The easiest and most profound changes to the tone of the OTS can be made to the tone stack. For instance: change the 150k slope resistor to 100k and change the 0.002mf treble cap to 1000pf or 500pf. This well drastically change the treble/mid/bass balance and you might like it. There are virtually infinite combinations that can be made to the tone stack that will alter the tone much more than adding 25mf capacitance to the filter network or upping the value of the preamp cathode bipass caps. Also, many people have stated that they hate the FET preamp. With a little tweaking mine sounds great. Just get the input voltage right (increase it by increasing the resistance to ground) and it sounds great. And after trying it; I really don't like the 470k screen resistor swap. Each to his /her own...

You can swap 6v6s for 6L6s as long as you adjust the bias. The impedance mismatch doesn't amount to much. I use 6v6s in my ots with switched/ fixed cathode bias (500ohms/10 watts) and it sounds great. I've done this swap with Bassmans, Twins, Bandmasters, a Blues Deluxe, and I've even run them in my Super Twin. As long as you get the bias half way close you'll be fine. Try it! Use good quality tubes that can handle the voltage. My Deluxe Reverb runs the plates at about 420 volts DC. you don't want to go much higher than that so I hear. i've run 6v6s (JJs) at 440v all night however, and nothing happened.

These amps utilize a pretty standard "Fender style" phase inverter and power amp. Decreasing the resistance to the screens increases the screen voltage slightly. This reduces the impedance of the tubes and causes them to run a little hotter. This has the effect of attenuating the high frequency roll off (warm sounding) that 6L6s are known for. This is just one of the reasons that fender amps often sound so ice picky. I am convinced that many amp techs (Leo Fender included) lost the high end of their hearing which caused them to design shrill sounding amplifiers. I have substituted a spectrum of values for the screen resistor in many Fender and home made amps. If anything I would recommend going to a higher value; maybe as high as 5k for the screens, this seems to yield a more grainy sounding texture. Higher value screen resistors also tend to protect the output transformer to some extent in case of tube failure. As always; the proof is in what you like to hear and how you want to sound , so experimentation will yield the "correct" configuration for the individual player. I would also recommend trying substituting good quality 6V6 power tubes along with cathode biasing. I love the sound of 6V6 power tubes and cathode biasing in Fender style amps. I built my OTS with a Fixed/Cathode bias selector switch and I seem to leave it in the cathode bias position. I'm in the process of adding spring reverb; which I seem to be addicted to...
It would be fun to add the mods that you recommend sequentially and analyze them on the scope to see what results.

The 220k resistor and the 270 mf cap don't do anything at all since there's a wire across those terminals as well. So snipping them off shouldn't have any effect. I talked (wrote) to NIK about this and he said he was going to correct the schematic. The 1k to 470 ohm screen resistor swap usually makes an amp harsher sounding and accents the higher frequencies.

Looks really good. What are those black cables? I built one from scratch a few months ago and now I  think I'm going to have to add reverb. I just can't live without it. 

John

That guitar sure does remind me of a Gibson Howard Roberts Fusion both in sound and shape. I have three, and they are the best guitar for the money that I've ever owned. Of course the Gibson has 4 knobs and the switch is on top on the original (where I like it).

Back to the ots. I'm having trouble finding the thread with your mod schematics. Give me a clue? Also, when I built my ots, the FET section cut out until I lowered the value of a resistor thus raising the voltage feeding the transistor. Have you heard of this happening to anyone else?

John

I built one of these ots amps from scratch a few months ago and I find the rock/jazz switch when in the jazz position to be substandard. The rock position sounds pretty good but the whole amp sounds too midrangy to me. i'd like a more balanced tone. any thoughts out there in netland?

JM

I'm in the process of building my own and I just want to say the first sound clip sounded real, real nice. The plying was excellent, no apology necessary.

Thanks