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Old 4th April 2006, 08:33 AM   #11
arnoldc is offline arnoldc  Philippines
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Quote:
Oh hi Arnold! I think Elso Kwak is talking about the increasing the value of the cathode resistor, less current.
Hi Wayne, I sure he's talking about something I don't know. But I said the same thing (increasing cathode)
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Old 4th April 2006, 09:49 AM   #12
lndm is offline lndm  Australia
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Default Re: Since we are touching biasing , can we dwell a bit on the "shouty" problem

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Originally posted by ashok
So would vocals with sections that tend to " shout" in some systems be a method of determining how good the amp / system is

Any thoughts?
This could be anything but reminds me of the dramas associated with electrolytics in the power supply, or the signal path for that matter, if it is actually the amp and not the speakers.
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Old 4th April 2006, 06:43 PM   #13
rdf is offline rdf  Canada
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I've found with many tubes two things happen with reduced plate current: 2nd harmonic goes up, higher harmonics go down. The second's another potential explanation for the 'mellower' sound of light biasing.
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Old 8th April 2006, 05:31 PM   #14
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I don't claim to understand exactly what it is about some amps that make them sound more laid back, but I don't think 'laid back' is the opposite of 'bright' or 'clean.' Consequently, I don't think that rolling off the high end or deliberatly trying to make the amp mushy is likely to get you the sound you're looking for.

Having said that, experimenting with different bias point on the 12B4 might get you closer to what you want, but try higher bias as well as lower.

I do agree that different coupling caps at the 2A3 grid can make a big difference. Tube rolling too, but that's a craps shoot unless you have a large collection of tubes to try. If you did then you would have tried them already.

Changes to the ps ... not familiar with the Sun Audio, but replacing electrolytics with motor run oil caps can help get a smoother more natural sound that can be more relaxing to listen to.
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Old 8th April 2006, 06:01 PM   #15
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I'll second the IMO, best piece of advice given so far, add/increase feedback, NFB, not cathode resistors. It's the simplest to do and if it has the desired effect, great
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Old 8th April 2006, 07:01 PM   #16
jj2 is offline jj2  United States
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Angry I have try bais, higher voltage & bypass cap.

Quote:
Originally posted by Dave Cigna
I don't claim to understand exactly what it is about some amps that make them sound more laid back, but I don't think 'laid back' is the opposite of 'bright' or 'clean.' Consequently, I don't think that rolling off the high end or deliberatly trying to make the amp mushy is likely to get you the sound you're looking for.

Having said that, experimenting with different bias point on the 12B4 might get you closer to what you want, but try higher bias as well as lower.

I do agree that different coupling caps at the 2A3 grid can make a big difference. Tube rolling too, but that's a craps shoot unless you have a large collection of tubes to try. If you did then you would have tried them already.

Changes to the ps ... not familiar with the Sun Audio, but replacing electrolytics with motor run oil caps can help get a smoother more natural sound that can be more relaxing to listen to.

You are right bright is the word with the 12b4.
it is a brett 12b4 linstage.
I have try changing the the bais form 15ma to 19ma to 25 ma.
change the from 265v to 300 v
change bypass cap to black gate.
try 3 other SET amp, they all sound bright.
can it be the out cap ? .22 beewax.
Thansk again for all the help.
John

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Old 8th April 2006, 09:10 PM   #17
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Default Re: I have try bais, higher voltage & bypass cap.

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Originally posted by jj2
You are right bright is the word with the 12b4.
[...]
try 3 other SET amp, they all sound bright.

Do you mean that you tried 3 other amps with your linestage and they all sounded bright? If so, that narrows down the problem quite a bit doncha think?

Have you tried skipping the line stage? A passive volume control might make you happier.

Brett's 12B4 linestage is a good example of a circuit that is so simple that pretty much every component will contribute to the overall sound. The output cap will definitely make a difference, though I don't remember anyone calling the beeswax bright. Are you using a cathode bypass cap? If so, try removing it entirely. Even the plate resistor will have a noticeable impact on the sound.

I'll stick with my recommendation of replacing any electrolytics in the ps with oil caps.


The output cap in the Brett linestage will definately make a difference, though I don't remember anyone calling the beeswax bright. For kicks you could remove the cathode bypass cap.

edit: it occurs to me that that some 12B4's are pretty microphonic. A fussy tube mounted on a thin aluminum chassis could result in some unpleasant resonances
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Old 8th April 2006, 09:31 PM   #18
Nafty is offline Nafty  Israel
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Default Mellow

Hello.
I do agree with Jim.
On the other hand, I dont like feedback circuitery and less, to increase it for some reason. Yes, it may give an less analytic type of sound but...I didn't like what I got. Feedback yes, if only local. Thats more or less my opinion.
Best regards to you all.
Nafty
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Old 8th April 2006, 10:08 PM   #19
jj2 is offline jj2  United States
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Default Re: Re: I have try bais, higher voltage & bypass cap.

[QUOTE]Originally posted by Dave Cigna
[B]


1) Do you mean that you tried 3 other amps with your linestage and they all sounded bright? If so, that narrows down the problem quite a bit doncha think?

yes it was the 12b4 preamp

2) Have you tried skipping the line stage? A passive volume control might make you happier.

yes I know ,I have a NOH TVC which sound better but I want to make this work

3) Brett's 12B4 linestage is a good example of a circuit that is so simple that pretty much every component will contribute to the overall sound. The output cap will definitely make a difference, though I don't remember anyone calling the beeswax bright. Are you using a cathode bypass cap? If so, try removing it entirely. Even the plate resistor will have a noticeable impact on the sound.I'll stick with my recommendation of replacing any electrolytics in the ps with oil caps.

I have change the beewax to a 3.3uf cap which sound much better, but still want to improve the sound.
will try the oil cap later.


4) The output cap in the Brett linestage will definately make a difference, though I don't remember anyone calling the beeswax bright. For kicks you could remove the cathode bypass cap.

the beewax was not bright by itself , I think my problem was using a .22uf cap that did not let the bass out.

I will like to know what parts other people used to build their 12b4 pre.

Thanks John
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Old 9th April 2006, 12:20 AM   #20
dimitri is offline dimitri  United States
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Quote:
Forget the rolloff. Rebias the preamp tube. Run it at much lower current! Then you will get the warmth and mellowness all at once. You have to run the tube where it distorts the most.
Jim, thanks for spreading "the cat's" ideas here
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