After the initial tests done with the 4P1L in pentode mode and filament bias, I thought a bit about how this driver could be implemented in practice. I may try this configuration in my 4-65a SE amp, but am not urged by this at all.
The screen supply is formed by a gas valve (SG3S) which provides a very stable reference when feed by a CCS. In this case the cascoded pair M3 and M4 will provide in conjunction with R4 and U2 a very low noise screen current to U1. R9 has to be adjusted on test to set a current of about 15mA on the CCS. The 4P1L will draw 1.8mA at 81V as screen current, so R4 may also need adjustment to set the right operating point.
The driver should provide about a gain of 150. Driving easily a transmiting vale (or why not a 300B 🙂 ) in class A2 with this configuration. My tests showed a maximum THD of about 0.27% at 200Vpp.
Gain could be reduced if needed by tweaking the RL. And if stacked supplies are not used, then a single +400V supply could be used with additional dissipation across the reference currents (M1 and M3/M4).
Keep thinking…
The high gain and A2 capability are just what i need to drive a gm70 or 813 amplifier with one stage, but how much current could it deliver for a2 operation?
A gm70 would pull up to 30mA @ +60V grid, would this be possible(guestimate) or should i turn to a ccs loaded mosfet source follower
Do you really need to run an 813 in A2? Blimey, who needs more than 20W in SE! Key point here is whether you are running in class A2 or un class A1 and switching to class A2 at specific points. See my 814SE design which implements this hybrid mu-follower in a DC-coupled configuration albeit with a triode driver. The top MOSFET will provide the current you need. if you are running the amp in class A1 then you will need the stacked supplies as per my design. For zero bias valves or A2 biased, then you will need a source follower to provide zero bias grid current.
If you put a cap to couple the grid, then you will not get the best out of this topology in what you want
Ale
need it…. not really. But if you’re building a gm70/813 amplifier why stop at A1, A2 doesn’t cost any more. Pure A1 @ 1200V/-120V would be max 30W, A1 and A2 @ 1000V/-90V could deliver 45W. It does need 300Vpp to get there, would this driver go this high?
I really need to learn how to simulate these things, I’m in over my head, but repeating what you already know is boring.
Or some topcap madness, 814 PP, I’ve got some cv6 triodes, topcap’s everywhere
Alex
I’ll just answer my own question, now I’m thinking straight 😀
@256V it can’t provide 300V swing, and the anode voltage can’t go high enough.
Alex
A driver which can swing 300Vpp or more is a challenge on its own. This design could do it, but you will need to optimise the gyrator load (and provide a big heatsink), use 1kV parts and check how linear it can be. The 4P1L can do 300V bias. It’s a stretch but can handle it. The question is though, handling the load in A2. That is the challenge.
I’d suggest you do the A1 implementation and then upgrade once you feel confortable. What you’re trying to do is quite a challenge. Are you well experienced on these designs and particularly HV?
ale
Further investigating confirms that the 300Vpp is indeed a challenge, it would need something silly like the 6HV5A @ 1100V and a interstage.
Your design fits the 211 so well that I should keep it simple in my first kV amp and go with that.
I wanted to calculate the anode current in your design and found this:
3.6V/5ohm=0.72A
0.72-0.67=0.05A=50mA
50mA x 256V = 13W plate dissipation 7.5W max?
what am I doing wrong? (not bothering you I hope :D) Or should I go to something like 25mA, -8V grid
Hi Alex,
Please read here: