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Old 7th October 2005, 06:33 AM   #1
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Default Cathode follower question

I saw a tube headphone amp on the Tube Cad web page that
had a feature that I don`t understand. Three triode sections
from 2 6dj8`s were parralled using a common cathode resistor
for the output ok. But these three triode sections also shared
a common 200 ohm plate resistor. What would this resistor
do? would the output impedence change?

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Old 7th October 2005, 08:07 AM   #2
dhaen is offline dhaen  Europe
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Got a link to the schema?
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Old 7th October 2005, 03:14 PM   #3
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I have built this with a few changes I omited the 150 ohm
plate resistor, subsituted a 6c33c-b and a 100ohm for the
3 paralled triode sections. I seem to have a prommising 1-2 watt
otl. I just wonder what that 150 ohm resistor was fore.

Thanks for looking

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Old 7th October 2005, 03:25 PM   #4
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Looks like, maybe, current limiting.


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Old 7th October 2005, 04:41 PM   #5
SY is offline SY  United States
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Most likely, it's to help squelch oscillation, analogous to a grid or cathode stopper.

It won't change the output impedance very much; an resistance in the plate is reflected back through the cathode divided by mu- for ECC88, the 150 ohm resistor would only add 5 ohms or so to the output Z.
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Old 7th October 2005, 08:09 PM   #6
cerrem is offline cerrem  United States
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I don't see how it would squash oscillation in the B+ due to the impedances / plate resistance/ cap involved....
Usually a unbypassed R in the plate circuit of a follower will introduce some harmonics.... This may sound off the wall but the ciruit may have been very sterile sounding and the designer wanted to add some slight coloring and thickness to the sound...

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Old 10th October 2005, 02:10 AM   #7
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Without further information I would say it is a rather pious repetition of what was seen elsewhere. I did not look up, but this seems to be a small percentage of the plate resistance anyway. I am sure that any tendency for instability could have been better addressed elsewhere in the circuit if necessary.
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