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January 15, 2021, 02:43:37 AM

Author Topic: CCS Loaded Parafeed Output - Design Considerations?  (Read 3445 times)

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Offline L0rdGwyn

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Re: CCS Loaded Parafeed Output - Design Considerations?
« Reply #135 on: December 29, 2020, 02:17:57 AM »
Would it be overkill to use one 6BX7 per channel with a resistive load and parallel the two triodes?
Keenan McKnight

Offline Paul Birkeland

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Re: CCS Loaded Parafeed Output - Design Considerations?
« Reply #136 on: December 29, 2020, 04:57:24 AM »
I don't think that would be a problem.  The two triodes in parallel will let you use a lower resistor value, but will be somewhat demanding on that B- supply.
Paul "PB" Birkeland

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Offline L0rdGwyn

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Re: CCS Loaded Parafeed Output - Design Considerations?
« Reply #137 on: December 29, 2020, 07:46:21 AM »
Yeah it would be demanding, always a compromise to be made.  I was planning on a TL431 shunt regulated B- supply, could probably spec it for 30-40mA.  I'm trying hard to use the 6BX7 since I have a bunch of them in pairs, but if it came to a point where I was using a single 6BX7 for both channels, I think it would make sense to switch to the 6AK4 or 6CK4 to take advantage of the higher single triode plate dissipation.  Also, maybe it is total tube amplifier voodoo, but in my experience I have found equivalent single-triodes to have better soundstage and imaging than equivalent dual triodes (6J5 vs 6SN7, for example), although I have not found a objectively measurable way to explain that (crosstalk, distortion, etc.).  Not sure if the same applies to cathode followers, haven't tried it yet.

I really like the second triode as a current sink if I can keep the 801A happy on startup, might really go with the time delay idea.
« Last Edit: December 29, 2020, 10:40:23 AM by L0rdGwyn »
Keenan McKnight

Offline Paul Birkeland

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Re: CCS Loaded Parafeed Output - Design Considerations?
« Reply #138 on: December 29, 2020, 09:20:43 AM »
Check out the 12GN7.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline L0rdGwyn

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Re: CCS Loaded Parafeed Output - Design Considerations?
« Reply #139 on: January 01, 2021, 02:00:11 PM »
Hey PB - I'll check out the 12GN7, thanks.  I'm still brainstorming some ideas on the 6BX7 cathode follower feeding 801A startup biasing issue.

What about this - placing a high-value resistor between the 801A grid and negative supply.  At startup before the 6BX7 is biased, the 801A will see the full negative supply and be placed at cutoff, then the grid will rise to its 0V bias point as the 6BX7 come up to bias.

Any reason you can see that wouldn't work?  My thinking is this would also protect the 801A in the event of a failure of the 6BX7, although the 801A will be fused as well.  I might be making a mental error though, I am experiencing post night shift brain.
« Last Edit: January 01, 2021, 02:07:59 PM by L0rdGwyn »
Keenan McKnight

Offline Paul Birkeland

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Re: CCS Loaded Parafeed Output - Design Considerations?
« Reply #140 on: January 01, 2021, 03:13:43 PM »
The problem with the high value resistor is that just a whisper of grid current will shift the bias substantially.  You can place a transistor between B+ and the primary of the OPT and have it sense current draw on the top 6BX7 in order to turn on and allow the output stage to work, but I favor a chunky resistor under the cathode follower as the simple/reliable solution.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline L0rdGwyn

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Re: CCS Loaded Parafeed Output - Design Considerations?
« Reply #141 on: January 03, 2021, 09:30:43 AM »
I gotcha PB, thanks for the input.  I think I am going to go with the resistor load and parallel triode sections.  I crunched the numbers, a 6BX7 as a current sink with a 160ohm cathode resistor only represents about a 2.6K AC load, not great!  Compare that to a 6K resistor on a single triode or a 3K resistor on parallel triode 6BX7 cathode follower.

Check out these curves PB - I recently purchased a curve tracer.  The first set are both triodes traced and overlaid on eachother.  With a single triode resistively-loaded cathode follower, operating point would be around 330V, 15mA, -35Vg1.

Compare to the second set of curves, this is both triodes in parallel, operating point would be 330V, 30mA, -34Vg1.  The linearity is much, much better!

So, think I will use the 3K resistor on the parallel cathodes of the two sections running at 15mA a piece, just have to spec my negative supply for it.

« Last Edit: January 03, 2021, 09:44:35 AM by L0rdGwyn »
Keenan McKnight

Offline Paul Birkeland

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Re: CCS Loaded Parafeed Output - Design Considerations?
« Reply #142 on: January 03, 2021, 02:42:55 PM »
Fancy.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man