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100 Watt sweep tube Circlotron OTL (WIP)
100 Watt sweep tube Circlotron OTL (WIP)
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Old 10th June 2021, 10:21 PM   #71
atmasphere is offline atmasphere  United States
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Quote:
Originally Posted by Jerdy View Post
I have attached the LTSpice files below, but I am prepared to abandon the design in favor of a more compact solid-state solution. The design is old, predicated on work I did (and thought I vetted) years ago when I was far less experienced. It's quite likely I overlooked some serious errors and even the performance I thought I was getting in the prototype was coincidental. At this point I am poised to cut my losses and redesign something safer, simpler, and more efficient.

The problem, near as I can discern, seems to be coming from biasing of the 6AU6. I can't find the adjustment point that draws enough current to lower the 6GY5 control grid to the ~370V required for 400V output while maintaining equilibrium with the other circuit elements. The output voltage is only 10V below the input and excessive screen current is drawn on the pass tube. The VR tube used is 85Vref, which I suspect may be too low relative to the desired output, causing issues with control tubes' voltage, current and biasing. Also possible is I'm using bad tubes and just don't know it. Most of my radio tubes were acquired in unknown condition. However, swapping among multiple 6AU6s changes nothing.

Any input is welcome, especially on suggestions for new HV regulated supply circuit designs.

A 6AU6 has a lot of gain and is capable of acting as an opamp entirely on its own. You don't need that extra tube for starters. I don't think you need to abandon the board but some mods could be executed. If this were mine I'd be taking advantage of the gain of the 6AU6 (assuming you don't have cathode/filament arcing issues).



You could run a zener stack in the cathode circuit of the 6AU6 and use a divider network to feed the grid; in this way you could have some feedback gain to more tightly control the output voltage.
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Old 10th June 2021, 11:51 PM   #72
Jerdy is offline Jerdy  United States
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Quote:
Originally Posted by atmasphere View Post
A 6AU6 has a lot of gain and is capable of acting as an opamp entirely on its own. You don't need that extra tube for starters. I don't think you need to abandon the board but some mods could be executed. If this were mine I'd be taking advantage of the gain of the 6AU6 (assuming you don't have cathode/filament arcing issues).



You could run a zener stack in the cathode circuit of the 6AU6 and use a divider network to feed the grid; in this way you could have some feedback gain to more tightly control the output voltage.
From what you describe, I'm picturing something similar to this circuit: (attached).

I can certainly pare my current circuit down by using just the 6AU6 and VR tube. I will mess around with some sims then try it on the bench. Thanks for the helpful suggestions.
Attached Images
File Type: gif reg30.gif (8.2 KB, 77 views)
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Old 11th June 2021, 12:12 AM   #73
TonyTecson is offline TonyTecson  Philippines
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in triode mode, the 6au6 has a mu of 60, still substantial..
if the sims did not jibe with real world, probably your spice model is wrong..
for G2 supply, nothing can be simpler than a mosfet voltage follower...
DCPP.....
Click the image to open in full size.

Last edited by TonyTecson; 11th June 2021 at 12:15 AM.
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Old 11th June 2021, 03:48 AM   #74
Jerdy is offline Jerdy  United States
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I recognize that schematic, Pete Millett’s engineer’s amp. I have borrowed that design before. I have a few more tricks to try on the tube regulator, but I do like the simplicity of the capacitance multiplier.
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Old 11th June 2021, 05:13 PM   #75
atmasphere is offline atmasphere  United States
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Quote:
Originally Posted by Jerdy View Post
From what you describe, I'm picturing something similar to this circuit: (attached).

I can certainly pare my current circuit down by using just the 6AU6 and VR tube. I will mess around with some sims then try it on the bench. Thanks for the helpful suggestions.

Although they look nice I wouldn't use a VR tube; IME they make noise and you need low noise on the cathode. I'd use a zener stack bypassed by a capacitance. Don't run them too hot.

Last edited by atmasphere; 11th June 2021 at 05:16 PM.
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Old Yesterday, 11:51 PM   #76
TonyTecson is offline TonyTecson  Philippines
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Quote:
Originally Posted by Jerdy View Post
I recognize that schematic, Pete Millett’s engineer’s amp. I have borrowed that design before. I have a few more tricks to try on the tube regulator, but I do like the simplicity of the capacitance multiplier.
i used his tricks in many of my tube builds, they work, and if you put 1/2 watt Zeners as gate reference, you have a voltage regulator and a cap multiplier...

i see you also used his A2 mosfet followers, they work good...
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