ViperDoc wrote: ↑Sun May 22, 2022 6:39 pmSo when choosing power rating for those zeners, would I be safe to calculate...
The MOSFET is doing all the heavy lifting. The Zener current is limited by the 1k resistor, which Aiken says should be sized for ~5mA to stabilize the Zener voltage drop.
If we say Vgs is 4V, a 12V Zener would get you a 16V drop, and you could make the resistor (16-12)V/0.005A = 800R (use 820). Then the power dissipated in the B+ reduction Zener would be 12V*0.005A = 0.06W. 1W is fine for that, and for the 12V Vgs protection Zener too.
martin manning wrote: ↑Sun May 22, 2022 12:35 pm
You've got it right. The 12V Zener is just there to protect Vgs. If you make the other one 12V you should get the ~15V drop you are looking for.
if you have room, an easy and compact layout can be made using a 3-lug terminal strip.
I just installed this with an 820 Ohm resistor (only difference) and two 12V zeners and I'm UP 12 volts. Are the zeners backwards?
EDIT: The Aiken diagram seems to make a connection with all three MOSFET leads. "1" to the tail of the 100R resistor, "2" to ground and "3" to the HCT. Maybe all I need to do is ground the MOSFET through the middle connector, "2"?
EDIT 2: I grounded lead 2 of the MOSFET with no change. The pin orientation is GDS when looking at it pins down, so it's in the right orientation as the drawing.
Edit: The powder coating on that heat sink is not conductive, but the grounded ends of all the components ground to chassis through the screw. I might pull this out and do it again after grinding off the coating on the flat back part.
martin manning wrote: ↑Wed Jun 01, 2022 11:46 pm
I would just leave the heat sink out. You are not dropping much voltage so not much heat to get rid of.
I took out the heatsink and re-installed the B+ reducer and I'm still up.
The layout looks correct, except I'm not sure where your reservoir cap ground is (should ground with the CT at the drain). Just to be absolutely clear it's wired as shown below, where I have the schematic arranged like the layout. Are those resistors wire wound? If they are inductive that could be a problem, especially for the gate resistor.
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martin manning wrote: ↑Thu Jun 02, 2022 12:24 pm
The layout looks correct, except I'm not sure where your reservoir cap ground is (should ground with the CT at the drain). Just to be absolutely clear it's wired as shown below, where I have the schematic arranged like the layout. Are those resistors wire wound? If they are inductive that could be a problem, especially for the gate resistor.
Thanks. The reservoir ground and heater CT are underneath the nut from the other side.
CF or MF resistors. The gate resistor value is not critical, so you can use 1k or even 10k. If that doesn't get it working, I would rebuild it with a new FET in case the existing one is damaged.
martin manning wrote: ↑Thu Jun 02, 2022 2:49 pm
CF or MF resistors. The gate resistor value is not critical, so you can use 1k or even 10k. If that doesn't get it working, I would rebuild it with a new FET in case the existing one is damaged.
I rebuilt the whole thing using CF 1W resistors, a new MOSFET and new diodes and I'm 420 VDC with 119 VAC at the wall. So up about 15V. That means I'm raising the 0V CT to 15V, right?
The average voltage at the CT should be negative with respect to ground. You are measuring 420V DC plate node voltage to ground, correct? Remember you will have to reset the bias for the new plate voltage and that will have some effect on the final plate voltage you get.
Try this to make sure that the reducer is working correctly:
With tubes out, measure the plate node voltage to ground.
Then connect the CT directly to ground and measure DC plate voltage again. You should see an increase of 15-16V.
martin manning wrote: ↑Thu Jun 02, 2022 7:12 pm
The average voltage at the CT should be negative with respect to ground. You are measuring 420V DC plate node voltage to ground…?
Yes.
I’ll check it out, thank you!
EDIT: Well, you're right. The power tubes with the B+ reducer were biased at 9 mA on the previous setting! I am only able to get 15 mA max cathode current on my current setup, but when I max the bias (turn the bias pot to zero), the plate node voltage drops.
Yea, as mentioned above, the bias voltage will be reduced (becomes more negative) along with the B+, so you will have to make some adjustment to your bias circuit.
I swapped out the 200R bias tap resistor for a 1K5 and that puts me at 400 VDC on the plate supply node with no wall attenuation. When I knock my Brown Box to it's first click (3% reduction), I get 389 VDC. So long as my bias pot is in the ballpark, I suppose I can fine tune with the zener drop diodes. Many thanks!
I've been trying to wax eloquent about what confuses me about bias supplies and negative voltage. No dice. I really want to understand this. I re-read Aiken to get the gist. What confuses me most is the relationship between Ohm's Law and the fact that when I reduce the tail resistance of my bias supply to a minimum (turn the bias pot to zero), the grid voltage becomes more negative. Conversely, when I increase the resistor value on the other side of the circuit, the bias AC voltage feed for example, the grid voltage also goes down. Please excuse me a 2 A.M. unicorn WTF, but do you see how confusing this is? So when positive voltage drops across a greater resistance, it becomes less positive. When a negative voltage drops across a lesser resistance, it becomes more negative at the source?
