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Novar Spud

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Novar Spud

The Novar Spud was discontinued in early 2019 and all remaining Spud circuit boards have sold. My decision to discontinue the Novar Spud was due to the dwindling sales of vacuum tube circuits and due to the enormous support cost associated with these circuits. Instead I chose to refocus Neurochrome as a manufacturer and vendor of state-of-the-art solid-state circuits. I have nothing against vacuum tubes. I had a lot of fun designing vacuum tube circuits. But I consistently find the low-distortion solid-state circuits to perform better – both on the lab bench and in the listening room – so I have chosen to direct my focus there.

You can find the design documentation, including the schematics, for the Novar Spud below. This information is provided as-is, where-is and is for personal, non-commercial use only. I am more than happy to discuss licensing of the circuit should you wish to produce the circuit commercially, though I will seriously question the sanity of anyone who wishes to mass-produce these Spud amps. 😀

I will continue to support those who have bought my Novar Spud circuit boards, but I will not provide support for DIY builds of this circuit. Should you need help with your DIY build, I suggest that you ask for help in the Tubes/Valves section of diyAudio.

*** WARNING ***

The Novar Spud operates at lethal voltage. At some points in the circuit you will find potentials of up to 350 V. This is plenty to kill you! If you are not qualified and comfortable working with such high voltages, please do NOT attempt to build this circuit. Should you choose to build this circuit, you assume all risk and expense associated with this circuit.

Spuds are neat circuits. A true Spud is a one-tuber, i.e. an amp with only one vacuum tube (per channel). I cheat a little by using a sweep tube with two dissimilar tube sections within the same glass envelope. A bit of searching reveals a family of 9-pin Novar tubes suitable for Spud duty: 6GF7A, 6KY8, and 6LR8.

The Novar (B9E) tubes are large, 9-pin tubes with 1.02 mm diameter pins arranged in an arc with a diameter of 17.45 mm. Do not confuse these with Magnoval (B9D) tubes with 1.27 mm diameter pins on the same 17.45 mm diameter arc. Both of these types are much larger than the Noval (B9A) socket type (1.016 mm pin diameter, 11.89 mm arc). 

Sourcing the tube sockets is the biggest challenge in the Novar Spud build. The Novar tube sockets are surprisingly rare. The supply of Magnoval sockets is abundant, but a Magnoval socket will not hold a Novar tube very well due to the difference in pin diameter. While it is possible to 'hack it' by squeezing the Magnoval pins with a pair of needle nose pliers, I strongly suggest finding a pair of Novar sockets for this build.

Those planning to use the 6LR8 are in luck, however. The Compactron (12-pin) 6LU8 is exactly the same tube as the 6LR8 and Compactron sockets are easier to find. Naturally, you will need to sort through the pin differences and connect the 12-pin tube appropriately, but at least you will be able to find tube sockets. Consult the tube data sheets for the pinouts of these tubes.

Output Transformers

There are many options for output transformers. Select output transformers that meet the following specifications.

Parameter Value
Output Power 5 W
Primary Impedance 5 kΩ
Primary Current Min. 60 mA
Secondary Impedance Match your speakers

Some may choose to buy transformers with multiple secondary taps to support both 4 Ω and 8 Ω speakers. The sweep tubes are not overly sensitive to the load impedance, however, so buying transformers with a single 6 Ω secondary would be a good compromise and result in an amp that can drive 4 Ω and 8 Ω speakers well.

At less than $20/each, the Edcor XSE10-5K is very tough to beat on the price vs performance ratio. In fact, my first Spud amp (a 6LU8-based one) used this exact transformer. That amp was my desktop amp for the better part of a decade until its rectifier tube finally went belly-up. For actual stereo use, I would go for the $95 Edcor CXSE25-5K. The James JS-6113HS looks like it would be a good fit as well and is priced similar to the Edcor. 
Doubling the price lands you in Lundahl territory. The LL1623/120mA appears to be a good candidate, though with 5.6 kΩ primary impedance, the max output power will be ever so slightly lower.

Design Documentation

You can find the design documentation, including schematics, here: Novar Spud Design Documentation

You can see my prototype build of the Novar Spud below.

Novar Spud


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