300b lovers

Last but not least, although four driver tubes are required, they don’t have to be matched quads. Matched pairs are fine. It doesn’t matter if the drivers in one amp are at 50 mA each and the drivers in the other amp are at 55 mA each. The difference can’t be measured and won’t be heard.

So if you have a stash of priceless MO-Valve KT66’s, which were manufactured in matched pair sets for the amps of the day, go ahead and use them! They will be operated very conservatively (far more so than most power amps).

There are a pair of pin jacks on the top panel to check if plate voltages match. Measure across the pin jacks with a basic DVM, set the DVM to measure DC, and if you see 3 volts or less, you’re good to go.

I see Lynn has spilled the beans on the driver tube change.  I have listened to the 6L6 (Russian 6P3S-E), the KT66 (Shuguang black treasure KT66), and the KT88 (My stash of a quad of the now unobtanium Shuguang WEKT88).  Of course I have spent almost two years listening to the 6V6 as the driver before this.  In my system the KT88 wins hands down.  It has midrange and bass that hits you in the chest at moderate listening levels.  That is in a good way.  It is vibrant and rich and tonally correct.  It has all the highs of the other tubes, but sounds less thin.  If you put any of these tubes, including the original 6V6 in this amp and had never heard any other driver, you would still think it was the best or one of the best amps you had ever heard anywhere.  But the KT88 is just superb.  That doesn't mean that in someone else's system they wouldn't prefer a KT66.  I also have EL34 to try, but haven't bothered because I think it the worst of the octal output tubes of that ilk and never understood why anyone liked them.  Mine are Mullards too....    

Let's just say that with the KT88 you FEEL the music in a way you do not with any of the other driver tubes.  You sort of LIVE the performance.  It is quite striking, and the larger chassis of the final design, coupled with the very conservatively designed power supply, allows for the amps to easliy handle the roughly 14 watt dissipation of the drivers vs. the 7 watts of the original 6V6.  It still runs very cool for a class A 25 watt tube amp.

What I will say is the output section is basically a supercharged amplifier of the driver section sonics.  You clearly hear the nature of the driver tubes.  This is of course true in other amps, but I have never heard the effect like this.  Usually a driver tube change is audible, but not night and day.  Imagine all the things you love about your classic KT88 amp with far less distortion and that KT88 rich full sound on steroids.  You are inside the music in a way that none of the other drivers do in my system, in my living room.  You get that KT88 sound without all the nonsense of feedback, RC coupling, whatever.  Instead you get an effortless, breathless, KT88 rich sound with lightning fast transient response.  This is related to slew rate and I will let Lynn discuss that if he chooses.  What matters is how it sounds:)  So now you have an amp with all large plate tubes, no feedback, that is lightning fast and will produce 25 watts all day long with tons of current.  The previous 6V6 version has easily driven 85 dB speakers.  The KT88 version... :)

Just to be clear, this is my system so those of you wanting to get a feel for it can know what I am using:

Lampizator Pacific DAC with 46 DHT tubes run XLR.  Dac is modded to eliminate a cap between DAC board and output stage resulting in a very slight pop when it changes resolution at the beginning of a track.  I find this benign, and the clarity is increased.  Just saying so you know this is better than a stock Pacific.

This drives the Raven preamp and Blackbird amps, cabled with Paul's best Anticable XLR v. 5 something.....   Speakers are Spatial audio X5 with seriously updated crossovers.  I never heard the X5 wake up like this....

@Donsachs I'm interested in the specifics of the X5 crossover upgrades please.

I have a technical question for tube gurus @lynn_olson @donsachs @atmasphere

I put the intrastage transformer Hashimoto A-107 between the 6sn7 input and 6f6 driver tubes in my SET amplifier. The load resistor connected to the 6f6 grid was 520KOhm. I measured a square wave output and it had a big overshoot. I changed the load resistor to 120KOhm, and the overshoot decreased by amplitude and attenuation time. But still, there is a notable overshoot. I measured a frequency response and there is a hump +1.7dB at 35KHz. There is -3db at 19Hz and  47KHz.

Should I decrease the load resistor more to remove overshoot completely?

If yes, in which value range should be this resistor? For example, if I take a resistor less than 50K it can increase distortions.

http://www.tube-amps.net/HP_A107.htm

I went through that with the Amity amplifier in the late Nineties. Nice square waves vs sonics. The amp sounded best with NO grid resistor, and the overshoot was a non-factor. Your amp may be different, of course. Keep in mind that music sources will never excite the overshoot, since it’s all in the far ultrasonic, and most studio mikes are all gone by 25 kHz.

The tube doesn't need that 510K resistor. The DC impedance of the IT is a few kohms at most, and that stabilizes the tube at low frequencies. If the driver is really unstable, maybe the 510K resistor helps, but that's usually the function of a grid stopper resistor, something quite different.