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In Reply to: RE: LM317 as a CCS , how to? posted by Alex Kitic on October 26, 2014 at 01:28:54
This is way off topic but since you are actively following this topic I hope you don't mind if I tap into your experience for some advice.
My next build will be a RHKT88 amp with a 6SL7 driver and I just wanted to know what would be changes needed to use that tube instead of the 12at7. My output trans is about 20 watts capable of 2.4K primary impedance into 16ohms. Regards, Dak
Follow Ups:
If you build an RH Universal v2, you can choose to install an octal socket instead of a noval socket for the driver. This will obviously limit the choice of driver tube to 6SL7 and 6SN7, plus the various ECC3x versions.
While the v2 circuitry is optimized for ECC81 (100k Rfb, 2nd 100k in series shorted by switch), and ECC83 (switch open, both 100k resistors in series for Rfb), you can use other tubes that fit the socket pin layout.
For 6SL7 and similar, the 200k position is fine. For 6SN7 you can try both switch positions and see which one you prefer. You can see on my blog the RH Universal v2 operated with a 6SL7 near equivalent (ECC35 if I recall correctly) using an adapter.
The OPT sre going to be fine if your speakers are 16 ohm nominal. For the rest, stick to the schematics including the power supply if you want to have all the output tube universality.
And, once you do, let us know the result!
******
http://rh-amps.blogspot.com/
Hi Alex and thanx for the help. I found that trying different values of the fb resistor in my other SE pentode amps can help with fine tuning the sound. For example to little fb makes the sound softer and the bass bloated. Does changing the fb resistor values in the RH amps have a similar effect? I am thinking that since the 6sl7 mu lies midway between the high mu of a 12ax7 and medium of the 12at7 It may be beneficial to use a fb resistor somewhere between 100 and 200k ? regards, Dak
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".
******
http://rh-amps.blogspot.com/
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