I don't think the amp tech was talking about it being biased cold he was talking about it being biased very hot.  Instead of -35 or so volts on the grid he was only seeing -20's and thus unable to properly bias the tube and reduce it's idle current to an acceptable level. 

Just my thoughts. 

In today's world I've found it to be quite common to have to change a resistor or two in the circuit to allow the bias pot to "scale" correctly for the tube of choice.

Here are a few pictures of a Plexi 100 watt head that I built using parts from Ceriatone.  My transformers came from AES and the choke is a Classic Tone.  The guitarist that uses this amp liked the sound quite a bit with a set of JJ E34L's but he LOVED it with the KT-66's which he's been using for a long time in a 50 watt Marshall. 

The favorite KT66 for us had been the Saratov piece that was sold by Groove Tubes as the KT66HP.  They got tough to get so we switched to using the Valve Art.  Now New Sensor has a KT66 out that's very similar to the Saratov version and it's sold as the Genelex KT66. 

Using this particular tube in this circuit is not difficult but it's not plug and play either.  The bias pot's range needs to be changed so that you can get enough negative bias for proper operation. 

What I did was replace the two bias "splitter" resistors which are 220k in the stock amp with two 100k resistors.  Then I added another 22k resistor in parallel with the existing 27k bias resistor.  I did this because it made it easy to fine tune the range on the bias pot.  Replacing the resistor with another value would be easy too. 

In my amp I have 485 volts B+ and the tubes seem to need about -60 volts to get them to idle at about 30ma current.  That's all there is to it.  Other KT66s or 6L6's might require some tweaking of these values but they will probably get you close.

You also will have to change from the "bear trap" tube holders to something else.  I just used some I had on hand but I had to stretch the springs a little bit to allow them to fit the chunky KT66.

Using the KT66 in this circuit is not a purist thing to do I guess but it opens up some cool sounds.  Myles the guitar player who uses this amp plays mostly rhythm guitar and he likes to use rather clean punch tones and the KT66 is a good tube for this because it gives a mixture of the EL34 crunch with the 6L6 punch.

I agree with the earlier post in that you need to buy a couple of pairs and try them to see what it sounds like and what you find that you like best.  The nice thing about the 50 watt head is that you only need two output tubes so it's less expensive to try different things.  Put them in there and bias the amp.

I've experimented with a lot of different output tubes in a 50 watt Marshall and I tend to like the JJ E34L's about the best if you're going to use an EL34.  I tend to like the EH tubes but I don't like their EL34 very much yet their 6CA7 is quite good sounding. 

It's really not any problem at all with the output transformer or the B+ portions of the amp.  The problems can come from overloading the 6 volt heater winding in the power transformer because the 6L6 and especially EL34, KT88 etc draw MUCH MORE heater current than a 6V6. 

This might not be any problem at all with the Ceriatone transformers but in a stock Fender or similar amp it can cause problems.

The little guy 6V6's do look a little weird in the amp but they don't need any tube retainers to keep them in their sockets. 



In my test the 6V6 just didn't have quite enough headroom for the application yet the tone was quite nice and I'd recommend giving it a try in an amp that just has too much headroom.  I've used this same 6V6 in several amps with very good results.  My Marshall was at right around 400 volts which is over the max rated voltage for a 6V6 yet just about the same as the voltage that Fender used in amps like the Deluxe Reverb, good quality 6V6's are a must for this swap.

Take a look at the racing world.  Right now you can build an engine that's a Ford or Chevrolet, Honda etc that contains ZERO parts from any of those companies, it's completely aftermarket yet it's still a Ford, Chevrolet or Honda etc design wise but possibly built with upgrades to make it more suitable for competition use.  I tend to see this as the same thing, an old amp built with modern components and some upgrades to make it more reliable and possibly perform better. 

I don't hear a lot of complaints when it comes to engines and I can't see how it should be any different with electronics. 

I guess I should have specified in my post that with each tube type I was always adjusting the bias to get the correct idle current for each tube type.

What's interesting is how "Marshall-y" the amp sounds no matter what output tube is in the socket in the one's I've experimented with over the years.  Including the 6V6's.  Overall tone is not incredibly changed, it still sounds like a Marshall but feel and overall playing dynamics are greatly altered for better or worse and that's why it's worth experimenting.  The 6V6 tube might just be the hot ticket for someone who's amp is way too dynamic for their playing situation.

I have experimented with placing the wire for the feedback loop on different secondary taps of the output transformer with different types as well. 

The lead guitar player in my band uses a Mesa 1x12 wedge cabinet at most of the gigs we do in small places indoors.  It originally came with a Celestion speaker that was about a 70 watter and never sounded very good.  He put up with it because of the convenience of having a small cabinet at the front of him pointing right at him instead of the audience and sound man. 

One day the Celestion finally gave up the ghost and blew so we replaced it with an Electro Harmonix(made by Eminence) 60 watt ceramic magnet speaker and it's tremendously better tonewise with a much smoother mid and top end and a tighter less clunky bottom.  Mic-ing it with a E609 Silver sounds spectacular with little to no EQ though the PA. 

I'm a believer in using a light weight speaker that the amp can really drive when it comes to electric guitar tones especially when working with dynamic mics and he's been hitting this speaker for the last few years with ahard running  Sovtek MIG60 with EH 6CA7's) and a few pedals.  This speaker was inexpensive from New Sensor and I bought a few spares should it get tired. 

I have a 1994 reissue Fender Bassman and my personal favorite output tube in the amp is believe it or not, the Chinese 6L6GC "Coke Bottle", they sound great in that thing.

A kit to build an attenuator box would be cool.  Something similar to the Marshall Power Brake would be awesome.  We've used one of those for many years in our band and it's always worked great.  But with that said it's getting pretty beat up(I've had to repaint the case) and they are no longer made by Marshall as far as I know.

You can also experiment with using the 6550 or a KT88 and then use whatever type you seem to like best.  I have two Marshall 1987 heads and over the years in my band we have used: The JJ E34L, Saratov KT66, Valve Art KT66, Electro Harmonix KT88 and 6CA7.  I've even experimented with a set of Electro Harmonix 6V6's just for fun. 

The amps get used through a Marshall Power Brake and predominately rhythm guitar and so far it seems as though the best results have come from either the Saratov KT66 or the EH KT88.  KT88 has a lot of punch and is excellent on stage when using a humbucker.  It's not quite as good with a single coil pickup just because it doesn't overdrive quite as easily. 

Right now we're testing the Russian Genelex KT66 reissue which so far seems quite cool and similar to the Saratov.

These circuits are quite versatile and it's worth experimenting to find what works best for you even if it's not historically accurate.