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12AU7 as a power tube

 
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sliberty



Joined: 26 May 2006
Posts: 1094

PostPosted: Tue Nov 09, 2010 3:43 pm    Post subject: 12AU7 as a power tube Reply with quote

I have a 12AU7 hooked up to an 8K OT as a push pull power stage. The sound is a bit fizzy. What causes the fizziness, and how can I correct?
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Phil_S



Joined: 23 Oct 2007
Posts: 3490
Location: Baltimore, MD

PostPosted: Tue Nov 09, 2010 4:35 pm    Post subject: Reply with quote

Steve,
What's your power stage look like?
Self split PP?
SE parallel?
LTP PI and bias method?
Something else?

How about some voltage readings?

Give us a little meat! All you gave us was bone!
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sliberty



Joined: 26 May 2006
Posts: 1094

PostPosted: Tue Nov 09, 2010 4:46 pm    Post subject: Reply with quote

It is a cathode biased push pull power stage (500 ohm cathode resistor). Preamp a Rocket (one tone cap change). PI is a LTP. Plate voltage on the 12AU7 is a little high for that tube - 345V (book says 330V is the max).
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Bob-I



Joined: 02 Feb 2006
Posts: 3674
Location: Hillsborough NJ

PostPosted: Tue Nov 09, 2010 5:01 pm    Post subject: Reply with quote

You might re-wire to a 6N1P. It can handle the voltages and has less fizz to the tone.
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sliberty



Joined: 26 May 2006
Posts: 1094

PostPosted: Tue Nov 09, 2010 5:06 pm    Post subject: Reply with quote

Is the fizz charachteristic of 12AU7 power stages? I could do the 6np1, but the 12AU7 is more common. Also, the VOX Lil Night Train uses one, so I assumed it could be done effectively. maybe I should I just drop the voltages. or isbthat not the real issue with the fizz?
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Bob-I



Joined: 02 Feb 2006
Posts: 3674
Location: Hillsborough NJ

PostPosted: Tue Nov 09, 2010 5:33 pm    Post subject: Reply with quote

sliberty wrote:
Is the fizz charachteristic of 12AU7 power stages? I could do the 6np1, but the 12AU7 is more common. Also, the VOX Lil Night Train uses one, so I assumed it could be done effectively. maybe I should I just drop the voltages. or isbthat not the real issue with the fizz?


Dunno... yes Sovtek is the only manufacture of the 6N1P and 12AU7 you have a ton... in fact I have bunch.

I also found that an LTP really drives these little tubes too hard. I lowered the output with a PPIMV and it cleaned up quite a bit. Maybe a cathodyne PI would be a better choice.

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Phil_S



Joined: 23 Oct 2007
Posts: 3490
Location: Baltimore, MD

PostPosted: Tue Nov 09, 2010 10:23 pm    Post subject: Reply with quote

IMO, the LTP drives the 12AU7 power tube much too hard. In my limited experience with triode amps, I found that triodes do not tolerate the sort of voltage swing that a pentode will. For starters, I want to suggest that you split the PI tube. Use half for a gain stage if you want or not, and use the other half as a concertina. This will clean up your problem. You might think about putting that extra triode gain stage on a switch that lifts it out of circuit.

In theory, at least, sort of, the extra triode will compensate for the gain that the LTP would have added. If I added a gain stage, I would bias it for low gain and low noise. I'm guessing the PI is a 12AX7. If you make a concertina, you might want to do that with a 12AU7 or 12AT7, both of which have better current handling capabilities and I think are better suited to the concertina.

You have a 12AX7 for the PI? I might try to simply ratchet down...5751, 12AY7, 12AT7, 12AU7. That might give you a hint as to whether it is the PI before you start heating up the iron.

You might look at biasing the 12AU7 power tube differently. I don't know what you used for a plate load R, but you might try increasing it and fiddling with the cathode R, too. The 12AU7 can take the current OK, but the over voltage might be problematic.

Of course, I don't need to mention, one change at a time....
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tubeswell



Joined: 27 Mar 2008
Posts: 1563
Location: Wellington. NZ

PostPosted: Tue Nov 09, 2010 11:55 pm    Post subject: Reply with quote

sliberty wrote:
It is a cathode biased push pull power stage (500 ohm cathode resistor). Preamp a Rocket (one tone cap change). PI is a LTP. Plate voltage on the 12AU7 is a little high for that tube - 345V (book says 330V is the max).


You need something like a 22k load resistance through your OT for a 12AU7 as an output tube. See the AX84 firefly
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Phil_S



Joined: 23 Oct 2007
Posts: 3490
Location: Baltimore, MD

PostPosted: Wed Nov 10, 2010 1:47 am    Post subject: Reply with quote

I lost track...I think Tubeswell pretty much nailed it. If the rest of the amp will support it, think about adding a 2nd 12AU7 so you are driving the OT with two triode sections on each half. That will should make notable difference. I have been told that triodes will tolerate a fairly wide range of primary impedance. Putting in a pair of tubes will cut the primary load in half.
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