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Old 1st April 2007, 01:56 PM   #1
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Default Help! Weak Bass!

Hi Folks!

Last September, I built an amplifier with EL34 tubes in Ultra Linear mode and fixed bias. It sounded really good until recently I changed the phase splitter from cathodyne to a long tailed pair, (because it would distort at higher volumes) and now the bass response is quite hollow. I checked and double checked my wiring and found nothing wrong. I fiddled with the size of the "tail" resistor from 22K to 10K to 6.8K and still weak bass. I then changed the inverter again from long tail pair to a floating paraphase and still weak bass.

There's 490V on the plates using a 250 mA PT, the the tubes are biased around 33 mA each. Is there's a solution?

Thank you,

Johnny
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Old 1st April 2007, 07:28 PM   #2
SY is offline SY  United States
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Have you measured the frequency response and distortion?
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Old 1st April 2007, 07:35 PM   #3
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Default Help! Weak Bass!

As far as test equipment goes, I don't have much. (My meter doesn't even measure true RMS) I would like to get a good scope someday, but for now I have is my ears and I don't like what I hear. Any ideas?

~~~Johnny~~~
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Old 1st April 2007, 07:44 PM   #4
SY is offline SY  United States
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Yes, get a scope and meter. ;-) Seriously, you wouldn't try working on a car without having a set of wrenches and a timing light, so you really need to have the right tools. Without them, you'll be going round in circles; with them, you'll have the problem nailed down in minutes.
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Old 2nd April 2007, 08:23 AM   #5
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Perhaps if we could see a schematic, we might be able to comment on possible problems. From what you say, there is no obvious reason why your bass response should be 'hollow' - BTW, I'm not sure what you mean by that.
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Old 2nd April 2007, 01:59 PM   #6
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When I say, "Hollow", I mean the speakers sound like they're out of phase, but like I said, I checked EVERYTHING. I'll post a schematic shortly.

~~~Johnny~~~
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Old 2nd April 2007, 02:19 PM   #7
bigwill is offline bigwill  United Kingdom
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Better damping factor causing less perceived bass?
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Old 2nd April 2007, 03:22 PM   #8
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Perhaps. Is there a way to get lower bass response out of a tube amp without using EQ or tone controls?
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Old 2nd April 2007, 03:28 PM   #9
bigwill is offline bigwill  United Kingdom
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Less NFB might work (lower damping factor)
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Old 2nd April 2007, 04:43 PM   #10
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The only NFB I used was already incorporated into the ultra-linear design. Do you think I would get better results if I change it to pentode operation?

~John~
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