300b lovers

Good to see someone designing this way, I see too much premature tube death from a lot of high end audio designs.

If we chose, we could have a switch that would alter the quiescent current for the driver section, so 6V6’s could be accommodated as well. All of these tubes have the same pinout, but the optimal current for the 6V6 is in the 25~30 mA range, while the 6L6 and KT88 are happiest in the 50 to 65 mA range. (By comparison, the optimal range for a 6SN7 is 8 to 10 mA per plate.)

If the switch is in the wrong position, the 6L6 and KT88 would be under-biased and have increased distortion, but no harm done. If it is the wrong position with 6V6’s installed, though, they would burn up in minutes, and if one shorted, would take out the interstage transformer as well. So a visit back to the factory, all from one user mistake. You don’t want to give the user the power to damage the amplifier, just from a single switch setting.

A limited range switch to trim between 6L6 and KT88 would not harm either (in the wrong position), so that’s an option. But optimizing current for the KT88 still gives plenty of current for the 6L6, and we’ve found these types aren’t all that sensitive to current settings, just so long as there is enough.

A general rule-of-thumb in vacuum tube design is setting the plate dissipation somewhere between 50% and 75% of max rating, with 65% to 70% usually considered a good balance between tube life and overall performance. Unfortunately, many KT88 amps take the tubes right through their ratings, so new types like KT120’s have been created for these amps.

Let’s do a little quick math to see how hard the driver is working, compared to an output tube. The gain of a 300B is 3.9, according to this Western Electric data sheet. The interstage transformer has a moderate gain of 1:1.2, so the net voltage gain of the output section is 4.68. In power terms, that’s a ratio of 21.9, or 13.4 dB.

The driver is working at 0.2137 times the voltage swing of the output stage, into an open load that only has 60 pF of capacitance, and only draws current when the 300B grids pass into the A2 region at 80 volts swing. The driver can push the 300B grids at least 20 volts positive, and surprisingly, the 300B remains linear in this region, with no visible transition when it goes from A1 (negative bias) to A2 (positive bias).

There are no charge storage effects, unlike conventional RC coupling, so the transition between A1 and A2 is seamless, and recovery is immediate on departure from the A2 region. In addition, if there are any nonlinear grid impedances from the 300B as the drive voltage goes up and down, it is swamped by the orders-of-magnitude lower plate impedance (700 ohms) of the driver section.

There are further subtle benefits of transformer coupling. The 300B grids do not go into the positive-grid region at the same time: they take turns. This means both driver tubes are available to drive whichever grid needs current, not just one. And the paired drivers aren’t just paralleled, but in a Class A balanced circuit, so most distortion is cancelled.

This is important with an output tube with distortion as low as the 300B; the driver should be as clean as possible, yet capable of peak-to-peak 200 volt swings. At 30 kHz (which is a 38 V/uSec slew rate).

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....