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-   -   Will this arrangement impact the input impedence of the cathode follower? (http://www.diyaudio.com/forums/tubes-valves/220980-will-arrangement-impact-input-impedence-cathode-follower.html)

Kenneth Zhu 5th October 2012 12:41 AM

Will this arrangement impact the input impedence of the cathode follower?
 
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Usually when DC coupling a common cathode and a cathode follower, a technique that connects the grid and the cathode of the cathode follwer valve with a semiconductor diode will be used, as the picture shows. It is used to prevent extrem large current through the cathode follower when negative grid voltage is to be established during the power-up. This diode will be reversely biased after the negative grid voltage buids up and then cut off.

However, I have a question here. As it apprears between the grid and the cathode of the cathode follower valve, will it be part of the input impedance of such valve. And more worsely, as the diode has very small impedance when being forwardly connected while very large impededance when being revsersely connected. Will this make the input impedance of the cathod follower rather unstable with regard to AC signal between the grid and the cathode?

Hope somebody can help to carify it. Thks.

ashok 5th October 2012 05:00 AM

The diode is reverse biased under normal operation by the gate cathode voltage. The gate cathode also remains with that polarity under signal conditions and doesn't go to zero under normal conditions. So the diode will be off and will not cause any unstable condition.

Wavebourn 5th October 2012 05:04 AM

What capacitance has 1N4007 diode? And how much it varies with signal?

Kenneth Zhu 5th October 2012 07:59 AM

It seems, according to listening test, the sound will be more appreciable and natural after the diode is taken away. Perhaps though in DC condition it is in cut-off, it still offers a path of varying impedance to AC signal.

disco 5th October 2012 08:14 AM

Seems the publisher of that circuit is pulling the good tube peoples' legs here Kenneth.
Or, could it be the operating point has changed?

http://imageshack.us/a/img35/6864/1n4007.gif

DF96 5th October 2012 09:31 AM

The capacitance of the diode will vary with signal, but it is bootstrapped by the cathode follower. For an ECC83/12AX7 it will see very little signal voltage. I have used a diode like that for an ECC83 LTP phase splitter, where one might expect greater signal voltage combined with much less bootstrapping, without any harm. However, I used a 1N4148 which has much less capacitance than a 1N4007. Just use any small-signal low capacitance silicon diode, not a rectifier.

SY 5th October 2012 09:52 AM

If you're worried about a reverse biased diode, use a neon lamp. They will clamp Vgk to 60V or so (if you choose one of the lower voltage lamps), which is low enough to prevent flashover.

DF96 5th October 2012 12:53 PM

As the neon lamp is likely to be bigger than a diode junction, will it inject more noise from ionising radiation passing through? (Only kidding! For an ECC83 the neon bias voltage will be too small to create any cascade effect.)

kevinkr 6th October 2012 01:12 AM

Call me what you will, but I have never used one of these diodes, I do use neon lamps and diodes for cathode clamping on LTP running on high voltage negative rails.

I've never had or seen a tube fail in an application where the modern trend seems to be to use that diode. I will admit I often, but not always use tube based supplies that warm up moderately slowly. (Regulators with a series pass tube or tube rectifier with an indirectly heated cathode)

I'd say if you must use a diode that the 1N4148 or 1N914 would be better choices.

Wavebourn 6th October 2012 01:49 AM

Call me what you will, but I never had problems with directly coupled tubes without any diodes: for example, if anode load resistor in the previous stage can provide too high grid current I better add in series with the grid something like 56k resistor with 100 pf cap in parallel with it, like I did in my Pyramid amp (schematic available in several topics).


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