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Old 10th May 2012, 10:28 PM   #1
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Default A friend gave me some random tubes...

So, a friend gave me a few tubes he pulled from some random equipment in a dumpster outside the EE building at my university. There are two 12ax7, two 6u8a, two 6x8, one iv2, and one 6aq5a.

I know very little about actual tube numbers (I know how they work in theory, and I have a grasp of tranistors from some classes, but nobody teaches vacuum tubes in EE classes anymore). I know the 12ax7 tubes are pretty common in preamps; can I use these tubes to make either a power amp or a phono preamplifier?
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Old 11th May 2012, 01:18 AM   #2
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6AQ5 is a low power audio output Pentode, good for say 2W.
6U8 is a combination Triode and Pentode good for small signal amplification
6X8 rectifier
1V2 I don't know

You can build a single channel single ended output amp with them.

If the dumpster isn't dumped yet, get the chassis for it's transformer(s).
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Old 11th May 2012, 01:37 AM   #3
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I wish the dumpster hadn't been dumped yet, but that was a while back.

So, do most preamps follow the push-pull setup too? Given that I can't build a powerful amp out of the components, the other thing my setup is missing at this point is a turntable preamp.
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Old 11th May 2012, 02:04 AM   #4
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The 1v2 is a low current high voltage rectifier used for focus voltage in older color television service. The 6X8 is not a rectifier, but a triode-pentode once used as an oscillator-mixer in old black & white TV tuners. The 12AX7s are really the only useful audio amplifier tubes. The 6U8 could be pressed into service if desired.
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Old 11th May 2012, 02:33 AM   #5
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Apparently these came from an old oscilloscope-y thing. And my friend did scavenge the transformers...and then tore them apart to figure out how they worked >.<

Anyway, I guess the next step for me is to figure out how turntable preamps work and which of these tubes I can use in the design. Obviously the 12AX7s, and probably the rectifier too.
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Old 11th May 2012, 02:43 AM   #6
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The 12AX7's can make a pretty good phono preamp. Refer to the uploaded schematic.

The 6U8s could be used in Dyna/Scott style small signal circuitry to drive either EL34s or 7591s in a push/pull O/P stage. 30+ WPC is the yield.
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Old 11th May 2012, 02:51 AM   #7
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Quote:
Originally Posted by LinuxMercedes View Post
and probably the rectifier too.
Nope, not that 1V2. It's only rated for perhaps a milliamp or two at most and the voltage drop is over 100 volts. (The tube manual says a max of 10mA, but it's never used for that much. Typically milliamp)
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Old 11th May 2012, 02:59 AM   #8
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Eli: Fantastic; I'll take a look at that. I may very well build a power amp too, then; 30 WPC should be enough to drive my bookshelf speakers. Do you happen to have a link to stuff about that amp style?

HollowState: My bad; I forgot the 6U8s aren't rectifiers. Anything that 1v2 would be good for?
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Old 11th May 2012, 03:01 AM   #9
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Quote:
Originally Posted by LinuxMercedes View Post
Anything that 1v2 would be good for?
Target practice.
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Old 11th May 2012, 03:40 AM   #10
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The Scott 299D uses a 6U8 and 2X 7591s in the power section of each channel. Just look at the right hand side of the linked schematic to see the power section. We'll talk PSU, if you think this sort of stuff is within your capability. Remember tubes run at deadly dangerous voltages and reading the "nube" safety "sticky" is important.

The Dyna ST70 was an incredibly popular item, back in the day. The linked schematic shows a 7199, but substituting the 6U8 is not a big deal. Perhaps some passive parts value changes will be necessary.

The basic theme of both designs is a pentode voltage amplifier DC coupled to a "concertina" phase splitter and on to PP "finals". Please notice the global NFB loop in both setups.
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