Posts: 182
Location: Serbia
Joined: June 25, 2014
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DAK,
I guess you are referring to "overall feedback" across the amp (from transformer output back to input).
Fine-tuning the Rfb in RH amps will not give the same result. While in simplified theory it may come to "softer sound, bloated bass", it is actually much more complicated than that.
With direct feedback from the anode of the output tube to the anode of the driver tube, there are several issues to take into account, like: - internal resistance of driver tube, - mu and gm of driver tube, - anode resistor as a means of determining the gain of the driver tube, - feedback resistor as a means of determining feedback, but also as in DC terms as current flow path, - cathode resistor of the driver tube, as a means of defining gain and as well feedback path, etc.
Thus the 100k option is optimized for ECC81, while the 200k option is optimized for ECC83. When I state "optimized" I refer to "maximum power output with lowest (or a pre-defined target) distortion".
Other tubes, like ECC82, 5751, E180CC, 5965... can be used as well with one of the settings (we cannot have miriads of settings, you know) with rather good results, although the settings are not what would be optimal for a given tube. On the other hand, it is not all that important whether you otuput 10W or 12W, the difference is rather small in terms of loudness - and if you like the sound, there is not reason why not.
Now, using the ECC81 with 200k will not lead to "bloated bass" or something similar - but the sound is going to be different and the output power not so optimal. Using the ECC83 with 100k, on the other hand, is going to give probably no more than 5W of output power... because at that setting the ECC83 just cannot drive the "output circuit" (the output tube and the feedback loop) any better than that: it needs "help" in the form of adjusted feedback.
That said, the 200k setting is not optimal for 6SL7, but it will do very nicely! I was very pleased when testing it with both 6SL7 and ECC35, although I did prefer the ECC35 - but because my 6SL7s are "garden variety" while the ECC35 has a much higher "intrinsic sonic value".
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http://rh-amps.blogspot.com/
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