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Help! Weak Bass!

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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
 
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.
 
TubeHead Johnny said:
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

Was this a Williamson topology? Usually, Williamsons used DC coupling into the cathodyne. If you changed that to AC coupling, you could have made the gNFB less effective at the lower frequencies, which will cause a deterioration of woofer damping.
 
Ex-Moderator
Joined 2004
The amp still is DC coupled using a 0.1 uf polypropylene cap between the voltage amp and the inverter.
That sounds like a contradiction to me. If it's coupled using a 0.1uF cap, then it isn't DC coupled.

To answer your earlier question, making it into a pentode will not improve the bass. On the contrary, it will make it "woolly" or ill-defined, because the output impedance will be much higher, giving very low electrical damping of the speaker. It is hard for the amp to control the speaker under those conditions. This is normally countered with pentodes by adding a significant amount of NFB - 26dB or more. Your UL arrangement already gives you enough NFB to lower the OP impedance and increase damping to near-triode levels, although it still might not be enough for your particular speakers, so you could possibly benefit from adding, say, 10dB of NFB.

What intrigues me, though, is how you managed to change from good bass to poor bass, just by changing your driver/splitter arrangement. There is nothing about the LTP splitter or the paraphase splitter that would cause the bass response to deteriorate, at least in a normal implementation. That's why a schematic would be useful, preferably showing the amp both before and after you modified it, otherwise we're just guessing here.
 
I'm sorry, I've got the two mixed up. The cap would make it AC coupled. I'll try eliminating it and see what happens. If that doesn't work, I'll try using bigger caps in the output stage, but I don't think using a 1 uf, 600V, non-electrolytic cap would be possible. Maybe a .33 uf Polypropylene?
 
Follow-up to "Help! Weak Bass"

Hello everyone!

Hope everybody had a great summer! I just wanted to do a follow-up to this problem I had with weak bass. (Originally posted 4/1) It actually sounded like the speakers were out of phase, but they weren't.

I tried everything I could think of to fix it, including larger coupling caps, different tubes, a lower power trannie with a higher current rating to name a few, but I couldn't figure it out until recently. (check out schematic).

Recently, I was going through Randy Aiken's site and found this:

"The transformer does, however, have a primary inductance, which has a direct effect on the low frequency response of the transformer. This means that if you want better low frequency response from your transformer, you have to increase the primary inductance, which means a larger core and/or more turns on the primary".

The trannies I was using were Hammond 1650N's with a 4.3K primary. I swapped them out for a pair of 1645's rated @ 30 watts with a 5K primary and WOW! Awesome deep bass! I then went to Alden at Heyboer and ordered a couple of 50 watt units with the same impedance, and now it sounds even better!

Thanks everyone for your help,

Johnny.
 

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Re: Follow-up to "Help! Weak Bass"

TubeHead Johnny said:

Hope everybody had a great summer!


Some of us had a great winter :)


Ty_Bower said:
Glad you fixed it. I wonder why it sounded good with the cathodyne phase splitter?


The "fixing" does not really make a lot of logical sense but perception of bass has little to do with actual bass level anyway. I certainly don't doubt that transformers with higher primary inductance will sound better in a zero NFB circuit.

Maybe the cathodyne had simply more second harmonics in the bass.
 
As gingertube pointed out:

Both Differential Splitter (LTP) and Paraphase splitter have significantly higher source impedance than a cathodyne.
That means those coupling caps will need to have lower impedance at bass frequencies - that is you need bigger caps.
Try 1uF in place of the 0.1uF
Cheers,
Ian

It's still a mystery to me, really, I'm just glad I can listen to it and not think about how I can fix it anymore.

I was actually thinking about ripping it apart and starting over!
 
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