Same Skin, Different Muscles
The 6V6 Lacewood amp was designed and built in the first half of 2011. At that time there were two important aspects to this build. The first was that it was partially an experiment to see if an alternating current filter supply would cause 60Hz hum in the output if the B+ supply was very clean. The second important aspect was that the UL power stage design was based on a pentode mode bias point pulled directly off the 6V6 data sheet. The amp turned out very well at it was my main listening amplifier in my office for several years.
Then in early 2016 I became acutely aware of the lack of good design data with respect to operating beam power tubes in the ultra-linear configuration. To remedy this, I performed a 6V6 UL optimization study to obtain some better design data for this tube. This is turn led directly to the redesign of the original 6V6 Lacewood amplifier using what I had learned.
The Electrical Design
Since I’ve always liked the look of the Lacewood amp I was not going to do anything to perturb the outside. This meant a redesign using the same transformers, main filter choke, filter capacitors and tube complement; a “stealth” redesign. Both the power and driver stages were to get a new design.
For the power stage, I took a bias point directly off the optimization table: Vp=275V, Vk=18.1v. Simple. For the driver stage, I no longer needed the soft (i.e. high 2nd harmonic) stage I had used before. Instead I opted for nice clean bias point with a much higher load. Here is the load line design.
Along with a few calculations for frequency response, these bias points led to the following schematic for the amplifier proper.
This design makes use of what I learned from the 6V6 optimization study and uses a more appropriate bias and load for the driver section.
The power supply also takes a slightly different approach from the original. Instead of just driving the B+ ripple to preposterously low levels, I opted for low ripple and very good (dare I say “great”) channel separation. Here is the power supply design I used.
This power supply uses the same transformer and main filter choke, but in place of the single 5H choke in the original, it uses separate chokes for each channel. This puts about 70dB of power supply channel separation between the power tubes at the primary ripple frequency. As frequency goes up, so does channel separation. There is also an additional 57dB of power supply filtering between the driver and power tubes of each channel. This design virtually eliminates any channel crosstalk due to power supply coupling.
As I said before, nothing changes on the outside, only the insides get a new look. Here is an overall picture of the rewire.
This really doesn’t look much different from the original. The 5H choke is gone; replaced by the two Triad 1H chokes. The old $2 alpha volume pot has been replaced with a nice PEC dual control. The test point wiring has been removed (although the test sockets remain). And finally, much of the wiring is different.
Here is a close up of the driver section.
This is actually much cleaner than the original. Some of the twisted pair wiring has been replaced by single leads. This change had no effect on coupling, hum, or noise; all are inaudible.
Here is the new power section wiring.
This is almost identical to the original. The only differences are the component values, and the addition of a small 5.1kΩ grid stopper resistor to control any bias excursions.
Finally, here is the primary power supply wiring.
This is virtually identical to the original. The only real difference being the addition of a 350Ω dropper resistor between the first filter capacitor and the main filter choke. Other power supply filter wiring is per the schematic above.
This amp got more testing than the original. There are two reasons for this. First I have begun to perform more testing on my amplifiers just so that I can hope to reproduce certain sounds I like in future builds. The second is that, as the first of my builds using the data from my study, I wanted to verify the optimization results in a complete amplifier build.
All the values for the amp look good. Voltages are about where I expect then to be and the output power, while about a dB low, is still a very acceptable level. The distortion at 1W is about 1.7%, mostly second harmonic. This is a good result. Here is what the distortion response looks like.
Ignore the 5th and 7th harmonics. These are a product of my signal generator.
The amp sounds wonderful. It is perhaps a littler cooler (i.e. lower 2nd harmonic) than the original but the sound stage is incredible. This amp is very fast and very detailed. I am very pleased with the result.
This is once again the primary amplifier in my work bench area. It is now getting regular use as I design, prototype, assemble, and test new designs. Like a Phoenix from the Ashes, the Lacewood has been born anew.