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drift in direct coupled tubes
drift in direct coupled tubes
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Old 9th July 2007, 10:38 AM   #11
ilimzn is offline ilimzn  Croatia
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Join Date: Feb 2005
Location: Zagreb
Originally posted by Wavebourn
Here you go:

Click the image to open in full size.

In order to shift levels I use bipolar B+ source.

Doesn't the high dynamic load resistance roughly equal R3? Obviously 'high' gets to be a relative term, then.

In general, if you have a ccs loaded gain stage, the cathode to plate voltage is the voltage drop on the tube's Rp at that working point. Changing the power supply voltage would not produce drift, but an aging tube will (towards higher plate voltage). If this is directly coupled to the follower, which is also ccs loaded, then it's cathode will go that much higher. If you are direct-coupling to a gain stage that follows, then you have a problem unless the whole thing is differential.
It;'s not terribly difficult to 'servo' this, by 'adjusting' the first tube bias voltage, from the follower output DC voltage. A bit easyer than servoing the plate ccs of the first stage as both input and output of such servo are ground referenced.
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Old 9th July 2007, 09:19 PM   #12
andrew_whitham is offline andrew_whitham  United Kingdom
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Join Date: Oct 2005
Location: Great Britain
Again thanks all,

Wavebourne, didn't that circuit come up when I was last asking about using CCS's in a cathodyne phase splitter ? (answer - yes, and it was nice then too) Anyone interested - http://www.diyaudio.com/forums/showt...threadid=95152 is where its explained... (going to build this one)

ilimzn - the Servo bit was what I was hoping to avoid. I know folks do this without it. Are you saying they just 'put up' with the drift? i.e. its mostly acceptable but there's a better way?

EC8010, great, and I'm convinced enough to just go build the thing with the cathode follower. I must admit I wasnt going to bother with a regulated heater supply, but I might now. HT regulation is part of the reason for building the amp though. testing, testing, testing...

Naturally results to follow, but genuine thanks for the help.

Cheers all

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