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November 25, 2020, 03:17:42 AM

Author Topic: CCS Loaded Parafeed Output - Design Considerations?  (Read 2295 times)

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Offline L0rdGwyn

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Re: CCS Loaded Parafeed Output - Design Considerations?
« Reply #105 on: November 12, 2020, 05:35:34 AM »
What do you think of this PB...

A friend of mine made a comment that got the gears turning on how to simplify my 801A A2 design.

Originally, I was using a source follower buffer to drive the 801A grid into A2 with V+/V- supplies such that it could swing from +60 to -70V. What I am experimenting with (in LTSpice) is eliminating the source follower, elevating the 801A filaments (~200V), and direct-coupling the 801A grid to the mu-output of the EF37A CCS. This does away with the V+/V- supplies, allowing the grid to swing positive/negative with respect to the filament, and eliminates a coupling capacitor. The output impedance of the DN2540 appears low enough to provide the grid current for A2 and does not seem to be adversely affecting the plate current of the EF37A. Could be different in the real world, I've attached my functional but not refined schematic below, 6J7 has been substituted for the EF37A for proof-of-concept purposes. Thanks for looking.
Keenan McKnight

Offline Paul Birkeland

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Re: CCS Loaded Parafeed Output - Design Considerations?
« Reply #106 on: November 12, 2020, 06:52:21 AM »
 I would rather see that 200V be a negative rail sitting under the driver so it doesn't have to deal with the craziness of the output stage.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline L0rdGwyn

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Re: CCS Loaded Parafeed Output - Design Considerations?
« Reply #107 on: November 15, 2020, 01:55:39 AM »
Hey PB - didn't see your reply for some reason.

Yeah, the more I have looked into this, I think that while it helps with some issues, it creates a host of other ones.  For instance, the bias of the output tube would be highly dependent on variances between different input tubes, changing each EF37A driver would require a manual adjustment.  Also, as the EF37A comes up to bias, the plate voltages initially swings up to 250-270V or so before settling down to 200V, meaning the 801A grid will momentarily be at +70V, which will likely blow my plate fuse.  Even if I increase the rating of the fuse, seems to me putting that stress on the 801A with each startup is not good design.  It also increases my B+ requirements, pushing up against the limit of 600V components...

I may just go back to the old setup!  Why didn't you tell me the 801A is such a pain, PB?!  (Just kidding, of course).

In your 801A A2 amplifier, what kind of harmonic spectrum were you seeing?  So far, with the NFB what I have measured as been a higher H3 than H2, not your typical SET dominant H2 spectrum.
Keenan McKnight

Offline Paul Birkeland

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Re: CCS Loaded Parafeed Output - Design Considerations?
« Reply #108 on: November 15, 2020, 07:01:53 AM »
I didn't capture anything that specific.  The future owner of the amp was very eager to get it!
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline L0rdGwyn

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Re: CCS Loaded Parafeed Output - Design Considerations?
« Reply #109 on: November 16, 2020, 11:13:29 AM »
I can see why, it's a real looker :) no worries, just thought I'd ask.
Keenan McKnight