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Folded/shunt cascode differential question
Folded/shunt cascode differential question
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Old 14th March 2018, 10:33 AM   #41
Rod Coleman is offline Rod Coleman  United Kingdom
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Folded/shunt cascode differential question
R-anode looks too small. 3W wirewound recommended.
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Old 14th March 2018, 12:36 PM   #42
mogliaa is offline mogliaa  United Kingdom
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Folded/shunt cascode differential question
You need a protection zener for the lower device if J112. A 15v one should work well

The IXTP08n100d as well as the other higher current versions from IXYS have a higher VGS value for lower current (<50mA) which will allow the J112 to operate at higher VDS so capacitances at play are reduced. Overall FR of the stage is improved at HF this way.
Btw: and they sound better!
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Old 14th March 2018, 05:38 PM   #43
john_tracy is offline john_tracy  United States
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From experience I'd use an IXYS 8N50D on the bottom of the anode cascode.
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Old 15th March 2018, 02:36 AM   #44
Sodacose is offline Sodacose  United States
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Epic tip on the ammo pack layout! Will also add zener for the JFET, up the anode resistor dissipation and make the anode load a cascode rather than a single depletion fet.
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Old 15th March 2018, 03:01 PM   #45
Sodacose is offline Sodacose  United States
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Updates! Board grew a bit, but I was able to route everything much more cleanly thanks to the ammo pack suggestion and (ironically) adding a few more/bigger parts. Now about 3.75" by 2.25" and accommodates a standard 1" spaced heatsink for the higher dissipation TO220.

Click the image to open in full size.

Click the image to open in full size.

Gotta say, laying out boards is kind of fun (in the way that ptp wire routing OCD is fun).
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Old 15th March 2018, 03:24 PM   #46
Ketje is offline Ketje  Belgium
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Can't help it, does remind me of this
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Old 30th July 2018, 08:31 PM   #47
Sodacose is offline Sodacose  United States
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Hard to believe that I've had boards for this on hand now for a few months. I am finally working my way back around to the project and implementation.

I'm wondering about start-up behavior. I have the option of powering this with a tube rectifier, which would delay and slowly ramp up the B+ (thinking 5AR4). This seems like a good idea because it will allow the tube to start conducting before HT is applied to the anode of the tube and emitter of the BJT through the shared CCS. Lacking that more controlled start up, I worry the BJT current will only be limited by the CCS with the tube heaters cold.

Anyone care to confirm this line of thinking?
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Old 30th July 2018, 09:06 PM   #48
Wavebourn is offline Wavebourn  United States
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Folded/shunt cascode differential question
I would suggest to return output through servo RC to R2, to stabilize the working point. The time constant should be far from R1/C1 to avoid possible instability.
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Old 31st July 2018, 11:29 AM   #49
Rod Coleman is offline Rod Coleman  United Kingdom
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Folded/shunt cascode differential question
Quote:
Originally Posted by Sodacose View Post
I'm wondering about start-up behavior. I have the option of powering this with a tube rectifier, which would delay and slowly ramp up the B+ (thinking 5AR4). This seems like a good idea because it will allow the tube to start conducting before HT is applied to the anode of the tube and emitter of the BJT through the shared CCS. Lacking that more controlled start up, I worry the BJT current will only be limited by the CCS with the tube heaters cold.
When you build the Shunt Cascode board, start with the CCS adjusted to the low end of the expected range. When it's warmed up, adjust the CCS until the output is at the total output swing. The swing is almost equal to the anode voltage of the triode.

The maximum current in the output is 2x the idle current, because the output voltage is then about equal to the anode voltage. The PNP does not allow the voltage to raise any higher, so that is a stable condition. The PNP dissipation is very low at this point, because Vce is very low.

So long as the output resistor is well-rated for the power, nothing bad will happen, even if it is left running in this state for hours.
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Old 31st July 2018, 11:43 AM   #50
Rod Coleman is offline Rod Coleman  United Kingdom
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Folded/shunt cascode differential question
Quote:
Originally Posted by Wavebourn View Post
I would suggest to return output through servo RC to R2, to stabilize the working point. The time constant should be far from R1/C1 to avoid possible instability.
I always plan to add a servo to my Shunt Cascode PCB.

But in practice, regulating the heater supply is usually sufficient, in my high-gain driver design. Of course, it depends on the quality of the valve, but I can say that the 6Э5П with OTК rhombus marking has very stable anode current, over a long operating time.

The last lot of 6Э5П I got from Ukraine is marked ПЕРЕПРОВ, so we will see if they are good enough.
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