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12au7 phase inverter (modern) tube??

F

fabchef

Well-Known Member
Hi gang. Well, recently, my nos RFT 12au7 tube was not working well, so i took it out. I do have other 12au7's that i use for the phase inverter tube slot. I have older Toshiba and Miniwatt (both made in Japan), as well as reissue Tung sol and Mullard. Of the bunch my least favourite is the mullard. The other 3 are ok. I was wondering what New 12au7!tubes, would you recommend as a phase inverter?
Thanks
Fab
 
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TriodeLuvr said:
What kind of phase inverter? What kind of amplifier? It makes a difference.
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What difference do you expect ? A 12AU7 that follows spec and is used withing it's limits is undistinguished from any other 12AU7 that follows spec ...
 
There was a Buzzing sound coming out of the speakers. When i looked at the tube, it was bright (hot) towards the bottom of the tube. When i cnaged it out, the next tube worked well and no more buzzing.
 
sounds like heater-cathode leakage. Not uncommon with split load inverters like this. The cathode voltage is fairly high and on this one the heater string is grounded.

If you'd like to reduce the chances of that happening again, disconnect the tie to ground at the junction of R54 and R55. Connect a wire from there to C24C. That will bias the heater string up by 24 volts which reduces the voltage difference.

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complete schematic for better context

 
Gadget..thanks for the info. I wish I understood "electronics" better. If i ever get this serviced i will mention it.
could it just have been a matter of that particular tube going bad. The other phase inverter tubes i have, work well.
 
Probably went bad, but the high-ish voltage difference probably made that happen a little earlier than it should have. Looks like this runs about 65 volts on the cathode and a 12AU7 is rated for 100 if memory serves. Its not being abused but sometimes things just go bad. I had a 12ax7 develop a heater-cathode short in a circuit that had it about 12 volts of difference.
 
I wouldn't change the circuit. The 12AU7 is rated for 200V h-k either AC or DC when the heater is negative WRT the cathode (which is the case here). Maybe the tube wouldn't have failed in this mode if the heaters were elevated, but who knows? It's best just to replace it and move on IMO. :)

Jack
 
Triod... that's what i did, swapped out tube and have had no issues and buzzing is gone.

Just wondering now, about new 12au7 tubes that might sound sweet in the phase inverter slot?
 
fabchef said:
Just wondering now, about new 12au7 tubes that might sound sweet in the phase inverter slot?
Click to expand...
The triode in this position has considerable degenerative feedback due to its cathode resistor. That - plus the large amount of negative feedback in the loop - tightly controls the characteristics of the driver overall. This is one of the applications in which the non-linearities typical of a 12AU7 don't come to the surface. In terms of various brands, I would be extremely surprised if there's any difference in sound quality from one tube to the next.

Jack
 
TriodeLuvr said:
In terms of various brands, I would be extremely surprised if there's any difference in sound quality from one tube to the next.
Click to expand...
Yep, in general, I don't think a phase splitter is going to impact sound quality. Gain maybe, but not tone.
Easy enough to find out!
I like Japanese tubes. Toshiba is fine; not sure who made your Miniwatt. Most all of my 12au7's are vintage. If you got to buy new, JJ will work.
 
shouldn't affect the gain either. In that config its a bit less than unity to ground, roughly 0.9 volts output for every 1 volt input. Within a feedback loop you'd never notice it anyway, the differences would be squashed.
 
Miniwatt 12AU7 tubes were also made in Australia, IIRC. Another tube you could add to your list is the RCA Cleartops. They are a great value for your money. Many exists in OS, ANOS and NOS. As far as SQ improvements go, I defer to others. But a tube you should look into as a future option. Tube rolling can become very addictive. GL.
 
This 12AU7 phase inverter is in the signal path, so I suppose it could make a difference to the tone.

The Japanese-made 12AU7/ECC82 that you have should work well. As long as they test high and are quiet.

These cathodyne (Concertina) phase inverters can swing healthy current depending on the setup, so the tube can wear out with time.
 
My VTA ST-70 us3s three 12AU7's. The only one that really makes a difference in sound or tone is the one voltage gain tube. The other two longbtail phase inverters don't really do very much at all for overall tone. I've used different 12AU7's NS and OS, E80CC's, ECC99's, and 12BH7's. I really didn't noticed much if any difference in the phase inverter position.
 
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an LTP inverter also has a pretty high amount of local feedback so it shouldn't have a large effect on how things sound, especially if its got a CCS involved to force equal output.

paraphase inverters probably do, but they don't have much of a mechanism to self-correct. Floating paraphase do to some degree but I expect its still going to be more sensitive to tube variations than other designs.
 
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