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Old 18th January 2008, 08:37 PM   #11
AJT is offline AJT  Philippines
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hi sheldon, could it be you got your zener diode backwards? i would use them back to back...
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Old 18th January 2008, 08:46 PM   #12
Sheldon is offline Sheldon  United States
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Quote:
Originally posted by Wavebourn


Ground loop may be?
I'll look closely, but I have no problem when the source is loaded with a 10K resistor instead of the CCS.


Quote:
Originally posted by Tony
hi sheldon, could it be you got your zener diode backwards? i would use them back to back...
The DN2540 is a depletion mode device. The gate is at lower potential than the source.

Sheldon
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Old 18th January 2008, 09:01 PM   #13
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One thing I found out accidentally is that DN2540 will smoke if you don't power it up in just the right order. For a follower like this, the depletion mode doesn't carry much advantage, so you might think about switching to a more prosaic enhancement mode device.
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Old 18th January 2008, 09:06 PM   #14
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Quote:
Originally posted by Sheldon



The DN2540 is a depletion mode device. The gate is at lower potential than the source.

Sheldon

But, it can be operated as an enhanced device > gate to source positive. Iguess something like 2 zeners to protect this FET ?
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Old 18th January 2008, 09:12 PM   #15
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Using a CCS as the grid 'leak' resistor for the 801A2 might be the reason for bias runaway. Try connecting a 100k resistor in parallel with the CCS, i.e. from the 801A2 grid to ground. This shouldn't materially affect the operation of the CCS but it will satisfy the maximum grid-cathode resistance requirement of the 8012A under fixed bias conditions. IF you're using a cathode bias resistor, a 470k grid 'leak' resistor will do.
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Old 18th January 2008, 09:44 PM   #16
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Quote:
Originally posted by Sheldon


I'll look closely, but I have no problem when the source is loaded with a 10K resistor instead of the CCS.

Different capacitances cause different currents on highs through ground loops.
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Old 19th January 2008, 12:31 AM   #17
Sheldon is offline Sheldon  United States
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Thanks for the suggestions. I'll file them for future use (maybe even tomorrow). Wavebourn was onto something - an extra ground issue. The immediate problem seems to have been a partial short between one of the FET's and heat sink. No conduction when I assembled it, but maybe with heat expansion, a little burr dug its way through. I replaced a cooked FET, lapped them all along with the heat sink. Seems fine now.

I'll take a break and listen to music, then see if I can implement a plate CCS on the driver tube.

Sheldon
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Old 19th January 2008, 12:35 AM   #18
Sheldon is offline Sheldon  United States
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Quote:
Originally posted by SY
One thing I found out accidentally is that DN2540 will smoke if you don't power it up in just the right order. For a follower like this, the depletion mode doesn't carry much advantage, so you might think about switching to a more prosaic enhancement mode device.
Interesting. In this case, the bias voltages are all derived from the same supply, so come up quickly and essentially concurrently. The bias will lag the rails just a little because of the extra RC sections.

If if problem becomes a recurring issue, I guess I could use a regular Mosfet as the follower and a DN2540 for the current sink.

Sheldon
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Old 19th January 2008, 01:33 AM   #19
SY is offline SY  United States
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Default DN2540

In a cascode current source or sink, I've never lost one, and I've done a few dozen.
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Old 21st January 2008, 08:13 PM   #20
Sheldon is offline Sheldon  United States
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You are right SY about these being touchy. I've lost a few now when I play with the amp under power. If everything is properly wired, I don't seem to have any issues. I've got the follower loaded with a resistor now, and it's fine limited swing. But I get significantly less distortion with the current source load when I try to swing more voltage, so I'm going to switch back. Interestingly, if I load the 6SN7 with a cascoded DN2540, the distortion increases slightly over the resistor load, even at larger voltage swing.

Sheldon
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