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-   -   Mods on leak stereo 20 (http://www.diyaudio.com/forums/tubes-valves/32848-mods-leak-stereo-20-a.html)

bronto64 23rd April 2004 09:29 PM

Mods on leak stereo 20
 
2 Attachment(s)
Hi, finally i choose this amp for my fisrt project. Mods/tips/etc. are welcome.
Thanks, Federico.
PD: post the schematic .

SY 23rd April 2004 09:41 PM

I rebuilt one of these when I was in grad school; absolutely gorgeous construction and HUGE transformers for the power rating.

In any case, I'd balance the two plate resistors in the long-tail phase splitter, then change out the cathode resistor (the long-tail) for a current source. In my rebuild, I added a fixed-bias supply, took the output stage cathodes close to ground, and converted the output stage to triode.

EC8010 23rd April 2004 10:22 PM

Do you intend to obtain a Stereo 20 chassis and rebuild it, or are you going to obtain transformers etc and use them to build the circuit? All the little capacitors (<1nF) are specific to the output transformers. If you modify as SY suggests, this could also change HF response. Could you obtain access to test equipment (oscilloscope, oscillator)? Do you know someone who could give you guidance in using test equipment?

SY 23rd April 2004 10:24 PM

Mmmm, good point that I missed- my assumption was that he was starting with an actual Leak. And you're absolutely right about having to redo the compensation and feedback- sometimes things are so "obvious" that we forget to mention them; mea culpa.

EC8010 23rd April 2004 10:28 PM

Fortunately, CD players etc don't produce square waves that upset amplifiers, but it would be best if he could set the compensation to suit the transformers.

bronto64 23rd April 2004 10:45 PM

Hi, thanks for yours guide, i start in this business with the help of a friend , he is an electronical tech and is he first time in tubes, but he knows about instrumental, work with high tension and other stuffs. Your guide is very apreciate, in this country exist the posibility of obtain tubes free or cheap, but the trafos is other history. I am newbie and learn fastly with the help of friends (like both of you). I have the schematic, and trafos, tubes , etc. What are the trafos values? I checkit with the trafos.
Thanks and sorry if my english is no very clear.
Federico

andyjevans 23rd April 2004 11:48 PM

I have a very nice circuit using 6N30, 12B4 and EL84. the schematic is too big to load but I can send it in an email. Andy

fdegrove 23rd April 2004 11:55 PM

2 Attachment(s)
Hi,

Andy, if you like I can resize it and post it.
Just push my e-mail button.

Here's a little amp I always like based on the Leak Stereo 20 but utilizing a pair of EL84s per channel:

Developped by Croft Acoustics:

Cheers,;)

EC8010 24th April 2004 12:10 AM

Quote:

Originally posted by bronto64
I'm starting in this business with the help of a friend , he is an electronic tech and this is his first time with tubes, but he knows about instruments, works with high tension and other stuff.
Good. You have someone to help, and he knows about high voltages. For one pair of push-pull EL84, the output transformer primary should be about 8k. If you parallel two pairs of EL84, 4k.

fdegrove 24th April 2004 12:18 AM

Hi,

Quote:

Here's a little amp I always like based on the Leak Stereo 20 but utilizing a pair of EL84s per channel:
That should read:

....I always liked but utilizing TWO pairs of EL84s per channel.

Note that the suggested improvements by the other members can easily implemented and of course you can drive a single pair of EL84s just the same but the value of cathode resistor in the output needs to go up to about 270 Ohm.

Cheers,;)


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