Tube DIY Asylum: Re: Best Driver Stage for a 10/10Y by Ejam

Thorsten

Finally I get to give you some advice. Myself and Lucas Cant mucked about with a set of VT25A (10Y) about four years ago but Lucas never got around to winding a 14k tranny for them. I manage to pick up 10 used VT25A but later sold them and regret that day ever since! Anyway on to the driver of choice. I used a 3A/167M (or you could use either the WE437, triode wired 7788 or 6C45Pi, they are all much of a muchness). The operating point was Vpk = 150V, Vgk = -2V and Ip = 20mA. Bias was a 2V Cyclon lead acid battery (sold by Farnell) in the cathode. Input grid resistor was 47k. Plate resistor was 6k - 10k and here is the tricky bit, you will have to vary the plate resistor to suit the 10Y and as well as the the 10Y's plate current. I even found for really bright 10Y's around 4k was required on the 3A/167M. RC coupling to the 10Y with a 0.1uF teflon cap and 68k. Nowadays I would use a grid choke instead of the 68k. I tried direct coupling but preferred the slight warmth that RC provided.

Now to the 10Y/VT25A, I tried both fixed and cathode bias for the VT25A. Forget fixed bias all together (my opinion ...) because the thing will ream your ears out. I used a cathode resistor but had more fun and flexibilty with a mosfet current source in the cathode. The mosfet I used was an IRF730 in parallel with a 3k 5W wirewound pot from the VT-25 cathode connected to ground and the wiper to the mosfet gate. The drain also goes to the cathode but put a 10 ohm resistor in series so you have something to measure the current. The mosfet source to ground. Bypass the mosfet with a 2uF film cap and you are ready to dial in whaterver current setting you like. I liked 30mA with 390V across the VT25A. We used one of Lucas's 11k 211 transformers for the load. Power supply was I think the test bench 6L6 regulated supply. Anyway the sound was way cool, unlike anything heard before. Oh, before I go, the usual precautions with the 3A/167M, it is a serious rf amplifying device... Have fun

Regards

Anthony