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Old 16th November 2014, 01:57 PM   #1
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Default Tube Preamp (my first)

Hello everyone,

I'm new here, and also young, and don't have much knowladge in eletronics. I'm learning to play the guitar and I've been looking for the ast months at some stompbox schematics and trying to learn how they work, yet with not much intent of building any, until past week, when my guitar teacher was talking about one of his solid state amps, which wa sone of the best he had, but really lacked that valvey sound. That was when it crossed my mind that I probably could build something like that, and we started discussing it. I've been reviewing some designs and trying to learn as much as I can, but still can't understand some principles. So, the idea: build a external stereo valve preamp. I came with this simple design (I'll put at the end), but my question is what formulae to use to find the cathode in the cathode resistor and in the grid leak resistor and if all the connections are made propperly. I tought of adding trimpot as cathode res. and then change the bias to change it's value according to my teachter taste (tell me if that's how it works). Also, to keep it at a low cost, I was thinking of not using a transformer for HT, since I live in Europe (+230V), and so that seems enough for driving the anode., and so only using a small transformer for the heaters. Sorry for the long post :/ and thanks form now on for any help you can give me.

Best regards,
Joo
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Old 16th November 2014, 03:31 PM   #2
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Quote:
I was thinking of not using a transformer for HT
We here frown upon direct mains connection as it has high risk of being lethal, up to the point that it's forbidden to discuss such issue. Anyway, assuming you use isolation transformer, 230VAC rectified using solid state would get you around 325VDC. Add the ripple filter and you will end up with something around 300-310VDC which is perfect for 12AX7.

As for your questions, have you tried reading valvewizard.co.uk? Start by reading the Triode Gain Stage section. That should give you a good basic understanding. Sorry if i don't answer your questions directly (it's about teaching how to fish, not giving fish )
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Old 16th November 2014, 03:49 PM   #3
rayma is online now rayma  United States
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Originally Posted by joodias View Post
Sorry for the long post :/ and thanks form now on for any help you can give me.
Best regards,
Joo
You'll need a 1M resistor from grid to ground for the second tube.

The volume control can't be directly connected to the high voltage plate circuit. Connect both ends of it after the two coupling capacitors.
That is, one end of the pot goes to the second tube's grid, the other end of the pot goes to the main output, after the capacitor.
The first plate resistor is too high in value, it should be around 100k instead. The second tube also needs a similar plate resistor.
The two cathode resistors will normally be in the range of 1k-2k.

And please, you must use a fused mains transformer for safety.

Last edited by rayma; 16th November 2014 at 03:59 PM.
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Old 16th November 2014, 03:49 PM   #4
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Originally Posted by ballpencil View Post
We here frown upon direct mains connection as it has high risk of being lethal, up to the point that it's forbidden to discuss such issue. Anyway, assuming you use isolation transformer, 230VAC rectified using solid state would get you around 325VDC. Add the ripple filter and you will end up with something around 300-310VDC which is perfect for 12AX7.

As for your questions, have you tried reading valvewizard.co.uk? Start by reading the Triode Gain Stage section. That should give you a good basic understanding. Sorry if i don't answer your questions directly (it's about teaching how to fish, not giving fish )
Thank you for your quick reply.
I didn't knew about this website; I'll be digging in it soon.
About the ripple filter, how many RC circuits is too much? I was thinking about 2-3 stages of filtering. And if you can give me any information about its values, things to be aware of, etc., it would be extremely helpful.
Thank you once again.
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Old 16th November 2014, 03:59 PM   #5
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Originally Posted by rayma View Post
You'll need a 1M resistor from grid to ground for the second tube.

The volume control can't be directly connected to the high voltage plate circuit. Connect both ends of it after the two coupling capacitors.
That is, one end of the pot to the second tube's grid, the other end of the pot to the main output, after the capacitor.
The first plate resistor is too high in value, maybe 100k instead. The second tube also needs a plate resistor, likewise.

And please, you must use a fused mains transformer for safety.
Understood, thank you. I originally had thought about a 100k plate resistor, but just when putting it on paper I commited an error. I believe I was thinking about R2, the input resistor, where I think it'd be suitable (correct me if I'm wrong). The plate resistor for the second triode, is also 100k a good value?
Thank you once again.
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Old 16th November 2014, 09:00 PM   #6
Merlinb is offline Merlinb  United Kingdom
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Old 17th November 2014, 07:11 PM   #7
tipetu is offline tipetu  Norway
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And with 1,5k ohm cathode resistors and 470k grid resistors you`ve got yourself the most common 12AX7 gain stage.
Adding a cap in paralell with the second cathode will probably give you abit more bass.
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Old 19th November 2014, 09:30 PM   #8
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Originally Posted by tipetu View Post
And with 1,5k ohm cathode resistors and 470k grid resistors you`ve got yourself the most common 12AX7 gain stage.
Adding a cap in paralell with the second cathode will probably give you abit more bass.
And for the second triode? Should I use the same values?
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Old Yesterday, 05:15 AM   #9
rongon is offline rongon  United States
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There's this book by a fellow named Morgan Jones, title is "Valve Amplifiers." It's on its fourth edition, but the third edition is fine for its introductory chapters on how to use valves (tubes) to make a basic amplifier. Highly recommended reading to get started. You'll probably find that you can use it as a guide for something really basic, then come back to it after you've gotten some experience and learn a lot more.
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