Fixed bias adjust for each final (what I do in my designs) has the advantage of allowing the balancing of the static (no signal) plate currents to reduce the possibility of DC core magnetization. This becomes especially critical for toroidal OPTs, as these cores include no air gap, and can't stand much DC magnetization.
One bias control for both finals is more "user friendly", but won't account for plate current imbalance any better than cathode bias. In both cases, you're likely to want matched pairs of finals.
If using a fixed bias to drive the grids on a 6080 am I looking at a 30-40 negative voltage requirement on each rail if the plate voltage is around 100-120 volts?
I heard when testing I should start at the most negative and slowly adjust until the required current is achieved.
I heard when testing I should start at the most negative and slowly adjust until the required current is achieved.
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Breadboarded an OTL last night, with mosfets as the CF drivers.
Used a 50K trimmer from a -130 V source for each rails bias.
One rail bias worked great, adjustable 0 to -90 volts on the Source output BUT the other stuck at -1.2 volts. thought it was the mosfet, change that, still the same.
Head scratching and testing resistors for hours. Have to check the trimmer, maybe that is gone but doubt it.
But very strange the one that is working, I measured the positive voltage on the drain and it measures 32 volts, but the voltage is 130 volts.
The voltage on the drain that does not adjust properly is 64 volts from the 130 volt .
Bizarre to say the least.
Lost sleep over it last night.
Have to to check it all over again tonight.
I'll check this when I get home tonight.
Gate stoppers need to be corrected. No they are not close up at the moment.
I wired it up as the same attached in this thread but yes have to double check. Maybe I didn't mirror both rails correctly.
I managed to blow up a mosfet last night. The negative voltage supply wire came lose and touched 2 of the pins of the mosfet. Sparks flew!!! Lol
Only have 3 left now
Have another question about the grounding scheme.
It's the first time I have used separate power supplies for negative and positive power supplies.
Do I keep the grounding separate also or do I include all of the grounding points to a star location? (including the negative and positive power supply grounding points to the B+ power supply grounding point).
As they are separate now it makes it tricky to measure all voltages as I need to change to location of the ground probe each time. (having to remember which circuit it's on)
It's the first time I have used separate power supplies for negative and positive power supplies.
Do I keep the grounding separate also or do I include all of the grounding points to a star location? (including the negative and positive power supply grounding points to the B+ power supply grounding point).
As they are separate now it makes it tricky to measure all voltages as I need to change to location of the ground probe each time. (having to remember which circuit it's on)
Help please.
If I'm using a trimmer to adjust neg voltage bias, where is the ground connected (3rd pin)?
To the ground of the common ground on the B+ or to the ground of the negative power supply?
Should the ground of the neg power supply circuit be connected to the common ground point of the B+ also? (or would this create a short circuit?)
If I'm using a trimmer to adjust neg voltage bias, where is the ground connected (3rd pin)?
To the ground of the common ground on the B+ or to the ground of the negative power supply?
Should the ground of the neg power supply circuit be connected to the common ground point of the B+ also? (or would this create a short circuit?)
Ok I've got a circuit diagram here showing the mosfet as CF.
If I use a voltage divider to create the postive voltage for the drain on the mosfet, around 120 volts from the B+, this drops to zero after powering up.
As you can see from the rough simulation I end up with a negative voltage on the drain.
I have the negative voltage derived from a floating power source to power the source and bias.
(actually a 220/6v tansformer in reverse connected to a 12.3v heater supply to give me around -130 volts.)
I didn't expect the postive voltage on the drain would become negative.
Some advice would be greatly appreciated. Thanks.
How do I measure the negative grid bias?
If I measure from the negative power supply (ground probe) I get my required -XX voltage but if I measure with the ground probe to the B+ circuit I get positive voltages.
If I use a voltage divider to create the postive voltage for the drain on the mosfet, around 120 volts from the B+, this drops to zero after powering up.
As you can see from the rough simulation I end up with a negative voltage on the drain.
I have the negative voltage derived from a floating power source to power the source and bias.
(actually a 220/6v tansformer in reverse connected to a 12.3v heater supply to give me around -130 volts.)
I didn't expect the postive voltage on the drain would become negative.
Some advice would be greatly appreciated. Thanks.
How do I measure the negative grid bias?
If I measure from the negative power supply (ground probe) I get my required -XX voltage but if I measure with the ground probe to the B+ circuit I get positive voltages.
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I suspect the issue is with the resistive divider from 350 to ground.
The thevenins equivelant circuit is 125V with a 64.3K series resistor to the drain of your FET.
You have 10mA of current flowing thorugh the circuit, so you have saturated your FET.
You need a lower impedance in the drain of the FET.
The thevenins equivelant circuit is 125V with a 64.3K series resistor to the drain of your FET.
You have 10mA of current flowing thorugh the circuit, so you have saturated your FET.
You need a lower impedance in the drain of the FET.
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hey guys, i've been out of the loop with this thread for a few months; had to put my project on hold, but i'm looking to get back to work on it.
I was wondering if anyone could give me a rundown of this schematic posted by tubelab, i believe its intended to allow negative feedback from the OPT to the right hand grid of the first stage LTP (in this case pentodes) while also allowing an adjustable DC bias to be applied to this grid for balancing the anode currents of the two tubes of this LTP.
I'm just not sure exactly how it works :S
Thanks guys.
I was wondering if anyone could give me a rundown of this schematic posted by tubelab, i believe its intended to allow negative feedback from the OPT to the right hand grid of the first stage LTP (in this case pentodes) while also allowing an adjustable DC bias to be applied to this grid for balancing the anode currents of the two tubes of this LTP.
I'm just not sure exactly how it works :S
Thanks guys.
Hi mnturner,
Fred here I think has a similar NFB idea and is well explained in his pages.
Hope this helps.
RA-100: Drivers and Power Amplifiers
Fred here I think has a similar NFB idea and is well explained in his pages.
Hope this helps.
RA-100: Drivers and Power Amplifiers
Thanks for the reply Brit01.
That schematic isn't exactly what I was talking about; Fred applies feedback to the grid of the left hand triode as though it were a simple common cathode amp. When using an LTP, I don't see why you would not use the grid of the other triode? Interesting read though.
That schematic isn't exactly what I was talking about; Fred applies feedback to the grid of the left hand triode as though it were a simple common cathode amp. When using an LTP, I don't see why you would not use the grid of the other triode? Interesting read though.
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