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PostPosted: 18 Nov 2011, 08:58 
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tmoble wrote:
Wow, much nicer drawings. Lots of stuff in that PS too. I was going to tube rectify mine, have several NOS 6CA4s but am having trouble with the non-center tapped Hammond. How would the ground plane be established?

Attached is an of example of how to use a 6CA4 / EZ81 with a transformer with no CT. But you will need a couple of diodes.
Attachment:
6CA4-EZ81-no-CT.png

Cheers


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PostPosted: 19 Nov 2011, 11:24 
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You can also try a "hybrid rectifier" half diode half tube, works very well were there is no center tap on a transformer. ;)
Attachment:
Hybrid rectifier.JPG

This is just a simplified version of what Gio has shown.


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PostPosted: 20 Nov 2011, 00:45 
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Wow, that's great. Maybe someday I'll understand how a silicone diode deal can provide a ground side but a tube can't. Thank you guys (peoples?) for taking time to help a noob out.

Any guesses on what value resistor I could use on that to simulate the circuit so I can figure out the dropping resistors on the B+ side? I was surprised to see that 195-200V of the diode bridge when I had it hooked up.with a cap and resistor. I was expecting something more like 120 * 1.41 ~= 170V? I actually saw 205 at one point. I had a 10 Ohm R and a 225uF cap hooked between the + and - of the diode bridge.


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PostPosted: 30 Nov 2011, 21:01 
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No answer over a week, should I just give up on this? I'm getting like 195V out of the hybrid PS, I would need help to get in the ballpark ob the B+, I think it originally called for 100V. I can run a CRCRCR type deal to drop the voltage. Also, I've got a little problem with the 12V deal. I can run the heaters on the 12AU7s off it, b ut I need to have a 6V source for the 6CA7. Can I just tap into the 12V output, use a series resistor to break it down?

Anybody got any hints?


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PostPosted: 30 Nov 2011, 21:26 
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You will have to try different values to get the voltage down to around 150-170 volts, just use CRCRC filtering, try the regulated supply that I have shown maybe to bring the voltage down, you can set it to what ever you want using a zener diode string, and then put a 4.7K CRC filter stage going to each preamp channel. I have no idea how to work out the "ideal" values of CRC type power supply circuits, I don't do enough reading and working out values to remember the math.

The easiest solution for the 6 volt supply is to just have a separate power supply, what are you doing with the 6CA7? Are you using the preamp to drive an amplifier that uses 6CA7's?

It would be nice to know more of what you are building, it would be helpful to know. ;)

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PostPosted: 30 Nov 2011, 21:44 
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I wanted to build your preamp, but unfortunately I have a bad habit of getting bright ideas. So I came up with the idea of using tube rectification. I have an EICO HF-81 that uses 6CA7 tubes and I have a couple NOS Mullard spares. Well, it's not working out that great. I think what I really wanted was another tube sticking out the top. I need to let go of that I guess. I actually did get the hybrid PS working, hay-wired on my bench. Had like 195V. I hooked a 10R and an 25Uf cap across the output pin 3 of the tube, through the R/C, on to the negative point where the 2 diodes meet.

Now I've got a guy wanting me to build him one of these with a phono pre added on. Jeez, I don't know.

Anybody need a car engine built? :)


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PostPosted: 01 Dec 2011, 15:32 
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I think you should try the ForeWatt tube preamp by Bruce using the hybrid bridge rectifier.

I am sure it is better in many aspects and likely sounds better too. ;)

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PostPosted: 01 Dec 2011, 17:47 
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I already looked at that. it looks good to me. I can't get his 215 B+ with the little Hammond I bought but maybe could work something out. I could always get off the cheapskate program and buy another.

Several years ago I bought a remote control setup for this build, looks to be the exact one Bruce got.

I'll check it out further. Thanks for being nice to the toob noob. :)


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PostPosted: 11 Jan 2012, 20:44 
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Hi,there.
This is my first post here,looks like a good place to learn[noob] about everything audio. What brought me here is I'd like to build a tube mic pre and your design looks both cheap and easy. I have a few questions though. Do you know what the total draw of the system is and whats the gain on the new design. Also I would like to make my own PSU using old PC PSU parts[ I have about 15] but when ever I use a fwb to rectify 120v I get 99vdc in multisim,I should be seeing around 170vdc right?
Thanks.


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PostPosted: 17 Jan 2012, 17:26 
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Hello unsmart, welcome to the forum, I think this question was asked in a previous post not sure, but I think this preamp will work as a mic preamp, but not sure how well. 100 volts will be fine for the preamp, likely not as much gain, and the estimated gain of this preamp was also mentioned in this thread too. ;)

How do you plan on building the power supply using old computer power supplies, they are all of switching type, and do not have a step up transformer to get the required 120 volts. Also, the transformer I used for the preamp puts out around 140 volts, a bit higher than usual.

Current draw is between 10 and 20mA, at 150 volts, not a whole lot, the filaments draw a total of 300mA at 12.6 volts.

Sorry for the long wait, but I hope this is useful information, and I do recommend that you read through this thread too because the project page has not been updated in about 3-4 years :)

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