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Author Topic: 12AX7 Tube Preamp  (Read 2433 times)
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metal
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« on: September 05, 2010, 04:47:51 16:47 »

Hi,

I am looking for the original article: http://www.siliconchip.com.au/cms/A_30804/article.html

Thanks
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pickit2
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« Reply #1 on: September 05, 2010, 04:53:55 16:53 »

http://diyaudioprojects.com/Tubes/12AX7_Preamp/
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metal
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« Reply #2 on: September 05, 2010, 06:26:53 18:26 »

This is not complete, there are other details I am looking for.
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solutions
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« Reply #3 on: September 05, 2010, 07:48:30 19:48 »

This is not complete, there are other details I am looking for.
Can someone please provide a Sonsivri link to the Mind Reader project?  

pickit2's link provides all the detail you need, IMO, except for the circuits boards (and a purist would NOT use a circuit board....or those transistor things in the power supply).  If you're looking to run this design from line inputs, the attenuation scheme is here:  http://www.siliconchip.com.au/cms/A_100628/article.html
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metal
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« Reply #4 on: September 06, 2010, 01:24:07 01:24 »

As long as "pickit2's link provides all the detail you need":

Tell me, for the PSU part, those couple of T1 windings on ETD29 in the PSU schematic, that are wound in the same direction, what is the wire gauge? I am blind and did not see where they wrote the gauge, show me where?

Solutions, I am not sorry for saying this, but the more you post, the more I see that you prove you have a void brain...
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solutions
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« Reply #5 on: September 06, 2010, 03:43:48 03:43 »

The windings are in the same direction, you take a tap off after 12T for the MOSFET, then an additional 80T from there. 

The 1N4004 is only rated at an amp, and will be a fuse in your circuit if you pull more than that through the transformer, so resistive loss (efficiency) and core fill factor will govern the wire gauge.  With a fill factor of 0.5, and winding window area of 90mm*mm for 92T means 0.5*90/92=0.489mm*mm wire cross sectional area.  20 gauge wire is 0.52mm*mm.

Just think if my void had a bigger fill factor than the zero you allege....

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borberk
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« Reply #6 on: September 06, 2010, 05:25:34 05:25 »

This is it
« Last Edit: October 12, 2010, 08:04:53 20:04 by borberk » Logged
metal
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« Reply #7 on: September 06, 2010, 06:40:13 06:40 »

Solutions, you know why they used different gauges, I presume. The 12T must cover the limb, you are right about the AWG 20 for the wire gauge, but if I follow your calculations for 12T only, I will end up using AWG 11 or 12, but they followed your calculations for the 92T, allowing the 12T to cover the limb. This is what I exactly thought of for a step-up auto-transformer, the 12 turns made me ask for the article because I don't know how to calculate the wire gauge, now I have an idea. This method of winding I saw in many transformers, this is the correct way when current is small. They opted using 31% smaller wire for the 80T winding because of the small current, good to know.

I think if AWG 20 has been used for the 92T, the core would be smaller, but not too much smaller, correct me if I am wrong solutions.

Just think if my void had a bigger fill factor than the zero you allege....

Now, this becomes a compromise, you deserve 0.5 only i.e. 50%



« Last Edit: September 06, 2010, 07:07:02 07:07 by metal » Logged

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borberk
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« Reply #8 on: September 06, 2010, 07:54:24 07:54 »

T1, primary is 12 turns of 0.8mm wire and 80 turns secondary is 0.25mm. Primary goes first.
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solutions
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« Reply #9 on: September 06, 2010, 08:33:00 08:33 »

Cool...20 gauge is 0.81mm...my calcs said that winding the primary and secondary with the same wire will fit on that core (lower resistance, less heat, no need to buy another kind of wire, etc).  Resistance of the secondary will be 10X by using that 0.25mm wire, though you'll get a bit better fill (less open space in the winding).

/of course if it doesn't fit just using 20 gauge, someone's gonna be mad at me after counting to 90
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metal
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« Reply #10 on: September 06, 2010, 12:02:37 12:02 »

But you know? There might be even easier way to do it:

For a transformer that is rated at 300mA, suppose we got two 9V:220V transformers. What if I replace the secondary in the 1st one with the primary from the 2nd one? I will end up with a transformer that has two 220V windings. After rectifying I should get 311V. Using http://giaime.altervista.org/maida.html things will be fine, this regulator is tested and works fine and outputs 260V, requires around 300V.

Seems i got easier changes in the plans this time Smiley
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borberk
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« Reply #11 on: September 06, 2010, 04:58:04 16:58 »

This goes this way: use two small mains transformers 15:230 and 12:230 (10VA+5VA could be enough) with secondaries connected in parallel 15V>12V. Filament voltage is taken from 12V and the last 230V is rectified and smoothed. DC current is only 5mA max and filament current is only 150mA.
Usually small transformers can not be disassembled and besides that you need extra filament voltage transformer. Next problem can be hum that enters with mains voltage. I think that original solution is better.
Althought stabilized anode voltage needs not to be stabilized it's tollerance can be 10% with no problems.
« Last Edit: September 06, 2010, 05:00:59 17:00 by borberk » Logged
pickit2
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« Reply #12 on: September 06, 2010, 05:00:15 17:00 »

Some more info on tube amps, worth a look
http://www.pimmlabs.com/
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« Reply #13 on: September 06, 2010, 10:25:53 22:25 »

Agree with borberk - you may get hum and your caps will be coke cans in size (uF and voltage rating, not to mention the $$$ you THINK you are saving) to filter the ripple to where the hum is acceptable, especially in a preamp.  We used the transformer/pi-filter method to get our A, B, and C Tube supplies for our superhet radios in high school.  Yes, the output hummed a bit when cranked up.

Cap size and supply noise frequency pushout is where the Silicon Chips switcher circuit shines.  Any amount of hum sucks, IMO....

/LOL - I know Mike Maida...weird to see a circuit referred to as "Maida-style".  Maybe we'll see a "metal-style" as well?
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metal
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« Reply #14 on: September 06, 2010, 11:52:20 23:52 »

May be, tests have to be carried out first. If the idea of the two primaries transformer works fine, then this is it; otherwise will have to go back to the switching solution. I don't like the idea of using two back to back transformers. This is because I know a friend who is capable of disassembling transformers and making this tweak I mentioned, so why not.

Someone suggested using a shield for the tube it self to solve the hum issue, in case there is hum, but still it is important to use a regulated B+ supply to avoid hum too. Still the fight would never end between those who like the analog style PSUs and those that prefer the SMPS style.

I noticed that people prefer 12B4 because it has less gain factor of 6.5 compared to 12AX7, this also plays a role in less hum and warmer sound according to those who tried both of them. sadly, 12B4 is not available locally.
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borberk
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« Reply #15 on: October 12, 2010, 08:07:01 20:07 »

Files I uploaded are renewed because of the complains.
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