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Old 10th November 2001, 04:36 PM   #1
tsz is online now tsz  Sweden
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I'm thinking about building a tube pre amp (actually only a buffer) and I would need some advice from some tube guys.
I was thinking of using a 12AU7 tube with a 250V power supply and biased at 10mA.
Here's the circuit:
<IMG SRC="http://w1.302.telia.com/~u30208505/tubebuf.gif">
Does it seem right? Any suggestions?

Cheers/Tobbe

[Edited by tsz on 11-12-2001 at 12:10 PM]
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Old 11th November 2001, 11:54 PM   #2
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The schematic doesn't show up.

Grey
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Old 12th November 2001, 04:23 PM   #3
tsz is online now tsz  Sweden
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Strange! It does for me..
I'll try a link instead: http://w1.302.telia.com/~u30208505/tubebuf.tiff

/Tobbe
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Old 15th November 2001, 01:31 AM   #4
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Tobbe,
I don't use the 12AU7 much, but shooting from the hip, I'd say that it looks okay. You could reduce R3 a bit if you wanted. I see that you've got both the input and output DC blocking tuned to the same frequency. Should you find that the circuit acts oddly, you might want to tune them to different frequencies.

Grey
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Old 15th November 2001, 11:29 AM   #5
tsz is online now tsz  Sweden
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Thanks Grey!
You say you don't use the 12AU7 much, is it a bad tube?

/Tobbe
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Old 15th November 2001, 11:42 AM   #6
AKSA is offline AKSA  Australia
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The circuit looks OK. However, a CF is a tricky beast to achieve good sonics, but when on the money, it is marvellous.

YMMV, but I would not run it so hot. I'd select a much lower voltage from plate to cathode, say around 80 volts, and around 40 volts across the cathode resistor. This effectively halves the present B+ to 120V, and greatly reduces current. Doing this will enhance H2, and add that sweet tube sound.

I'd run it at 2.7mA. Thus the cathode resistor should be 15K. There is no telling what the cathode bias resistor should be; start with 220R and work upwards towards about 350R. It might be necessary to use a composite resistor.

You must gun for 120V on the plate, and 40V at the cathode.

BTW, use a 100nF polystyrene for the input cap, and a Hovland or RTX 1uF for the output. Unless you are driving 10K loads, it will not need a 4.7uF output coupler.

Cheers,

Hugh
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Old 15th November 2001, 02:39 PM   #7
tsz is online now tsz  Sweden
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>> AKSA
Thanks for your reply.
120V sounds great, that would make it a lot easier to design the PSU.
Regarding the current, isn't the tube more linear at 10mA? It looks that way in a grid voltage, plate current diagram I found.
Would perhaps the 12AX7 be a better choice? It seems more linear at lower currents.
Any other tube suggestions?

/Tobbe
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Old 15th November 2001, 03:20 PM   #8
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Tobbe,
Any tube can be used for a follower, but low to medium mu tubes are prefered; the 12AU7 is a good choice.
Cathode followers seem to have earned a bad reputation with some people. Just for the record, I don't count myself among them. Followers are fine, just make sure you don't set too low an operating current.

Grey
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Old 15th November 2001, 05:28 PM   #9
tsz is online now tsz  Sweden
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>> Grey
Do you agree with Hugh about using 120V and biasing to 2.7mA instead of 250V and 10mA?

>> AKSA
Would the biasing you suggest handle 10k loads? I think my current power amp has 10k input impedance, it's not that unusual, is it?

Thanks again!
/Tobbe
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Old 15th November 2001, 09:31 PM   #10
AKSA is offline AKSA  Australia
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Hi Tsz,

You speak of linearity. This goes to the nub of design.

We have here a vacuum tube, which is chosen because it adds a lovely, tubey sound. This is, in fact, largely low order distortion. You like it, and so you want a tube preamp.

Let's not design a preamp which has NO distortion, or, to put it less controversally, NO harmonic enrichment. You want the tube, therefore, to be a little non-linear. Grey is right; don't operate it too low, because then you have too much of a good thing, and it sounds terrible. Just a little.

In light of your heavy 10K input impedance, I suggest increasing B+ to 150V, increasing current to 4mA, and this will probably not much change the cathode bias resistor, but it will change the cathode tail resistor from its present 15K to 18K. Then, the effective AC impedance seen from the CF cathode will be 6.42K with a 10K load, and this is OK. Once again, vary the size of the cathode bias resistor to achieve 72 volts dropped across the 18K resistor. Then the tube will have just under 80 volts between its plate and cathode, and the sound will be just dandy.

Cheers,

Hugh



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