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Can someone give some info on this tube circuit please?
Can someone give some info on this tube circuit please?
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Old 21st October 2019, 01:31 PM   #71
DF96 is offline DF96  England
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Join Date: May 2007
Quote:
Originally Posted by disco
So, when impedance drops due to FB, the current to voltage ratio goes up for the same output voltage.
Almost always the output current is set almost entirely by the load impedance. The output impedance just affects how much voltage gets dropped at the output.

Adding feedback to a circuit does not render the amplification "useless"; it has been used to reduce distortion, output impedance etc.
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Old 22nd October 2019, 05:13 AM   #72
VaNarn is offline VaNarn  Australia
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Join Date: Dec 2010
Location: Ivanhoe
Can someone give some info on this tube circuit please?
Quote:
Originally Posted by disco View Post
According to your schematic (#57) the right ECC82 has its A=140V G=60V K=62V. A new tube should be at 12mA what is plenty to drive any amplifier. If the circuit is not to your taste you'd better move on to another. I would, as the amplification is rendered useless by the high level of global feedback.
If the voltage figures for the cathode follower are accurate then 62 volts/33kohms results in a current of 1.878 mA through the valve in the absence of an audio signal. Clearly the ECC82 load lines cannot be used in this manner. I have no idea of what the complete circuit schematic of the preamplifier looks like, as it would be of interest to know if there are amplifying stages preceding the 50k pots or if there are just switched inputs provided. It is relevant in that signals should be kept in the correct phase as the source material by balancing inverting and non-inverting active stages to satisfy this criteria.
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