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CFB for tubes in parallel, PSE. Any surprises?
CFB for tubes in parallel, PSE. Any surprises?
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Old 15th January 2021, 12:16 PM   #31
mondogenerator is offline mondogenerator  England
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Personally, I would use LED self bias, which to my mind, is somewhere between self bias and fixed bias only because the Vgk voltage is pretty much fixed at the Vf of the diode string.

my mind might be totally wrong though

I'd then probably try a BJT cascade CCS anode load, one for each device.

Things become complex very quickly!

I may be completely off tangent, but that's just my 2 pence.
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Old 15th January 2021, 03:36 PM   #32
Kay Pirinha is offline Kay Pirinha  Germany
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LED bias also would require different heater supplies with directly heated valves. Not different from resistor self bias in this respect.
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Old 15th January 2021, 04:04 PM   #33
SpreadSpectrum is offline SpreadSpectrum  United States
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Quote:
Originally Posted by Kay Pirinha View Post
This will go tricky enough, due to the necessary individual current sensing resistors, won't it?
Best regards!
I have always wanted to see if I could use an optical isolator or something to make a good current sense on an electrode that isn't referenced to gnd.

The dual cathode windings that were discussed above could make this easier if the OP is making the transformer. But as you said, the directly-heated cathode complicates things.

I'm currently experimenting with directly-heated transmitting triodes as output tubes but I never considered combining with CFB, just because it seemed like it would really complicate things having the cathode of a directly-heated tube (running on DC) swinging around.
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Old 15th January 2021, 04:30 PM   #34
Kay Pirinha is offline Kay Pirinha  Germany
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Quote:
The dual cathode windings that were discussed above could make this easier if the OP is making the transformer. But as you said, the directly-heated cathode complicates things.
Yes, the CFB winding halves need to be isolated, not CT'ed. The urge of individual heater supply would remain anyway.
Directly heated valves really are a PITA to deal with, unless you restrict yourself on applying only a single one .

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Old 15th January 2021, 06:45 PM   #35
50AE is offline 50AE  Bulgaria
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CFB for tubes in parallel, PSE. Any surprises?
1. Increase of chance of oscillation due to electrode intercapacitances increasing with the amount of paralleling?

2. Why the idea of separate CFB windings? No problem doing it trifiliar even, for 3PSE, but why do it? Separate heater supplies? Why?
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Old 15th January 2021, 11:23 PM   #36
45 is offline 45  Italy
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Quote:
Originally Posted by 50AE View Post
1. Increase of chance of oscillation due to electrode intercapacitances increasing with the amount of paralleling?

2. Why the idea of separate CFB windings? No problem doing it trifiliar even, for 3PSE, but why do it? Separate heater supplies? Why?
Separate windings are mandatory if you use DHT or DHP. Not necessary for indirect type.

Oscillations? Not sure it will be a real problem.

Last edited by 45; 15th January 2021 at 11:25 PM.
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Old 17th January 2021, 10:00 AM   #37
50AE is offline 50AE  Bulgaria
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CFB for tubes in parallel, PSE. Any surprises?
As far as I can self explain, one should start with separate heater supplies if one wishes to have separate resistances in each cathode. Otherwise one heater supply low resistance will bind them all. Or you might get away with a CCS per each heater?

A similar idea comes in mind about the separate CFB winding perhaps?
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Old 17th January 2021, 11:15 AM   #38
45 is offline 45  Italy
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Quote:
Originally Posted by 50AE View Post
As far as I can self explain, one should start with separate heater supplies if one wishes to have separate resistances in each cathode. Otherwise one heater supply low resistance will bind them all. Or you might get away with a CCS per each heater?

A similar idea comes in mind about the separate CFB winding perhaps?
Heaters with CCs? Why? You need a stable voltage if anything.

The CCs is a more refined circuit that also does voltage regulation like Rod Coleman's or TentLabs boards for directly heated types is different thing because the filament is the cathode as well.
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Old 17th January 2021, 12:29 PM   #39
Kay Pirinha is offline Kay Pirinha  Germany
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Quote:
Originally Posted by 50AE View Post
As far as I can self explain, one should start with separate heater supplies if one wishes to have separate resistances in each cathode. Otherwise one heater supply low resistance will bind them all. Or you might get away with a CCS per each heater?
A similar idea comes in mind about the separate CFB winding perhaps?

One CFB winding for SE and two winding halves for PP is sufficient even with DHT's or DHP's. You need to connect an individual heater supply per tube to the CFB winding's hot end(s) via the bias RC combination (for cathode bias) or the current sensing resistor (for fixed bias).


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