Spudsky

Flea-powered capless spud amp, or universal line-out/driver stage.

PREFACE

This "project" is just a brainstorming session to combine a few ideas and get my system off the ground. I have so much stuff going through my headthat I want to do, that I will get nowhere in terms of actually building something. This stage might get me moving to both play around with the 6E6P and also start building my DHT amps. The first itteration might be just the 6E6 with an OPT to test the biasing method, aka the spud amp part. This would be followed by adding new iron in the form of a 2+2:1+1 tranny to allow the stage to be used as a preamp with 4:1 output (still 8x gain), and even later as a 1:1 (SE) or 2:1+1 (PP) interstage to drive an output tube.

THE IDEA

Thorn in the eye with all the spud-elligable tubes is that they tend to be high-mu and often high-gm with a low bias point and high current, meaning a cathode resistor needs a large bypass cap to keep impedance down and LF response adequate. There are all sorts of fancy bias options like diodes, batteries and leds, but they all have their own issues. So I thought I'd try out the same method as on my DHT amps, namely filament bias.

Filament bias uses a current source to light the filament, or heater in this case, and also runs through a resistor in the cathode circuit. The current through this resistor generates a voltage over the resistor, creating difference between cathode and grid, or bias. If we calculate the resistor to drop about 2-3V (3.3 to 4.7 ohms), then we will have the required bias for the 6E6P. We can set the current source to generate a little more/less current to fine tune 6.3V across the heater.

As the 6E6P will only output about 0.4-0.5W, although super linear and low distortion, it might not be enough for everyone. These amps are mainly intended for 96dB or up speakers and most people will have a seperate amp/speaker system for the low end (as is in my case). This amp would power either the Tangband speakers (97dB) from 150Hz up or the eventual JBL2451 (110dB) from 500Hz an up or the tweeter for that system at 7kHz and up. Who knows.

Regarding the current source on a tube like this: I might actually try a voltage source on the filament and resistor, as I'm not too keen on the changes it may bring when the filament degrades with the tube being so sensitive to bias.

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