[tomchr]

Folks,
 
The goal of this project is to provide an amplifier with high performance that can be built by a relative beginner on a beginner budget while also provide options for experimentation for the more advanced builders. The build budget starts just shy of $200 (not including chassis). 
 

 
This amplifier supports three tubes: 6GF7A, 6KY8, and 6LR8. With some creative wiring the 6LU8 may be used as well. The 6LU8 is the 12-pin Compactron version of the 9-pin Novar 6LR8 tube.
For headphone use, the 6GF7A is an obvious candidate. It provides a bit over 2 W into an 8 Ω load (so a 4 V RMS swing). Into a 300 Ω headphone, this is about 50~60 mW. Plenty loud... The 6GF7A is a dual dissimilar triode tube. It contains a small signal input tube and a power triode. I really like the sweet triode sound from this tube. It doesn't quite have the power needed to drive my 85 dB/W/m efficient speakers, but for headphone use, it's a rockin' tube.
When driving speakers, I use the 6LR8 in triode mode. In this configuration, the amp provides 4.7 W into 8 Ω (6 V RMS swing). This would translate to 125 mW into 300 Ω headphones. Probably enough to blow the headphones off your head.... Above shows the amp with the 6LR8 tubes.
 
The schematic for one channel is shown below.
 

 
And the power supply:

 
To make life a bit easier for DIY builders, I do offer PCBs for sale on my website.
I hope it's OK to post this in the DIY forum. It's not like I'm getting rich and famous doing this... The board sales fund the development of new circuits. It's a self-sustaining hobby more than a business. My boards also make it much easier for the relative beginner to achieve success assembling their own amplifier.
 
The board measures 3.8 x 5.8 inches (97 x 147 mm) and is optimized for signal integrity (low hum, low noise, etc.). It's professionally fabricated on FR-4 fiberglass material. It features plated trough holes, solder mask, and silk screen.
 

 
Thanks,
 
~Tom

 

[nikongod]

Is the "power" stage triode or pentode (looks like ultralinear)? 
 
What does the connection "HTR Bias" do on the cathode of the power tube?

 

[tomchr]

The board provides options for using either Ultra-Linear (UL) or Triode connection on the output tube. That's accomplished by a wire link option (OPT1).
On the input stage, you have the options of using resistive bias, resistive bias with local feedback, and LED basing. So you have options...
 
With the 6LR8 tubes, I get up to 4.6 W out in triode mode. Nearly 6 W in UL mode. The difference between 4.6 and 6 W is negligible (seriously, 2.6 dB is "a little bit louder" not "much louder"). I really enjoy the triode sound so I use triode strapped 6LR8s. 
 
In this design, the heaters need to be elevated to avoid violating the Vhk spec of the tubes. That's accomplished by the HTR_BIAS point and a couple of resistors. Pretty standard fare in the tube world.
 
I have a lot more circuit description on my Novar Spud page. Even more circuit description and information comes in the design document that accompanies the board.
 
~Tom

 

[kstlfido]

Hi Tom-

The H-K max DC spec for the 6FG7 and 6LR8 is 200 volts (heater negative to cathode). I can't imagine the cathode bias to come anywhere near that. No need to lift the heater supply.

Thanks for this article, BTW.

-Kent