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    the safety precautions around high voltages.

12au6 preamp

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Joined 2004
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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.



Joined 2003
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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.
*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 :cool:
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...
haha yeah. however, I have established by now that neither tube "sounds great" for audio purposes, then again, neither did my first lm386! :p

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! :D

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.


Joined 2003
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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.


Joined 2003
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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.
*Sigh* I'm not planning on building a complete tube guitar amp YET.

I'm only wanting to build a PREAMP so I can run my accoustic guitar's pickup into a chipamp ( TDA2040 used in fender champs( I think it was the champ, dont hold me to it)

anyways... yes the tubes are out of a '67 rca superhet 6 tube radio.

I would really like to know if anyone (else) has general plans or conversions so I can use it in someone elses circuit. I have a 12v,13.5vac,18vdc, and 32v psu to use. I would really like to be able to power it off of a small voltage ( if it will work at all) so it can be portable ( no internal transformer) and so I can use a wall wart, cuz I dont wanna mess with higher voltages untill I get a mm AND I dont have any bigger psu's. As for the dissing the not wanting to dish out 20 bucks, I'd gladly do it if I actually had some help on this project- IF I had money. I'm only 16 and currently looking for a job, gotta save up for insurance and a new interior for my 63 falcon, and start saving for a house...etc etc all that good stuff. I'm here to ask questions and learn, and not be discouraged from my favorite hobby. ( no offence on PRR's part, you seem like a very knowledgable person and I respect that)

p.s. thanks for the tube specs on the 6au6 and 6be6 prr! :)
haha maybe not :p

I've been interested in electronics since I was 6, taking apart every transistor radio, hair dryer (those things have nice motors!) speaker, rc car, etc I could get my hands on. Though I messed a lotta stuff up, I sure learned from it! O and Ive also learned the hard way not to "cut the red wire to the cathode on that tv tube, make sure you use your most unsulated pliers" :xeye:

but seriously, besides model building and working on engines, electronics is my favorite hobby! Now I'm into building circuits and modifying them. I love audio, so I build more speakers and chipamps than anything. Someone on the board just taught me ohms law too, which reallly helps! Now I can learn to bias resistors and more safly work on tube stuff! I love tube stuff btw, if you havent noticed. I love the beauty and grace of a tube amp, and the sound. I had an old tube am/fm radio, and boy was that thing sensitive! all stations were crystal clear and full, now I dont think very highly of my fm-857 fisher tuner after hearing that one. Also have a 1936 rca superhet, got it working and decided to play around with the phono inputs. I was surprised it took a high line level input instead of a really small signal like for phonographs. ( Is this because phonographs back then had preamps on board already?) anyways, I plugged my mixer into it and my guitar into the mixer. My mixers a cheapie (some radioshack brand from the 70's, but hey it works) and the output is very loud. So I turned it up 3/4 volume and turned that radio to full. I'm spoiled now! I want a tube amp for my guitar so bad I cant see straight! I loved that 4 watts of overdrivven tube sound into that old 8 inch feild coil speaker! If 4 watts is that loud, I think a simple 10 watt or 15 watt tube amp for my guitar would rock! :)
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?

I wouldn't do that. The 'AU6 family of small signal pentodes have amplification factors of right around 5200. In audio work, you seldom need that much gain. Since any radio needs ~100db(v) from antenna to loudspeaker, of course you will find these used as IF amps, usually run ballz-to-the-wall to maximize gm, and thus, voltage gain. Since IF amps at least include double tuned RF xfmrs (if not crystal BP filters) distortion is not a factor.

For audio work, pentodes are problematic. Though much more linear than transistors, like transistors, they tend to produce objectionable higher order harmonics. That means lots of tweaking to get that distortion down. Furthermore, it would be way nice if the screen grid simply acted as an electron accellerator to horse up the gm. Unforch, it doesn't, being very much a second control grid. This means that the usual series voltage drop resistor and screen bypass capacitor also introduces an extraneous pole into the transfer function. Pentode amps can sometimes have characteristics that you aren't expecting because of that. You have to be careful what you hang on that screen grid.

Since you say you have these radio tubes, look for a 6AV6 or 12AV6. Use that for your preamp. This is a high-u triode that can easily give gains of 50 -- 70. If your source is 10mV(P), a gain of 50 gives 0.5V(p) -- that should be sufficient for any respectable amp.

Frank's has data on the 'AV6, 'AU6, and plenty of other tubes.
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