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Old 4th May 2008, 11:48 AM   #1
fedecv is offline fedecv  Argentina
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Join Date: Dec 2007
Question First tube amp

Hello all!!!

I want to build a tube audio amplifier (very first xD), i had never worked with tubes before so i dont know almost nothing from they. I had build solid state amps and some chip amps, many people had told me about they great sound more natural than solid state amps. I had two 25L6GT tubes, it would be great if i can use them, i want to build one single tube amp if its posible and very basic, anyone had a easy schematic i could build??


PD: sorry my english is bad.
Many Thanks!!!!
Thanks!!!
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Old 4th May 2008, 01:00 PM   #2
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Location: Monroe Township, NJ
Other than the awkward heater voltage, employing the 25L6s is easy enough. What are your local AC mains like (voltage/freq.)?

The simplest circuit you can execute will employ triode mode "finals". Assuming a stereoblock HIFI amp, approx. 2 WPC is what you will get. Do you have access to the highly efficient speakers such low power O/P requires? 2X the power O/P of triode mode is available in pentode mode, but you get involved with more complex circuitry.

BTW, are you prepared to deal with the expense of acquiring the necessary magnetics?
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Old 4th May 2008, 02:46 PM   #3
fedecv is offline fedecv  Argentina
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Thanks for your reply Eli Duttman!, i had read an article of valve basics (http://web.telia.com/~u85920178/begin/tube0.htm) written by Harry Lythall, and builded the basic pentode amp here described, it works!!, here ac mains are 220v 50hz, i used a 220v to 110v tranformer and then a rectified it getting something arround 150v for HV, i used a 220v to 9v to match impedance to the speaker(wich probably is really bad) and the sounds are quiet good... and all components values are aprox!
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Old 4th May 2008, 02:46 PM   #4
fedecv is offline fedecv  Argentina
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Join Date: Dec 2007
My spekears are not very efficient , but i dont need much power, a mono amp will be ok xD, can i use STD transformers(like 220v to 9v) to match speaker impedance? i used one to test the amp but im getting very low output anyway maybe tube is dead, if you have any simple schematic it will be appreciated (triodes are more simple?). Thanks!!
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Old 4th May 2008, 04:09 PM   #5
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Location: Monroe Township, NJ
What are you trying to accomplish? Do you want a HIFI amp or a guitar amp? Can you get a hold of additional 25L6s?

Please post a schematic of the power supply you built. I hope you didn't use a step down autotransformer to get the "110" VAC. Doing so is deadly dangerous.

For the most part, power transformers, like those you describe, are unsuitable for coupling power O/P tubes to loudspeakers. Perhaps another Argentinian member will provide a convenient source for tube amp O/P transformers. Shipping magnetics over long distances can be costly, due to their weight.
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Old 4th May 2008, 09:23 PM   #6
fedecv is offline fedecv  Argentina
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Join Date: Dec 2007
Well i want to build a very basic amp(low power, only 1w rms @ 8 ohms would be nice) just to learn some about tubes and try a tube amp, i builded a basic pentode amp(http://web.telia.com/~u85920178/begin/tube8b.gif) but im not sure if i caculated well the values for the 25l6 and well the matching tranformer that i used is not suitable as you said. Then maybe i will try some hifi stereo tube amp for my pc xD. the big proublem is getting the matching audio transformers here, i will try to wind one. The schematic of the power supply is attached.

Thanks man!!! sorry i dont explain myself well...
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Old 4th May 2008, 09:23 PM   #7
fedecv is offline fedecv  Argentina
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Join Date: Dec 2007
i forget a part of the power supply , im using another transformer 220v to 24v for the heater of the tube.
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