Yet another 12B4 line stage, or is the 12B4 better than the Grounded Grid..... - Page 11 - diyAudio
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Old 12th December 2005, 01:40 AM   #101
mach1 is offline mach1  Australia
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Quote:
Originally posted by Planet 10
The Broskie reg linked above is a shunt reg.
Actually it's a series reg. A really good shunt reg is shown here http://www.tubecad.com/july99/page13.html

The improved version is definitely better - however you need a really good cathode bypass cap as this component really affects the sound. I would also recommend an Auricap or better to feed the grid of the tube.

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Old 12th December 2005, 01:50 AM   #102
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Quote:
Originally posted by mach1
Actually it's a series reg.
Indeed it is... i downloaded the article and just had a glance at it...

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Old 12th December 2005, 04:15 AM   #103
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Dave, thanks for your top three. I just finished downloading the RDH 4 Manual, and I'll get Morgan. Much thanks for the link. I found the RDH from the '30s, but I had given up on finding this one. These will keep me busy.

I don't want to pull this thread off topic, but in case any one is interested, I have also found some other old tube books on line at Pete Millett's site. Also, Karl Spangenberg’s, On Vacuum Tubes is up on Ken Gilbert's site as jpegs and one huge pdf.

OK, I'll shut-up now and see what I can learn.
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Old 13th December 2005, 12:36 AM   #104
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I picked up all the parts for this line stage today... but as usual I have to go out of town on account of King Kong. At least its to a place that I like to go to...... A short 8 hour drive.... But I might tale all the stuff with and build it up in the motel room......

Mark
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Old 20th December 2005, 12:05 PM   #105
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hi guys,
i have build this....i know that this is to low plate voltage for the tube but it was the easiest way for me to try it (i have converted my old 12b4 preamp).
The problem is that on the first session i was out of my shoes how good it was but a little bit later i figured that it is not sounding that good... i was kind of strange and the high tones were not so good .... strange sound... could anyone explain me why this cirucit might sound less good than grounded cathode with resistor?
I smell that this circuit might be good but don't know what to do to make it sound right... i can't go up with the high voltage - at this current this is my maximum - well maybe a little bit more i could squize out (transformer limit)

Also the cathode to ground voltage is not to bright - all in all it is not so good but why it is sounding that strange....high and to much treble, not that good soundstage, bass not precise and so on....
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Old 20th December 2005, 01:45 PM   #106
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First off you need more cathode bypass. About 450 uf give or take plus a film bypass cap across that for improved HF bypass. Electrilytics don't behave the same way at high frequencies as they do at lower so a film cap is definately needed. You might consider placing two of your transfromers secondaries in series OR using a voltage doubler rectifier circuit to get higher B+ which you also need.

Hope this helps....

Mark
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Old 20th December 2005, 08:17 PM   #107
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Dear Mark,
thanks for the reply... i don' think that 450uF is needed. Even Brian Beck said that in this thread.... he said that 220uF or even 100uF would be good enough... Bypass on this capacitor is an option that i didn't explore .... i don't know if this would remove my strange treble....
Question - how would this tube work if i would have 10-15mA ccs.
In this way I would need something that is not LM317 (well 15mA would go with LM317) ... any suggestions (simple and effective)?
Would LM317 have problems in high frequencies (arround 200kHz) - oscilations?
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Old 21st December 2005, 12:12 AM   #108
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I have not heard of any one else running that tube at 15 ma, its not a really linear region of that tube anbd may be why yours sounds funny. They are running them at 25 ma though. I really feel that your power supply is not high enough voltage to operate this tube properly. it really needs at least 100 volts on the plate and 25 to 30 ma to operate in its most linear region. Can't you make your rectifier into a voltage doubler or is it already that way?

Also, There is a big difference between 220and 440 uf of cathode capacitance and an even larger difference with and without the small film bypass. Also.... unless you're driving a very high input impedance your output capacitor is also way too small. I drive a 10K input impedance with mine..... Aleph 2's with a 6mfd film cap paralled by a .01 Wonder cap. My cathode electrilytics are paralleled with a .22 Wonder Cap SETI.

Mark
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Old 21st December 2005, 12:37 AM   #109
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Quote:
Originally posted by sunrise

Question - how would this tube work if i would have 10-15mA ccs.
In this way I would need something that is not LM317 (well 15mA would go with LM317) ... any suggestions (simple and effective)?
Would LM317 have problems in high frequencies (arround 200kHz) - oscilations?
~15-18mA works fine and will let your B+ rise up to a decent level. Personally, I'd lose the silicon hanging onto your cathode and just go with a traditional grounded cathode stage. It's could be causing your treble problems.

One other trick is a little plate to grid feedback ala John Broskie and his 300B T-Rex design. In my particular circuit, it did wonders.
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Old 21st December 2005, 02:31 AM   #110
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pedroskova,

Well, I can definately say that the addition of the LM317 was a worthwhile improvement, especially in the low end behavior of the line stage. I originally built mine as a "stock" version with only the resistor in the cathode. Me ears and my froends agree that running the 12B4 with the LM317 in the cathode and 30ma of plate current is really pretty hard to beat. Feedback... you don't really need any running it at 30ma.

Mark
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