Speaker impedance roughly 6 ohms, 2A3 SET... - diyAudio
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Old 15th October 2004, 06:17 PM   #1
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Default Speaker impedance roughly 6 ohms, 2A3 SET...

... Which OPT tap should I use, the 4 ohm or the 8 ohm? And why? The speaker's impedance is fairly flat, probably +/- 1dB across the frequency range it is used for. The OPT is a One Electron UBT-3, which I believe is a 3kohm transformer. The tube is a Sovtek 2A3, being run somewhat higher than the classic 250V/60mA operating point. Speakers are roughly 95dB, so I can't quite say that I definitely want reduced distortion no matter how much that reduces my power. At the same time, I also don't need to maximize power no matter how much distortion that creates either.

I'm more curious about the reasoning behind whatever your answer is And this isn't a hypothetical question, this is my current situation with my amp/speakers.

Thanks,
Saurav
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Old 15th October 2004, 06:31 PM   #2
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Doesn't matter... 4 ohm will work better if you increase voltage, 8 ohm if you increase bias current (and perhaps reduce idle voltage). Triodes are inherently tolerant of load swings.... which BTW isn't worth worrying about anyway as speakers have such wild impedance in the first place.

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Old 15th October 2004, 06:39 PM   #3
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This speaker's impedance is fairly flat (should have said +/- 1 ohm, not 1 dB). It's only used above 500Hz, so the box resonance stuff doesn't come into play, and my current crossover is minimal enough that I could keep the impedance flattish.

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4 ohm will work better if you increase voltage, 8 ohm if you increase bias current (and perhaps reduce idle voltage)
By voltage do you mean B+ to the tube? I don't think I can increase that any further, I could add dropper resistors to bring it down though (or use a smaller first cap). Which I've been thinking of doing, I wonder what I'm missing out with the Sovteks that I might get with better 2A3s. But that would mean backing off my operating point so I don't burn through expensive tube too fast.
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Old 15th October 2004, 11:15 PM   #4
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Bah, tubes can take it... or maybe you shouldn't use such an expensive tube

Yeah, I mean output operating point.

Tim
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Old 15th October 2004, 11:26 PM   #5
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Well, the Sovtek isn't all that expensive I have no idea about any others, really.

Thanks for the help.
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Old 16th October 2004, 12:52 PM   #6
mcs is offline mcs  Denmark
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I would just try both options. Using the 4 ohm tap, and getting a bit more distortion, may sound better (to you). Who knows?

Best regards,

Mikkel C. SImonsen
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Old 17th October 2004, 01:51 AM   #7
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I thought putting a 6 ohm speaker on the 4 ohm tap would increase the load seen by the tube, which would reduce output power and reduce distortion. Do I have that backwards?
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Old 17th October 2004, 03:10 AM   #8
Colt45 is offline Colt45  Serbia
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Quote:
Originally posted by Saurav
I thought putting a 6 ohm speaker on the 4 ohm tap would increase the load seen by the tube, which would reduce output power and reduce distortion. Do I have that backwards?

yep.
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Old 17th October 2004, 03:29 AM   #9
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Hm. OK
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Old 17th October 2004, 05:16 AM   #10
DougL is offline DougL  United States
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Just a thought:
What is the primary impedance of your output transformer.
If its on the low side for a 2A3, the 8 ohm tap might be better.
If its on the high side, the 4 ohm tap may be more appropriate.

Also, if the amp uses feedback (not likely for a 2A3) than amps tend to sound better on the secondary tap that the feedback is connected to.

My 2 cents;

Doug
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