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Old 20th July 2005, 02:45 AM   #1
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Default 12au6 preamp

hey I have some old tubes lieing around from an old radio. I've heard that the 12au6 can be used as a preamp, if so, how?
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Old 20th July 2005, 03:07 AM   #2
kevinkr is offline kevinkr  United States
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More to the point why would you want to? The 12AU6 is not particularly linear and is extremely noisy, hummy and microphonic compared to say an EF86.

It was commonly used in relatively cheap power supply regulators in pentode connection as an error amplifier as well as the obvious IF amplifier duty in AM/FM radios for which it was originally designed.

Kevin
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Old 20th July 2005, 03:17 AM   #3
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well its all ive got, id like to get my feet wet building a tube preamp. what will a 12be6 do?
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Old 20th July 2005, 03:54 AM   #4
PRR is offline PRR  United States
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> what will a 12be6 do?

Convert 1210KHz to 445KHz?

These are radio tubes which have very different optimizations than audio tubes. The 12BE6 is a flippin' hexode! Whatever you've heard about or from good audio tubes is probably not the same when using the radio tubes.

Also, nobody has ever really looked at using these tubes in audio. You have to figure out proper circuit values. Dudes who can do that well cost more than a tube, so it makes economic sense to just go buy a 6SN7/12SN7 or other well-known widely-used audio tube so you can use existing designs.

However, a tube is a tube. Even the wild wacky 12BE6. Here's my free design advice: Connect grids 1, 2, 4 and the Plate together. Treat it like a half a 12AU7 or 6SN7. It will work: audio output bigger than audio input. The radio-oriented specs for 12BE6 don't even let me guess what the audio distortion is, but I suspect it will be large because some forms of "distortion" are actually "good" (or "not-bad") in tuned radio circuits.

6AU6/12AU6 is actually sometimes used for audio. The BBC made outside broadcast consoles out of buckets of 6AU6. Hmmmmm.... strap grids 2 and 3 to Plate, pretend it is a half-6DJ8, it won't be too very far off. You may have to fiddle the cathode resistor a bit. You can also run 6AU6 as an audio pentode: workable values are in the RCA book. Gain can be VERY high; like 400. There are not many places today where you want that much gain, and the high-gain conditions lead to very high output impedance that can't be carried more than an inch, so you need buffering.

As a starter project, I'm not keen on either of these. But go ahead and try.
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Old 20th July 2005, 07:33 AM   #5
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*AU6 is just fine in line-level and driver stuff. Not really good for a phono preamp mind you.

*BE6 is really cool for making line-level compressors or as a mixer in sound generation equipment.

The hobby is all about experimenting - there is no such thing as a useless tube
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Old 20th July 2005, 10:50 AM   #6
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A simple question was asked by ThSpeakerDude88 and that was could a 12au6 be used as a preamp and if so how. Why not just answer the question and leave out the verbal bashing? Maybe its not the best suited tube. Maybe its all he has and therefore he just wants to try it. Maybe he is new to this sport.
How about just giving the guy a break with a simple two sentence
answer to his question?

Just a thought...
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Old 20th July 2005, 03:32 PM   #7
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haha yeah. however, I have established by now that neither tube "sounds great" for audio purposes, then again, neither did my first lm386!

I guess I kinda forgot to mention...I wanna use this tube preamp for a guitar, not anything that has to have low distortion factors. If anything, why not a preamp with built in distortion? I dont know too much about tubes, thats why I'm posting ,sos I can learn!

p.s.: can this tube preamp i want be ran on a low voltage, such as 12 volts or so ( lower maybe?) I've heard that the 12au6 will run on a 6 volt heater current because they used them in car radios also.
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Old 20th July 2005, 07:32 PM   #8
PRR is offline PRR  United States
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> A simple question was asked .... Why not just answer the question and leave out the verbal bashing?

I did not "bash" TSD88. I slung a few stones at the idea.

> How about just giving the guy a break with a simple two sentence answer to his question?

