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6au6 uses?

janikphoto

janikphoto

Lunatic Member
I'm stealing the dynaco 12ax7's out of an fm3 tuner I picked up, and I'll replace them with cheapies I have sitting around. It also has a pair of 6au6's in it, but I'm not sure what I could use those for. Where would I find 6au6's used most of the time?
 
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FM radios as limiter stages, don't care for them in audio apps. Heathkit tried using them them and they sucked big-time.
 
IF strips mostly. Not much outside of tuners used them and what did seems to have done so poorly as I understand it.
 
OK. Well, I changed them out for some old westinghouse and rca tubes I had, anyway. I also pulled the pair of 6ba6's, in case I need a matching pair of those, too.
 
With your info, I was able to do a bit of reading on another website and see that these are in fact best used in tuners only. That's fine by me. I'd rather have a matching set of Dynaco tuner tubes in my storage than some random Westinghouse, RCA, and Sylvania tubes. This is the second FM-3 I've owned now. Too bad I didn't keep the 12ax7's out of it, too. Here's the tuner in action, after the tube changes...

http://youtu.be/k4z2kzIdrjA
 
nashvillebill said:
Considering Dynaco didn't make tubes, they merely bought other people's tubes and rebranded them, so you could still have random Westinghouse, RCA, and Sylvania tubes in there.
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True, but I know the 12ax7's are telefunkens. Not sure about the 6au6's or 6ba6's, as I didn't look closely. But, I meant more that I had clearly-labeled matching tubes by Dynaco, not that they were the actual manufacturer. My stack of tubes had two westinghouse, an rca, and a sylvania before... all looked rough on the outside, too!
 
One thing with changing out tubes in tuners is that it can slightly affect the alignment. Basically its the same thing that swapping tubes in an audio chain does, but tiny changes in capacitance and tube construction tend to affect RF circuits even more than audio circuits. It probably needs an alignment by now anyway, but if you ever want a tube tuner to be at its absolute best, it should be aligned with the tubes you plan to run in it.
 
Yeah, that crossed my mind. It seems to be pulling in really well, as is. But, I'm sure a good bias adjustment wouldn't hurt!
 
With due respect for disagreeing members, I used 6AU6As in audio before the days of EF86s and the like, and did not find much wrong with them, provided they were properly used. One cannot mention 6AU6 and 6BA6 in the same breath: the latter is a variable µ tube where that characteristic is of advantage in gain-controlled r.f. stages. It can thus be quite non-linear in audio use. The 6AU6 is a sharp cut-off pentode - admittedly old - but with no recognisable disadvantages for audio.

These days one would not prefer it compared to lower noise less microphonic later pentodes, but as relatively high gm pentodes go, what were the problems compared e.g. to 6BL8 or 6U8?
 
I fully agree with the comments of Audiovet above: 6AU6's were actually used in many old audio equipment with very good results, comparable to a EF86 in performance. Check the Audio Anthology (re) issues for many exemples. 6AU6's were also used in professional audio gear and tube microphones (ALTEC M11 "coke bottle" mic) which is a very demanding and critical application for a tube. Of course, 6AU6's vary widely in quality and sometimes need to be selected for best noise performance, but this is true for many other tubes. For those who want the best there is also a premium/industrial version called 6136.
I can't see any valid reason preventing the use of a 6AU6 in an audio application. Actually much worse tubes were recycled by "audiophiles" for audio use.
 
Not the most linear tube I agree, but for small voltage swings in low level preamplifier circuits (the intended application) the linearity is quite acceptable and no "vari-mu" or compressor effects are noticeable. The 6AU6 is a "sharp cutoff" and not "remote cutoff" (variable mu) pentode and not well suited for compressor circuits. (there you should use a 6BA6)
The 6AU6 may also be used as a triode with earthed plate and suppressor grid, using the screen as the anode. In this configuration it is an excellent low noise preamp tube.
Many old tube databooks also recommends the use of the 6AU6 as: "RF/IF amplifier, FM Limiter and Audio voltage amplifier."

(References: Radio Designer's Handbook, Langford-Smith, 1953 and CBS Electron Tubes Technician's Handbook, 1957)
 
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janikphoto said:
Yeah, that crossed my mind. It seems to be pulling in really well, as is. But, I'm sure a good bias adjustment wouldn't hurt!
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There's not much to adjust on an FM tuner that isn't in the instructions that won't pretty much make it worse if not non-operative. Generally any adjustments must be done by a technician with a sweep-signal generator and oscilloscope.
 
The FM3 instruction manual has alignment procedures without using a sweep generator and scope,it uses mostly the tuning eye and a dvm.
 
s-petersen said:
The FM3 instruction manual has alignment procedures without using a sweep generator and scope,it uses mostly the tuning eye and a dvm.
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Yep, and it even says not to do it the "fancy" way. I did one once and it turned out ok as far as I could tell. A little tube like the 6AU6 that is good in audio and just as cheap is the 6AV6.
 
s-petersen said:
The FM3 instruction manual has alignment procedures without using a sweep generator and scope,it uses mostly the tuning eye and a dvm.
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Good to know, I think I remembered something like that, hence my "in the instructions" qualifier. Otherwise blindly twiddling the IF alignment hoping for an audible improvement without guidance is inadvisable.
I had one of these once, decent sound when it worked tho' suffering from SM shunt capacitor disease for which 'K' can IFTs are notorious.
 
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A sweep alignment, if you have the gear for it, will give better performance than a fixed frequency peak and zero volt tune on the discriminator. Those instructions are in all of the really old service literature but you tend to end up with an asymmetrical IF response. It'll work, but sound quality may not be optimal and its probably even more important for stereo gear to get the pilot tone through correctly.
 
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