchen table for a couple of years now that I kept telling myself I'd get repaired. Well....it's FINALLY repaired! I installed it into my computer system, and I think it had some kind of allergic reaction. It DEFINITELY doesn't like running the surrounds (which are Paradigm Atoms.)

What I don't want to do to remedy this situation is run one of those transformer based speaker switching boxes. I'd like to keep the signal path to the main speakers simple and pure. What I was thinking would be the least intrusive to the front speakers would be to run another set of wires off the binding posts of the tube amp and send it (via some type of step downlike they do in the high level inputs of a subwoofer plate amp???) to another amplifier dedicated to the rears. I'm guessing that the type of impedence that the tube amp saw in this situation would be negligible. (And no, I don't want to run a "Y" connector from the computer output. Despite it being one of those tweaked out sound cards from parts connexion:

http://www.partsconnexion.com/prod_pdf/Prodigy.pdf

I'm not convinced that it's gonna want to drive 2 sets of interconnects. Unless you know something I don't know...? Ok, I'm sure you know a LOT that I don't know

Yes, I know this sounds more complicated than it needs to be, but the tube amp and main/front speakers are high enough quality that I'd like to make the effort.

Thanks!

Last edited by dsrviola; 28th October 2010 at 07:54 AM.
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Old 28th October 2010, 04:16 PM   #2
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Are you handing the tube amp a reasonably matched impedance at its output?

"resistors in series with the surrounds" is gonna change the load impedance, and not seeing the "correct" impedance is gonna toss your amp all out of whack. (i.e. it "wont like driving" a mismatched load, higher OR lower than what the OPT is tapped at.
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Old 28th October 2010, 08:13 PM   #3
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what do you mean by matched? L and R channel matched? If so, then yes.

I think it's more a matter of the total impedence with all four speakers running is just too low. As soon as I turn up the volume to "gaming" levels (mind you, I'm only 2 feet away from the front speakers), the distortion goes through the roof.
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Old 28th October 2010, 10:28 PM   #4
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By "matched" I mean, matched between the output impedance of the amp, and the actual impedance of the load, i.e. anything hung off of the "speaker" terminals.

I'll assume that your tube amp does not have several "output taps" at the speaker connections? (Many tube amps would provide facilities for 4, 8, or 16 ohm loads off the output transformers) A tube amp needs it's load impedance to be "matched" to it's output impedance, otherwise power is not efficiently delivered to the load, frequency response can be all borked up, and distortion could go up.

It's all because of the complex too-smart-for-me electromagnetic magic inside the output transformer, which is there so the output tube(s) can drive speakers. The transformer matches the (high impedance) tube to the (low impedance) speaker. In order to work effectively, and properly, the transformer "secondary", i.e. winding that feeds the speaker terminals needs to have a proper load across it.

It would, however, take a bit of a bad mismatch to cause what you're hearing.

So, Tube Theory Lesson aside.... if your tube amp is wired with an 8-ohm secondary, it REALLY WANTS TO SEE an 8-ohm-ish load. They're not happy to keep dumping, or forcing, power into a mis-match, either too high or too low.

(That's why many older amps used multiple transformer "taps" at the output... you could parallel 2 8-ohm speakers on a 4 ohm tap, or feed them in series from the 16-ohm tap, or use one 8, 4, or 16 ohm speaker on an "appropriate" 4, 8, or 16 ohm tap.)

Measure the total combined impedance at the wire that hooks up to the "speaker" terminals, with everything hooked up as normal, except don't actually connect to the amp. If this doesn't match the output impedance of the amp (likely 8 ohms, if you only have 2 output terminals per channel) you found at least PARt of the problem.
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Old 29th October 2010, 11:11 AM   #5
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Well, I fixed the problem: I went around it. The sound card happens to have a headphone output, so I ran a low level line from there into a little digital amp. Works like a charm....with one little exception: I can't run the psuedo surround circuit I'd originally intended. The circuit only runs off the positive terminals, and the digital amp goes into protection wired that way. I've got it wired with the negatives switched and that seems to be working ok.

Thanks for your clarification. The tube amp is a very tweaked Golden Tube SE40, and the output transformers are set up for 6 ohm loads. I never did mention the combined load of the 4 speakers, but whatever it was was NOT close to 6 ohms. The amp's happy as a clam now!
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