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Novar Spud : : A DIY Sweep Tube Headphone (and Speaker) Amp

post #1 of 3
Thread Starter 



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.






post #2 of 3

Is the "power" stage triode or pentode (looks like ultralinear)? 


What does the connection "HTR Bias" do on the cathode of the power tube?

post #3 of 3
Thread Starter 

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.



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