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Protection for DC-coupled cathodyne stage?
Protection for DC-coupled cathodyne stage?
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Old 17th September 2018, 07:28 AM   #21
petertub is offline petertub  Sweden
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Quote:
Originally Posted by aros71 View Post
I use a 30sec delay circuit - at power-on, the filament voltage comes on immediately, but there's a delay before the AC is applied to the rectifier diodes.

This means that the moment the tubes see HT they're already conducting and voltages won't surge.

The other option is the arc-protection diode as described by Blencowe in the Cathodyne chapter of "Designing High-Fidelity Tube Preamps"
This start sequence will prevent the excess Vgk . Tube problems seems to have

other reasons. Maybe try another brand of tubes ? JJ makes E88CC
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Old 20th September 2018, 10:41 AM   #22
MrSnrub is offline MrSnrub  Australia
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OK, I think I have the remaining issues resolved now. A new pair of Reflector 6N23P are installed, and the input stage's cathode resistor has been reduced to 270ohm. This brought the anode down to 90V, and because of that, the cathodyne now has a Vak of about 100V at idle. Before, it only had 20V or so. The amp now sounds fine.

I noticed that the idle Va varied between valves. I tried a few, and it varied anywhere between 60 and 90V for the same cathode resistor. Vgk sits at around -1.2V, which I'm hoping gives enough headroom on the positive excursions. Some playing with loadlines suggests that replacing Ra with 33k and Rk with 330ohm would still give me Va of about 90V, but a more comfortable Vgk of -2V or so, but for now I think I'll just enjoy the amp as it is.

Again, thanks for the suggestions. I can wield a soldering iron, but I'm no expert at the design side of things - though fixing up this amp has given me ample opportunities to learn
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Old 20th September 2018, 10:49 AM   #23
TonyTecson is offline TonyTecson  Philippines
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Protection for DC-coupled cathodyne stage?
FWIW, i have been using NE-2 neon lamps instead of diodes to protect the tube...
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