I've seen the KT66 mentioned here a handful of times but there has never been a follow up on any of those threads.  One guy even said his were running a little hot even after turning the bias as low as he could...  No reply to his post.

I'm just curious because I ordered a pair hearing rave reviews about them in general and would like to know what others think or problems they've run into.

Thanks much guys!!!

just a heads up you may not like the 12AT7 in the PI spot. Significant loss of volume and tone (i speak from experience)

Here are some notes from guitarampliferblueprinting.com:

Some basic tube facts
• 12AX7
o Has a published spec gain of 100
o Has a publishes spec current output of 1.2 milliamps
• 12AT7
o Has a published spec gain of 60-70
o Has a published spec current output of 10.0 milliamps (ten times that of the 12AX7 as a side note)
As a third example, a 5751 has a gain which is almost identical to the 12AT7 but it’s standing current is 1.0 milliamps (about the same as the 12AX7). But, there is a third factor, transconductance, to be considered. The 5751 has a transconductance of about 1200. A 12AX7 has a “TC” of about 1600 and a 12AT7 has a TC over 5000. These three tubes act quite differently. A 5751 and 12AX7 are much more closely related than the 5751 and 12AT7.
We will stick to the basic 12AX7 and 12AT7.


When you push your amp hard it is not as much the output tubes distorting as it is the phase inverter breaking down and distorting. We are talking output stage distortion here. We are not talking about how you may have messed up the signal with preamp tube distortion and compression already. The phase inverter may be the hardest worked tube in most amps. I cannot begin to count the times when I have found phase inverters that were long past their service life. When you change your output tubes change that phase inverter. At the least change it every other output tube change.

Many folks think that when they want to have their amp have more clean headroom they can insert a 12AT7 in place of the 12AX7. Very true. (By the way, the 12AT7 in a first gain stage is an awful tone generator in a guitar amp. If you want to drop front end gain use a 5751 (gain of 60-70) or a 12AY7 (gain of 40). The 12AY7 was the first gain stage in the classic Fender Tweed Bassman, Deluxe, etc.
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

Greg, that was awesome!  And I love the way some AT's sound in some of my favorite amps.  None are Fenders...  

Sorry, I only have old stock ones.   Some of my old British Tungsrams ones have the Mullard tone going on in spades.  Probably the nicest tones can be had out of the Sylvania triple mica, gold print gray plate, the Telefunken, Siemens 6060, Tung Sol blackplate, and RCA blackplate.   The Mul;lard cv4024 is very nice, big bottom and great clarity on top.  That is my fave PI 12at7 type, they are very long lived and very nice sounding.

I'm glad and my ears are glad I spent the money and have them in hand insted of believing what I read. 

And as a final note, huge thanks to Greg again for his careful investigations with DVM!

Mullard's version of a 12AT7