Ceriatone Forum

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!!!

Ok, here's an update!

I put the KT66 Gold Lions in and WOW!  More depth, articulation, bit more sustain, smoother, and a deeper/richer tone.  Really to my ears it's almost like crossing a 6L6 with an EL34... 

I set the bias to around 33~34mV which is roughly 60%

Overall it feels more lively :)

(Next step, change PI out to Jan 12at7)

Specs and settings:
V1: Amperex - Holland 12ax7a
V2: RCA - 12ax7a
PI: tung sol - 12ax7
Output tubes: gold lion KT66

Bright: off
Deep: on
Rock: on
Vol: 7
Tone: 8
Mid: 6
Bass: 3
OD: 10
Ratio: 7
Presence: 10

Guitar: SG standard w/ Phat Cat P90s and rewired internals to the '59 Les Paul circuit
Cab: 4x12 Marshall very clean high wattage speakers

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

Glad to hear about the new production KT66 experience.  Which amp? 

12at7 is most definitely NOT clinical and lifeless, depends on the circuit.   Have collected quite a variety of vintage ones, tons of experience with them in 12ax7 positions in single ended and high end PP boutique amps, clean to filthy distortion levels, to the point of being able to make some good recommendations tone wise, just like my collections of 12ax7 and 5751.  Some tube makers exhibit a family tone through different models of their tubes, the 12ax7, 5751, and 12at7 black plates from Raytheon are a perfect example of this.  Experience is the best teacher, not repeated dogma.

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

Also, in my testing i found there was a pretty large voltage difference between having a 12AX7 vs a 12AT7

V3   12AX7 GE Long    Pin1: 239      Pin 3: 39.2         Pin 6: 232     Pin 8: 39.4



V3   Mullard CV4024  (12AT7)   Pin1: 185.1   Pin3: 55.2       Pin 6: 180.2  Pin 8: 55.1

The 12AT7 sounded pretty good with about a 8 volt swing between Pin 1 and Pin 2, and with the JAN Phillips 6L6WGB's in the output spot, pushed hard and biased hot. With standard biasing, 6 volt swing, and standard 6L6's, not so much.

Gregg

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!

I may have to try the CV4024 in the PI spot again for shits and giggles. I'll have to drop the chassis again to adjust the PI trimmer though... sigh


Gregg

Mullard's version of a 12AT7

Hey guys!  I dropped the Jan Phillips 12at7 into the PI spot and I noticed more sensitivity, more sustain and honestly didn't notice all that much volume drop... 

Also, gotta say to an earlier post in warning of KT66s.  (Add your "I told you so" here) They are Really big an do add warmth...  What I've ended up doing is...
-dropping the bass to almost 0
-raising the Treble to 9
-Mid is at 6
-vol at 7
-gain 10
-ratio 7 (but needs to come down to 6 or so {due to 12at7 I assume})
-presence 10
--cable in FET
--jazz on (verdict is still out on this - helped smooth the cleans a bit but that may be obtainable by just rolling back "vol" on guitar)
**SG with P90s thru 4x12 Marshall**


This is not the fuzzy tone a lot of guys buy this amp for but it is a SICK classic rock tone somewhere between a Fender and a Plexi that is just sweeeeet and full of harmonics!
Perfect tone if you really want your guitar to sing in Allman Bros, Govt Mule, Steppenwolf (though Steppenwolf is really trebly) kind of rock n roll!

:)