Hi,
I am trying to calc a tiny power supply for a tube preamp.
Supplying 2 x 6n2p (12ax7) on a 10mA +HT 😀, one triode as a cathode follower.
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Reading the Blencowe's book, I got a bit lost on the CF chapter.
How much current would the cathode follower demand in this schematic?
Is smth like 250/56k - around 5mA max?
---
How it would compare if I substitute the cathode follower with a power mosfet? (f.ex. here)
Would the current drawn go lower?
Best Regards
I am trying to calc a tiny power supply for a tube preamp.
Supplying 2 x 6n2p (12ax7) on a 10mA +HT 😀, one triode as a cathode follower.
---
Reading the Blencowe's book, I got a bit lost on the CF chapter.
How much current would the cathode follower demand in this schematic?
An externally hosted image should be here but it was not working when we last tested it.
Is smth like 250/56k - around 5mA max?
---
How it would compare if I substitute the cathode follower with a power mosfet? (f.ex. here)
Would the current drawn go lower?
Best Regards
Last edited:
The CF grid has the same voltage as the plate of the previous stage (144V). Grid to cathode voltage is negligible in comparison (1V or less), so current is 144V/56k = 2.6mA.
Current will be the same for a MOSFET, performance will be better. The thing to watch for is choosing a MOSFET with a low drain to gate capacitance.
Seems that max power dissipated in a mosfet CF would be 144v*2.6mA = 0,37w
(considering another schematic topology and no free triode for the CF)
So for a 30-40w device, to-220 body, I would like to leave it without a heatsink.
---
But what about the start current through the CF?
With a cold heating there would be little to no current through the previous valve and a higher reading on the plate (>144v).
Don't know exactly how much higher (doing some research now).
So there might be a higher current and power through the mosfet at starting.
----
p.s.
If at cold start the resistance through the valve is huge and we exclude it from the calcs, there would be a voltage divider:
250v in, R1 100k, R2 56k and a result of... 90v across the mosfet.... So less voltage...???
---
Or, the load through the valve not working, goes to the mosfet chain. So considering 2*2.6mA on the CF.
Don't quite get what is going on 🙂.
(considering another schematic topology and no free triode for the CF)
So for a 30-40w device, to-220 body, I would like to leave it without a heatsink.
---
But what about the start current through the CF?
With a cold heating there would be little to no current through the previous valve and a higher reading on the plate (>144v).
Don't know exactly how much higher (doing some research now).
So there might be a higher current and power through the mosfet at starting.
----
p.s.
If at cold start the resistance through the valve is huge and we exclude it from the calcs, there would be a voltage divider:
250v in, R1 100k, R2 56k and a result of... 90v across the mosfet.... So less voltage...???
---
Or, the load through the valve not working, goes to the mosfet chain. So considering 2*2.6mA on the CF.
Don't quite get what is going on 🙂.
Last edited:
At startup, the gate of the MOSFET will be near the supply voltage, so Vds will be near zero. It's safe for most devices. For some reason, depletion mode MOSFETs will sometimes pop on start-up or shutdown, but I've never lost an enhancement mode MOSFET in this use.
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