I have a negative supply connected to a CCS in the tail of a LTP in a simple pp amp. The amp works fine, but when I measure the voltage at the output of the negative supply I get only 3.7mv. Out of circuit, the supply measures approximately –40v. I don’t understand this result. What am I measuring?
I don’t have a diagram, but the circuit is easy to describe. It’s a two stage amp with an ECC99 LTP driving 6V6 output tubes. The ECC99 is operating at roughly 150v/8ma per section. A pair of 10M45S in coscode is used for the CCS. The negative PS uses a simple pi filter.
I discovered the voltage anomaly because I wanted to use the negative supply to try fixed bias as opposed to the cathode bias currently used in the output stage.
Maybe I should suspect my autoranging Radio Shack multimeter.
I discovered the voltage anomaly because I wanted to use the negative supply to try fixed bias as opposed to the cathode bias currently used in the output stage.
Maybe I should suspect my autoranging Radio Shack multimeter.
I don't suppose you could have misread a resistor code in your pi filter and ended up with a much larger value than intended. I haven't used the 10M45S-depletion mosfet I presume?- but the cathode would perhaps be positive enough for your cascode to work even if the bottom were simply grounded.
A single 10M45 should sink the 16mA total cathode current without any problem. The device does need at least 10V across it to get into operation, but that doesn't explain why your negative rail is near zero.
If you hook a resistor across the negative supply to simulate your design-current (about 2.5k), does the negative supply still output around -40V?
As piano3 requested, a sketch of your circuit would be helpful to the group here.
If you hook a resistor across the negative supply to simulate your design-current (about 2.5k), does the negative supply still output around -40V?
As piano3 requested, a sketch of your circuit would be helpful to the group here.
After leaving the amp powered up for several hours, the 100 ohm resistor in the pi filter of the negative supply got very very hot. I’m not sure why the PS failed. I used a full wave rectifier (two diodes) with a small toroid transformer. Is it not recommended to use a full wave rectifier with a toroid?
No problem using a toroid. 16mA through a resistance of 100 ohms dissipates 0.0256 watts which would not cause any resistor to get even slightly hot. However if you dropped the entire supply voltage at 16mA across the resistor, then you would get 0.64 watts which would overheat and eventually might destroy a 1/4 watt resistor. This would happen for a 2k5 resistor, for instance. So, as I suggested above, are you sure that the resistor was 100 ohms? Also, are you sure that the current was 16mA? Did you actually measure it?
You question got 313 "views'. LOTS of people looked at it, saw there was no schematic then decided they wouldn't help. Could you post one?
The next thing to do when you have a problem is take a photo and post that too.
The problem is one of three things and with not schematic and phot no one can know which it is
1) The design is flawed
2) The design is OK but you made a mistake in assembly and the the schematic is not implemented
3) Bad parts
From what you say I suspect 2 or 3. You can see that most of the 313 people who looked, where unable to help, not enough information.
Problem solved. I constructed the CCS on perf board and the standoffs I used were too short to keep one or more of the component leads on the bottom of the board from shorting to ground. I now measure -38V at the output of the negative supply when connected to the CCS. The amp sounds better than ever now. Thanks to all for generously taking time to advise this amateur builder.
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