The Caged Frog II-- A Pentode Based Differential Transconductance Amplifier for Headphones

Caged Frog ][

I really like transconductance amps for driving headphones. The first one I built, the Caged Frog, has remained a favorite of mine. Even the Baby Frog is great for what it is. But, since I have a definite preference these days for differential amps, the next obvious thing to do was make a differential gm amp.

The Caged Frog -- A Pentode Based Transconductance Amplifier for Headphones

Caged Frog -- Single Ended Pentode

The truth of the matter is that after years of playing around with all kinds of single ended triode (S.E.T.) amps, I'm not sure that they are really my cup of tea. Sure, they do a some things really well, they have that sweet midrange after all, but some other things just never seem quite there. Indeed, I generally feel like they lack coherence when things get complicated. The usual alternative is push-pull, which certainly has it's place, and indeed a good differential circuit is hard to beat. But, there are other solutions as well.

One solution that is often ignored is the lowly pentode.

D3a Experimenter's Printed Circuit Board

This project is a circuit board which, through various jumpers, can be configured for various circuits. Specifically, it is designed around the D3a vacuum tube, a high Gm, high mu pentode. Additionally, there are several other tubes such as the E180F and E810F that share the pin arrangement with the D3a allowing their utilization as well. 

Low Voltage, All Tube, Guitar Amplifier for Headphones

I was browsing though some space charge tubes (these were tubes designed to operate at low voltages -- usually for car radios) recently and happened upon the 12K5 which is designated as a power output tetrode. I thought it might be useful for driving some headphones, and when I went looking, it turned out that others had been here before me.