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Old 15th September 2014, 01:17 AM   #1
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Default 300B Tubelab SE

Hi,

I told my friend that I would improve the amount of bass on his 300B SET. I am not expecting to have lots of bass (it's a SET...) but I'd like to improve it somehow.

These are the mods I have/planned to do :

1. I added a PCB that provides a 20 sec delay (via relay) before B+ is apply to the filtering cap C5. The filaments for all tubes and the - bias are being powered.
2. I added 500uF in parralel to C5 for a total value of 600uF after the choke. Before the choke I have a 47uF which is OK for the bridge rectifier.
3. I added a switch (DPDT) to allow my friend to choose from tube rectifier or SS diode rectifier, thanks to the tube rectifier model, using the SS diode will only increase the B+ by 10Vdc therefore the bias of the output tubes won't be affected
4. I am now using the 4 ohm tap of the OPT instead of the 8 ohm tap, speakers are 8 ohm so reflected impedance on the primary will double so as the damping factor, I'm hoping this will provide a better control of the bass while reducing very slightly the output power.

Any more ideas are welcome.

Thanks in advance.
Eric

Last edited by e_fortier; 15th September 2014 at 01:19 AM.
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Old 15th September 2014, 09:43 PM   #2
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Please share your tricks to improve the bass of this nice SET.

Thanks,
Eric
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Old 15th September 2014, 10:32 PM   #3
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What about the output transformer, that is often the limiting factor on good LF performance.
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Old 15th September 2014, 10:39 PM   #4
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And what about speakers?

jeff
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Old 15th September 2014, 11:50 PM   #5
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Thanks guys,

OPT is Hammond 1627, too bad he didn't use the 1630.

Speaker wise it's a 3 way DIY spkr about 88db eff. called Astasia designed in Europe (France)

BR,
Eric
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Old 16th September 2014, 12:17 AM   #6
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Quote:
Originally Posted by e_fortier View Post
OPT is Hammond 1627, too bad he didn't use the 1630.
They both have the same bandwidth.

Quote:
Originally Posted by e_fortier View Post
Speaker wise it's a 3 way DIY spkr about 88db eff. called Astasia designed in Europe (France)
If these are the speakers, IMHO you picked the wrong amp to drive them.

Astasia Prject - 3 ways loudspeakers

jeff
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Old 16th September 2014, 03:10 AM   #7
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Hi Jeff,

I haven't heard the 1630 but the primary inductance is nearly double the one of model 1627.(according to Hammond spec sheet)

This is my friend's amp and speakers, I'm just trying to help him. I am aware that his inefficient speaker is not really the best match for a 300B. He really enjoy the combo at low volume like Jazz trio etc but when it's time to listen to music with more guts the amp just cant' keep up with the rhythm.

BR,
Eric
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Old 16th September 2014, 04:56 AM   #8
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Quote:
Originally Posted by e_fortier View Post
This is my friend's amp and speakers, I'm just trying to help him.
Maybe suggest getting some more efficient speakers. A friend here has a pair of Audio Note AN-E's that he drives with 300b mono-blocks. They are approx. 95dB/w/m efficient, and will play to satisfying levels. Not the only option, of course.

jeff
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Old 16th September 2014, 06:54 AM   #9
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Quote:
Originally Posted by vinylkid58 View Post
Maybe suggest getting some more efficient speakers. A friend here has a pair of Audio Note AN-E's that he drives with 300b mono-blocks. They are approx. 95dB/w/m efficient, and will play to satisfying levels. Not the only option, of course.

jeff
I think this would be the best way with the best result too
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Old 30th September 2014, 03:28 AM   #10
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Hi,

Mods are done but I still have the same issue I had at the beginning ;

On the right channel, if I move the 300B gently by the base the bias goes through the roof..450mA and the output transformer actually moves a bit it actually feels like connecting a battery to a woofer, it bottom up and goes ploc.

It only happens on the right channel, I redid all the soldering (complete board), I don't have much time to take measurement cause the 300B is red plating but I checked and the fixed bias voltage is at -150V and on the gate I have -65V with respect to gnd on each tube.

Any idea what could be causing that sudden huge increase in bias while I gently move one of the 300B ??

This happens with either SS rectifier or 5AU4.

Thanks in advance,
Eric
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