12at7 in the phase inverter?

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Mr. Willy

Mr. Willy

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Anyone running a 12at7 or equivalent in the phase inverter? I tried in a few amps I owned, specifically a 2204 clone and a 2 channel dual rectifier and I didn't seem to notice much difference. I slapped one in the pi slot of my killer Kali 2203 and really noticed a difference. The tone seems clearer and more defined. I guess it really depends on the specific amp. Anyone else have good results using a 12at7 in the pi?
 
It depends on the circuit design and how much voltage has been stepped down at the PI. A 12AT7 will output about 70% of a 12AX7. The KK's signal is probably still pretty hot passing through the tube loop to the PI and the AT7 cuts the output to the power section somewhat. Whereas, your 2204 is probably below the service voltage of the AT7 anyway. It won't hurt anything. If you have a DMM, you can check the pin out voltages and see. Every once in a while, I get a mod that needs a little taming. Instead of an AT7, I have a few of those Jan-Phillips 5881's. Run these anywhere from the input and gain stages to cut a little bit of the edge off or quieten the amp a bit.
 
steve_k":1pyu851g said:
It depends on the circuit design and how much voltage has been stepped down at the PI. A 12AT7 will output about 70% of a 12AX7. The KK's signal is probably still pretty hot passing through the tube loop to the PI and the AT7 cuts the output to the power section somewhat. Whereas, your 2204 is probably below the service voltage of the AT7 anyway. It won't hurt anything. If you have a DMM, you can check the pin out voltages and see. Every once in a while, I get a mod that needs a little taming. Instead of an AT7, I have a few of those Jan-Phillips 5881's. Run these anywhere from the input and gain stages to cut a little bit of the edge off or quieten the amp a bit.
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Exactly what I thought. The KK's got a lot of preamp gain, so the at7 does tame it a bit. I don't know if I like it better than an ax7, it's just the first time I've actually heard a difference.
 
I used to have one in my Recto's PI spot. Not a lot of difference there.
Today I put it in the 2nd (gain) spot and really like it there. The Recto's gain is so over the top, now I can raise it higher and get more clarity and less mud. No boost needed. A real cool trick for Recto's IMO.
 
Yeah, the Recto's are very responsive to various graded tubes. Most people don't pay too much attention to the PI tube because it doesn't make gain or affect the tone stack. But, as the output tube from the preamp to the power section, some amps respond much differently and can really change the feel of the amp. If I have one that is sort of dark and tight, usually something like a JJ or a Tung Sol will open it up a bit. For an amp that is too loose, saturated or bright, the Sovtek LPS is usually the one to tame it. An LPS in V1 will also calm an amp down a bit.

Steve
 
some great info here:

http://www.guitaramplifierblueprinting. ... verter.pdf

"Going from a 12AX7 to a 12AT7 in the PI (phase inverter) will yield a change in output tube distortion, touch, and output dynamics in most amps. Is this because the gain is lower in the 12AT7? 10% yes perhaps. The lower gain is a factor but the larger factors are:
• We have almost 10 times the current available to drive the output tubes before the phase inverter starts to break down.
• We have a transconductance of 5500 vs 1600 of the 12AX7. Keeping this simple, it means it takes a lot less input signal for a given output signal.
"

i use a 12at7 in my jcm800 , it feels better for want of a better term and sound wise too, going to a 12ax7 afterward for a short time (the tube was already quite old and stopped working ) just didn't feel and sound right at all

for most of the amps i've tried tube swaps they work really well in the PI slot, along with say 5751s in V1 when there's too much gain on tap

there's maths/electronics explanations, but as always trust your own ears and whatever works best for you
 
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