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Old 11th September 2005, 06:59 PM   #1
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Default thoughts about a 'universal' preamp

About two years ago I built Frank’s pre-amp (12BH7A in a WCF configuration).
Frank's Ultimate Tube Preamp.

After that he published a schematic with a little gain, using a SRPP 12BH7A into a 12BH7A WCF. I was tempted to build it…but than the Aikido already made his entry in the scene and I didn’t exactly know what to build anymore. I remember Frank posting a comment that the Aikido was new for him, but short after that he posted the schematic of the SRPP 12BH7A + WCF 12BH7A again, to show it had lots of things in common (at least in the wiring). So I thought about the possibility to build something more universal, which implies in the possibility to easily change cathode/plate resistors for different tubes/current settings. I didn’t want to solder and desolder it every time and I finally found a solution for this, inspired through http://www.artemislabs.com/docs/PL1_Manual_1.pdf amplifier - the ZIF socket. With this one it’s possible to easily change resistors, and it probably will work very well. For other connections it is possible to use jumpers (well, I will build a prototype on veroboard, so it is easy to use jumpers).

Today I spent some time in designing the more universal preamplifier. Well, it is not actually designing, I just put some freely available schematics together. With different settings of the jumpers it should be possible to 'create' a SRPP amp, a WCF, a SRPP + WCF, an Aikido and a Gomes amplifier.

Issues like the power supply and exact values for the cathode resistors (and the plate resistor in the SRPP) I haven't put here. Power supply van vary a lot. A 6BL7 in SRPP will be happy with 390VDC, almost the double of what the ECC88 can take...so power supply depends on used tube (B+ aren't tied together, but should be). The Aikido design allows one to change tubes without changing cathode resistors, but I like the idea to easily change the settings of the used tubes - that's why I will use the ZIF socket (in the schematic a 'ZIF connection' is shown as two parallel lines, a bit like a capacitor). Jumper pins are shown with the dots, the actual jumpers with red lines.

I post it here and I am sure comments will arise, given the popularity of the Aikido and the 12BH7A WCF, about two years ago.

First schematic is that of the SRPP 12BH7A + WCF 12BH7A

Erik

PS: and if it all seems to strange - I got the 'valve amplifiers' book as a gift for my graduation and I can take some theory from there.
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Old 11th September 2005, 07:02 PM   #2
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the 'universal' amplifier - all those dots.
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Old 11th September 2005, 07:03 PM   #3
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Jumpers set for SRPP operation. Note that the second tube is not needed here.
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Old 11th September 2005, 07:04 PM   #4
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Jumpers set for WCF. Now the first tube is not being used.
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Old 11th September 2005, 07:06 PM   #5
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SRPP + WCF (gain + output buffer)
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Old 11th September 2005, 07:07 PM   #6
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The Aikido - safe version
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Old 11th September 2005, 07:08 PM   #7
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And why not - jumper settings for the Gomes
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Old 11th September 2005, 07:14 PM   #8
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Wow..great Erik...I would call it universal gain stage though..because you could just as easily use it to drive an output tube...etc..etc.
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Old 11th September 2005, 07:22 PM   #9
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WOW.
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Old 11th September 2005, 09:46 PM   #10
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Quote:
Originally posted by Bas Horneman
Wow..great Erik...I would call it universal gain stage though..because you could just as easily use it to drive an output tube...etc..etc.
Sure. I am full of ideas of building different DAC's and riaa amplifiers, I/V and amplifying stages, solid state and tube based etc. But than there is a myriad of designs that can be used for each aplication, and each I want to try. But building each individually would take to much time and, as I like to work from scratch, I would use one kind of capacitor in one and another in the second, so comparisions wouldn't be fair.

Attached is a version with three more ZIF 'slots', allowing the assembling of the 'simple line stage' from John Broskie. Only the first tube is used (it should be a 12BH7A). And so you can go on...

Erik
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