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Speaker impedance roughly 6 ohms, 2A3 SET...

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Saurav

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Joined 2002
#1
... 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
 
#3
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.

4 ohm will work better if you increase voltage, 8 ohm if you increase bias current (and perhaps reduce idle voltage)
Click to expand...

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.
 
#10
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
 
#13
What is the primary impedance of your output transformer.
Click to expand...

Good point, didn't think about that. I think it's a 3000 ohm transformer. Which is slightly high, right, 2500 ohms is the typical transformer used? Or are you asking about the DCR or primary inductance of the 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.
Click to expand...

Now I'm confused again. The 4 ohm tap will take 4 ohms and reflect that back as 3000. So if I put 6 ohms there, it'll be even higher than 3000, right?

And thanks everyone for clarifying the distortion issue. I think this boils down to "try both" 🙂
 
#14
Saurav said:
Good point, didn't think about that. I think it's a 3000 ohm transformer. Which is slightly high, right, 2500 ohms is the typical transformer used?
Click to expand...

Yes, 2500 ohms is mentioned in the 2A3 datasheet. But anything from 2k5 to 3k5 is typically used.

Now I'm confused again. The 4 ohm tap will take 4 ohms and reflect that back as 3000. So if I put 6 ohms there, it'll be even higher than 3000, right?
Click to expand...
Yes, the impedance ratio of the transformer is 750 on the 4 ohm tap and 375 on the 8 ohm tap. So you will get either 4500 or 2250 ohms on the primary with a 6 ohm load. Both are OK.

And thanks everyone for clarifying the distortion issue. I think this boils down to "try both" 🙂
Click to expand...
If all you have to do is move the speaker wire from one terminal to the other it's a simple test... 🙂

Best regards,

Mikkel C. Simonsen
 
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