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Old 26th August 2005, 02:59 AM   #1
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Default 6AU6 to do EF86 duty?

Does anyone know whether the 6AU6, used in pentode connection as a voltage amplifier, can give acceptable results? For instance, how would it behave as the first stage in a Mullard 5-20 amplifier, instead of a "low noise" EF86?
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Old 26th August 2005, 03:22 AM   #2
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I have briefly experimented with the 6AU6 and found two interesting phenemenon. They are not all the same, and they are a bit picky about their operating point.

I have about 300 of these so I tried them as audio amps. There are many different brands for these tubes. If I set up the amp for one tube, and plug in a different one, I have to readjust the cathode (and sometimes the plate) resistor.

I have also found that the sound changes pretty dramatically with the bias point. They sound lifeless at low current, then sound OK and begin to distort as the current increases. Some are worse than others in that respect.

It should be noted that this was a brief test since I have about 100000 tubes to go through. I am trying to find out which which ones are suitable for audio. I put this one on the "investigate further" list, but haven't had time to get back to it. I wan't to try it in triode mode. Most of my tubes are used and may be defective, although all of the ones that I used show good on my tube tester.

I have tried the similar 6BA6 tube and found it useless for audio due to high distortion.
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Old 26th August 2005, 03:52 PM   #3
Jaap is offline Jaap  Netherlands
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Default Gary Pimm thinks 6au6 are fine

http://home.pacifier.com/~gpimm/Tabor3s.gif
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Old 26th August 2005, 04:13 PM   #4
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LQQK at the specs: 6AU6 Specs. Extensive range of pretty good linearity. Also has good linearity operated as a triode.
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Old 26th August 2005, 05:22 PM   #5
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"tubelab", thanks for that. I wonder if it's particularly noisy, as pentodes are reputed to be (partition noise)? I have noticed, BTW, that a GE 6AU6 (in triode mode) was very microphonic, whereas an RCA tube doing the same job was not.

Jaap, you're right, Gary Pimm used 6AU6 in quite a few variants of his partial feedback amp.

Miles: Yes, I've seen the specs and I agree it sounds fine as a triode. I used it for a while with g2 as the anode, g3 and plate grounded, plate curent 2mA. This was as a preamp stage, following a James (aka passive Baxandall) tone stack that attenuated about 12dB, so the signal was pretty small. For line-level signal or bigger, I'd use conventional triode connection with about 5mA or more plate current.

I'm using the 6AU6 successfully at present as a pentode CCS in the tail of a 6SL7 LTP splitter, with a neg voltage rail of 110v. What I'd really like to know is if it works OK as a voltage amplifier in pentode mode, i.e. not too noisy and sufficiently linear: as good as an EF86, in fact, with all that lovely pentode gain!
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Old 26th August 2005, 07:37 PM   #6
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Hi Ray. Curious why you wouldn't use an EF86. The Winged C version measures and sound nice as a front end tube in my amp. Reasonably priced as well (compared to NOS.)
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Old 26th August 2005, 11:57 PM   #7
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What I'd really like to know is if it works OK as a voltage amplifier in pentode mode, i.e. not too noisy and sufficiently linear: as good as an EF86, in fact, with all that lovely pentode gain!

Certainly does. You can easily have THDs below 5%. However, it's still a pentode and that means noisier than a triode, and the harmonic distortion will have more higher order products than what you'd get with a triode. There are lots worse pentodes for small signal audio work, but none better that I can think of. Like tubelab said, pick your Q point carefully.
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Old 27th August 2005, 01:06 AM   #8
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Default Also......

many are microphonic.
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Old 27th August 2005, 05:18 AM   #9
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Quote:
Curious why you wouldn't use an EF86.
Convenience, really. I already have a few 6AU6s as well as the holes drilled in the chassis for the bases, from when I was using them as a triode preamp stage. I don't have any EF86s or B9A bases.

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Certainly does.
Thanks, Miles, that's good to know.

Quote:
many are microphonic
Yes, I was afraid of that. My RCAs seem OK but my GEs are very microphonic.
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Old 27th August 2005, 10:34 AM   #10
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Default 6au6

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