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Simple Low Voltage Tube Preamp?

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CarlosT

Member
Joined 2006
Have never done anything with tubes but I am intrigued about the "sound" of tubes. I was thinking about a simple tube pre-amp to an LM3875 chipamp.

I came across this low-voltage tube 512AX

CK512AX

Pinout: 1(red dot)=anode, 2=screen, 3=filament, 4=g1, 5=filament Vf=0.625V, If=20mA. Va max=45V, Vsg=45V. No power limitation, since you can't draw enough current to do anything bad.

Pentode mode: gm=160uS. Triode mode:gm=250uS, Ra=35k, MU=9.

Reasonable amplifier bias points:

Pentode: 0.7V bias. Va=22.5V, Vsg=27V, Anode supply=45V. anode resistor=180k. Ia=.12mA, Isg=.06mA.

Triode: 0.6V bias. Va+sg=22.5V, Anode supply=45V, load resistor=100k, Ia+sg=.2mA.

Could this be simply implemented with 25V transformers? Are there other simple miniature or low-voltage tubes that would sound good? Since this would be a preamp, a lotta gain is not important or even preferred.

Thanks!
 
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Audio_idiot

Member
Joined 2006
Hi CarlosT,
What is your intention of the tube pre? do you need some gain or just a tube buffer?

Ecc88, 6922 can do at fairly low plate voltage of about 90V, low enough Rp to give low enough Zo, with a suitable plate choke, I think you can use ~100V B+ for the whole stage.

6AS7 or 6080 can also work in the 40~60 B+ zone... cathode follower if you must have that super low output mpedence, even with normal ground cathode, the impedance would still low enough for most applications.

6GM8 is good candidate, but From my experiments, I think the current is too small to produce good sounding cathode follower... I've not tried other method...

Cheers
 
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CarlosT

Member
Joined 2006
WOW! So much to learn...so little time :D

That Aikido stuff was cool...as was the blog.

I got a buncha 25V + 25V toroidal trannies around so that's why I was thinking about low voltage tubes. I also currently have a combined channel mono LM3875 design which coulda benefitted from a buffer.
 
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Sheldon

Member
Joined 2003
davidlzimmer said:
Dang you guys!:D This stuff is SOOO tempting!

Could someone elaborate on the two outputs of the Aikido pre? Could a similar circuit be used with just one tube per channel?

A schematic for mm phono also would be great.

Thinking about building one as a gift to my brother in law.

Thanks!
Click to expand...

Not one tube per channel, but you can use a single section on the input side - so three dual triodes total for two channels.

http://www.tubecad.com/2004/blog0013.htm

Sheldon
 
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Sheldon

Member
Joined 2003
davidlzimmer said:
Thanks, but I was wanting to stick with the 6GM8.
Click to expand...

Broskie's three triode input example applies to any triode - either for the first or second position. The only requirement is that the triodes in the second position be identical to one another. If two triodes sections are used in the first position, they also need to be identical to one another. The point was that you can build a full Aikido amp with three triodes. I believe 6GM8 is a dual triode with identical sections, so it should work for either or both positions. And you can use three tubes total for a stereo pre-amp.

Sheldon
 
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