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Old 12th June 2013, 07:21 PM   #1
Gost22 is offline Gost22  Bolivia
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Lightbulb "Diamond" tube preamp with 12au7

Hi guys!
I want to create the tube preamplifier or tube buffer under this schematic. If anyone has some schemes comments let go. Any help is welcomed.
Just please tell me what is wrong what's wrong with this tube buffer (preamplifier)?.
Thank you for your cooperation!
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Last edited by Gost22; 12th June 2013 at 07:52 PM.
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Old 12th June 2013, 07:34 PM   #2
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I'm not really a tube guy, but the plate voltage looks too low to be hi-fi.

Mike
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Old 12th June 2013, 11:10 PM   #3
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I don't think this is going to work:
- With 60V B+ and a predetermined quiescent voltage of 40V, the 12AU7 will be biased at almost 0V. Positive swinging input signals will almost clip immediately.
- Taking the output from the anode makes for a high output impedance, with all kind of issues as a a result (attenuation, noise)
- Any impedance connected to the output will be in parallel with the 100k in the schematic. Assuming you have to drive impedances as low as 5k, you end up with 4k7. The frequency response with the 330nF cap will be high-passed at 102 Hz. With these values, you'de need at least a 1.7uF cap for a 20Hz response. You can play it safe with a 2.2uF or loose the 100k, so you'll end up with just the load impedance, and you can get away with 1,5uF.
- The cathode bypass cap is unnecessarily large. 22uF will probably do it.
- 300 ohms is not much of a grid stopper.

Edit: The 12au7 draws 150mA when the heater is connected in series. With that current, your 30Vrms transformer output will want to drop around 10,3 volts per resistor pair, leaving 9,4 volts for the heater. It's being starved. Consider a regulated DC heater supply at 12,6V for best performance.

Last edited by funk1980; 12th June 2013 at 11:39 PM.
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Old 12th June 2013, 11:57 PM   #4
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I would do a hybrid mu-follower myself. This can swing 5+ volts into 10k and has a much lower output impedance.
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Old 13th June 2013, 05:10 PM   #5
Gost22 is offline Gost22  Bolivia
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Hi,
Do you like this schematic to work, ... or need some changes?
thanks!
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Last edited by Gost22; 13th June 2013 at 05:21 PM.
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Old 13th June 2013, 06:32 PM   #6
00940 is offline 00940  Belgium
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What do you need it for ?

With a cap at the cathode + a ccs, you'll have a lot of gain with an 12au7. And since you don't have much voltage, that also means you will have to seriously limit the input levels. If you just need a buffer, you'll be much better served by a cathode follower.

Make your heater voltage negative and put a 3 to 4ma ccs in between your negative rail and the cathode and you don't even need an input cap. You'll be able to pass 2vrms into a 10K load with minimum distortion.

There might be a voltage surge at startup though (output cap has to charge and the voltage doubler is slow to rise). A relay with a simple delay circuit might be useful.
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Old 14th June 2013, 05:11 AM   #7
Gost22 is offline Gost22  Bolivia
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Hi, 00940!
I want to connect the output CD player to the input audio amplifier with LM1875.
So this input resistance of the audio amplifier is about 33...47k (into a 33...47k load)
Led lamp with which to replace QTLP690C, because the ice is hard to be found?
ECC82 want to replace with ECC88JJ!?
Please provide changes to the schematic?
Thank you!

Last edited by Gost22; 14th June 2013 at 05:36 AM.
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Old 14th June 2013, 04:03 PM   #8
00940 is offline 00940  Belgium
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So, let's assume that your cdp output is about 1vrms (so 2.8Vp-p). Your lm1875 has max rails of 60V, probably more like 50V. With +/-25V rail, power into 8R is 20w, which means about 43Vp-p. 43/2.8 gives us a necessary gain of 15. The datasheet implementation of the lm1875 has a gain of 21. Ok, there should be no need for gain.


As for the ecc88, it won't work. First, your transformer is too weak for the 365ma required by the ecc88's heater. It's already a bit weak for the ecc82. Second, the ecc88 runs on a 6.3v heater. Even with a more powerful 15-0-15 xformer, that's a lot of voltage to burn from the 21vdc rails you get.

If you really want it to work with the ecc88, add a second transformer for the heater, something like a 9V/1A will do fine. If we have a second transformer, we can make things simpler, so let's get rid of the dual supply. The 470uf caps in the doubler must be at least 50V, preferably 63v, the 100uf must be 100V ones. the qtlp690c is just any red led (that's the only one by default in ltspice). The pot goes in front of the input cap. As said before, there is a thump at startup with such simple buffer, be careful not to blow your speakers, turn the preamp on before the amplifier or add a relay with a delay on the output.
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Old 15th June 2013, 05:47 AM   #9
Gost22 is offline Gost22  Bolivia
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Hi 00940!
Can this red or blue LED diodes be used (replacement) for QTLP690C?
see datasheet: http://learn.adafruit.com/all-about-...-led-datasheet
thanks!

Last edited by Gost22; 15th June 2013 at 05:52 AM.
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Old 15th June 2013, 07:10 AM   #10
hpeter is offline hpeter  Europe
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maybe framegrid more appropriate?
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