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Author Topic: 12at7 PI  (Read 6080 times)

jpfamps

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Re: 12at7 PI
« Reply #15 on: April 26, 2011, 08:55:29 PM »
There's quite a lot of twaddle in that article, so I would not regard it as an authoritative source of info. Bear in mind that the author works for Groove Tubes, ie a company that sells (expensive) valves.

As stated, a 12AT7 has a lower gain than a 12AX7, and can source more current than a 12AX7.

The main effect of using a 12AT7 instead of a 12AX7 in a long-tailed pair (LTP) PI is to reduce the gain. Unless the circuit has been optimized for 12AT7s, ie the plate load resistors have been reduced in value from the typical 100K (Fender used 47k in the later Silverfaced amps), then additional current sourcing capability will be negligible.

Bear in mind that most amps inject negative feedback into the bottom of the LTP, so any reduction in gain of the LTP will also reduce the feedback factor.

Also, assuming that the balance of the LTP has been optimized for a 12AX7 (and many haven't), then using a 12AT7 will unbalance the outputs; whether this is good or bad

I've had good results with 12AT7s in the LTP, but not so much in pre-amps. 12AT7 experience heavy grid currents when the input is overdriven, and I think this is why they don't work as well in pre-amps.

sgmypod

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Re: 12at7 PI
« Reply #16 on: April 26, 2011, 08:59:29 PM »
The only way to find out if it works for you & your sound is to try it yourself.

I think that's how most amp designers do it anyway :)

shhh!!!!, don't be giving the secrets away  :lol:

LOL
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jpfamps

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Re: 12at7 PI
« Reply #17 on: April 26, 2011, 09:00:08 PM »
It is a noticeable difference in tone as well as output level. while the voltage level is reduced, the AT7 can provide more current in its output which is beneficial in some electrical situations.

It's more a matter of output impedance - 12AT7 has much lower plate resistance, therefore lower output impedance and lower Zout means more high frequency response.

The lower plate resistance won't really have much effect on output impedance in the standard LTP with 100k plate load resistors.

The standing current (and hence output impedance) in the LTP is determined by the value of the tail resistor and the plate load resistors.

Regardless the increased bandwidth would be of little benefit in a guitar amp anyway.  

sgmypod

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Re: 12at7 PI
« Reply #18 on: April 26, 2011, 09:00:31 PM »
There's quite a lot of twaddle in that article, so I would not regard it as an authoritative source of info. Bear in mind that the author works for Groove Tubes, ie a company that sells (expensive) valves.

As stated, a 12AT7 has a lower gain than a 12AX7, and can source more current than a 12AX7.

The main effect of using a 12AT7 instead of a 12AX7 in a long-tailed pair (LTP) PI is to reduce the gain. Unless the circuit has been optimized for 12AT7s, ie the plate load resistors have been reduced in value from the typical 100K (Fender used 47k in the later Silverfaced amps), then additional current sourcing capability will be negligible.

Bear in mind that most amps inject negative feedback into the bottom of the LTP, so any reduction in gain of the LTP will also reduce the feedback factor.

Also, assuming that the balance of the LTP has been optimized for a 12AX7 (and many haven't), then using a 12AT7 will unbalance the outputs; whether this is good or bad

I've had good results with 12AT7s in the LTP, but not so much in pre-amps. 12AT7 experience heavy grid currents when the input is overdriven, and I think this is why they don't work as well in pre-amps.

Had to read that a few times.....not been messing with amps for a while, thanks frank..this was to try in my orange rocker
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jpfamps

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Re: 12at7 PI
« Reply #19 on: April 26, 2011, 09:06:36 PM »
The only way to find out if it works for you & your sound is to try it yourself.

I think that's how most amp designers do it anyway :)

I thought a lot just copied marshall or fender.
Apart from high F response (thanks for the explanation by the way) isn't their a benefit where by blocking distortion is decreased if you're driving you amp hard while using a AT or AU over an AX? I can't remember why I think that.


EDIT: Also the lower plate resistance means that they bias up more... naturally (couldn't really think of a good word) than a 12AX7 when used as a cathode follower? Which changes the tone that you get from the normal "poor" design of a cathode follower.

Blocking distortion due to power valve grid conduction can be reduced by reducing the output impedance of PI, however this would require the PI to be redesigned to take advantage of valves such as the 12AU7 or 12AT7.

Blocking distortion can be completely eliminated by using a DC coupled cathode follower to drive the output valve grids (a la SVT). This works well (I've tried it!), but complicates the amplifier somewhat.

sgmypod

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Re: 12at7 PI
« Reply #20 on: April 26, 2011, 09:17:41 PM »
so in easy to understand terms probably wouldnt do much oin the rocker
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