Back when.... there were no mosfets. A VR tube will work over a fairly wide current range so buffering is not necessary on most circuits. A simple mosfet buffer will however allow the VR tube to function over a small current range improving its regulation.
If you were totally anal you could feed the VR tube with a CCS then LPF it with an RC network and then use a mosfet buffer. Oh, wait a minute that is the circuit that I am using in my Tube Cube amp. I guess that makes me........
If you were totally anal you could feed the VR tube with a CCS then LPF it with an RC network and then use a mosfet buffer. Oh, wait a minute that is the circuit that I am using in my Tube Cube amp. I guess that makes me........
That's because it isn't finished yet. Life has thrown me a lot of curves lately so I have several unfinished projects. In fact the Tube Cube has been sitting around for nearly a year. It is two Simple P-P boards, each wired for mono PPP, a screen regulator board, a board with 8 constant voltage sinks, a big Antek toroid and two OPT's all stuffed inside a 12 inch cube. It is nearly finished, but I managed to blow up the CVS board about 6 months ago and haven't touched it since.
Important details:
8 X EL84 makes 50 WPC!
Regulator tubes have current limit around 40 mA. If screen grids draw higher peaks when your output tube saturates you will get nasty distortions (sharp peaks of screen grid voltage: VR tubes have negative resistance when switch on/off) instead of saturation that sometimes is forgivable.
In my first Pyramid amp I used SG-4S tube and IRF740 source follower to avoid such a problem.
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