Please feel free to do so, especially if you have used 12AU6 or 12BE6 in audio. I'd guess that you know more, at this time, than TSD88 does right now.

I gave some suggestions for using these odd tubes as semi-equivalents of very common types.

Yes, it ran to six or more sentences, way over your two sentence limit. Sorry, it would take 3 times longer to boil six down to two. I donate my time here "free", but I don't work cheap. What TSD88 is really asking for is a several-hour research and development to avoid his spending $5-$20 for other tubes (nevermind that a tube amp is much more than tubes). Considering the world and web is full of well-tried plans for inexpensive tubes, I'm not inclined to invest my hours and coffee just to suit one man's collection. Giving the sorta-equivalents could be enough of a clue to get a "working" amp going.

I now regret even that much, because TSD88 has been coy about saying what he really wants. It was my-bad for ASSuming that "preamp" in this forum context meant "hi-fi amp"; we now learn that TSD88 wants a guitar preamp. Fairly different problem, as he knows. However we also learn that TSD88 may need to learn more about general tube systems; the "heater current" question implies he has more learning to do. That's cool, and I'll throw some more crumbs and stones later (my day-job needs attending NOW). But the simple question "12AU6... how?" turns out to be a much bigger question than it seemed.
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Old 20th July 2005, 07:36 PM   #9
Giaime is offline Giaime  Italy
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Quote:
Originally posted by PRR
the "heater current" question implies he has more learning to do. [/B]
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Old 20th July 2005, 10:07 PM   #10
PRR is offline PRR  United States
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> can this tube preamp i want be ran on a low voltage, such as 12 volts or so ( lower maybe?) I've heard that the 12au6 will run on a 6 volt heater current because they used them in car radios also.

If the 12AU6 was used in cars, it would have been the 12 volt cars from after roughly 1960. The heater voltage has to be close to right. GE says 11.34 to 13.86 for 12AU6.

It is very likely that the 6BE6 was used in car radios in 6-Volt cars: this is the dead-standard AM radio converter tube.

To get off-track: there was a short-term series of car-radio tubes that "worked" with 12V on the heaters and 12V on the plates. They did not work very well: 12V is not enough to make a tube plate happy. But once we had a transistor that would drive a speaker with only a 12V supply, it was economic to make the RF and IF stages limp along with 12V plate power rather than buzz-up a 250V supply. In less than three years, transistors improved enough to displace tubes from the RF and IF stages too.

The 12BE6 is THE standard converter in US-market five-tube AM radios.

With a 12AU6 next to it, I'm thinking you have an AM/FM radio. 12AU6 is unlikely in an AM radio (maybe a good short-wave radio RF amp) but common in FM radio IF strips.

> can this tube preamp i want be ran on a low voltage

Heater voltage is not a big deal. Sort through old wall-warts, you can find 6V or 12V power easily.

"Good" tube operation needs a high plate voltage, 50V-300V. Is that a big problem? It is often a major part of any tube-project's cost, and potentially dangerous.

There are several tubes specified to work at lower voltages. The low-volt car-radio series was pretty pathetic, needing tuned circuits to give significant gain. The "power" tube in the series gave teeny power at tremendous distortion. Only the low expectations of car-radio let them get away with such lame tricks. There were ship-radio tubes rated for modest performance on 24V power, such plans are around the Web.

> I wanna use this tube preamp for a guitar

You "can" build a complete guitar amp with just a pentode like 6AU6 and an output tube like 6V6 or 7591 or 6BQ5. However the very high gain of the pentode makes this VERY tricky: think of putting a V-12 Ferrari engine in a Toyota. They typically work (if at all) on the edge of instability. This can "add tone" to guitar sounds, or just squeal supersonically. Triodes are MUCH better bets for guitar preamps until your feet are much wetter than I think they are. 12AX7 are not very expensive.

But who knows what "crazy" plans you may find if you search the many guitar-amp sites. Perhaps someone has already put in the time to develop guitar-amps using radio tubes.
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