• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Can someone give some info on this tube circuit please?

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So far as theory goes: the higher the feedback (lowest resistor value), the lower the output impedance, the better the drive is.
Output impedance has nothing to do with drive capability.
You can have a big tube amp with 1Ω output impedance or a headphone amp with the same.But obvious a great difference in drive capability.
The drive capability of an cathode follower depends on the current it can pull (both directions).
More current is no option here, the supply is too weak.
Other possebility is replacing the cathode resistor (33k) with a CCS.
Like that you can load the output with 33k and still have the output level you had without load.
Mona
 
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It's rather simple to put in a CCS.
Here with one side modded, other side original to see te difference.
Mona
 

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Drive ability is set by both the output impedance and output stage quiescent current, with the latter often dominating and often overlooked. With enough loop gain and enough feedback you can have vanishingly small output impedance but still have poor drive capability. This is not a valve issue, but a circuit issue as exactly the same thing happens with transistors.
 
Pure mathematically looking at impedance as the voltage/current quotient, for the same tube higher impedance implies a higher voltage to current ratio. So, when impedance drops due to FB, the current to voltage ratio goes up for the same output voltage. Agreed, the tube must be able to deliver the current, that's for sure. Maybe a high GM video penthode may be useful. What is the load?
 
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.
 

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disco said:
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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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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