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Old 29th September 2010, 10:53 AM   #521
Brit01 is offline Brit01  United Kingdom
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Quote:
BTW Chris and George and everyone, this has been a fantastic and educational thread, thanks heaps!
ditto

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If you are fussy about bias, i.e. each tube of each pair biased within a couple/few milliamps, then you would use a pot for each tube. If you are less fussy, or use matched tubes, then the second method, a pot for each pair.
maybe the fussy bias I need to opt for.

It's actually for the drivers of an OTL (circlotron), in parallel.
Fixed bias controls before the CF's that will drive the output tubes (6080).
So 1 bias control before the grid of the cathode followers I'm thinking.
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Old 29th September 2010, 07:17 PM   #522
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Originally Posted by Brit01 View Post
I have a question regarding fixed bias.

I see some designs with individual fixed bias on each split signal.
Others I have seen fixed bias connected in the middle a matched resistor either side of the opposite rails.

In what situations do you use individually controlled fixed bias and just 1 bias control in the mid point between rails?
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.
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Old 30th September 2010, 10:30 AM   #523
Brit01 is offline Brit01  United Kingdom
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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.
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Old 30th September 2010, 09:29 PM   #524
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hi guys,

am I along the right lines with my bias calculations?

cheers
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Old 4th October 2010, 09:22 AM   #525
Brit01 is offline Brit01  United Kingdom
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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.
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Old 4th October 2010, 10:08 AM   #526
SY is offline SY  United States
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I'd guess "wiring error." Can you sketch a schematic of what you THINK you wired?

You used gate stoppers, with the bodies of the resistors right up on the gate lead, right?
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Old 4th October 2010, 10:38 AM   #527
Brit01 is offline Brit01  United Kingdom
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Quote:
I'd guess "wiring error." Can you sketch a schematic of what you THINK you wired?

You used gate stoppers, with the bodies of the resistors right up on the gate lead, right?
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
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Old 4th October 2010, 11:22 AM   #528
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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)
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Old 5th October 2010, 12:53 PM   #529
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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?)
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Old 5th October 2010, 08:21 PM   #530
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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.

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