ps. I'm Mark
Well, I'm not really loading the ECC40 with a CCS. I actually have an aversion to solid-state in or near the signal path , I find a lot of the ideas don't work or sound quite right to my ears - however I slowly accept these things where it's done well and I have proven it myself. For instance I have not found a CCS I fully like when used as an anode load. I have not tried a CCS in a cathode, but then I'm not sure how well that would work in SE. I can see the benefits in PP, but have heard some poor implementations that did not interest me.
So, going back to the ECC40, it's loaded by the choke really - the CCS and VR tubes form a stabilised supply with shunt-regulation. I'm hoping the CCS and the choke will not interact in a detrimental way. It's rather like the method used by Lynn Olson ( +Gary Pimm ) for the driver stage in the Amity & Aurora amps, although they are PP.
I really should do a Spice sim now before it's too late. I just haven't got the valve models downloaded or working yet ( I had it before, but lost all the set-up in a computer crash ) . But explanation as above - I believe the CCS and VR tubes form a little 'wall' from the main supply and then the ECC40 interacts with the choke load. Note the choke value is too low for good full-range action - this is where I have concerns that something odd might happen when feeding this amp some bass - which it will get. After the coupling cap, the output stage operates only in the upper mids or HF.Wolfgang wrote: ↑Mon Jun 18, 2018 4:04 pm How do you calculate the AC load line with a CCS, a choke , and a resistor? Do you use a Spice simulation? The linearity (and benefits) of the CCS depends after all on the impedance to which it is connected but if you want to adjust this as RC filter how do you make all these factors fit?
But anyway, at 500Hz upwards the choke will look like a CCS in terms of load line - so a horizontal line across the ECC40 curves.
As far as the 45 goes there aren't too many choices on load line etc if you want to get decent power out of them - ie. 1.5W or more. I've not spent a lot of time playing around with load lines over the years, it's usually a trade-off between 2nd harmonic and 3rd-harmonic at the end of the day - both have their pros/cons depending on the music you play - and I play a pretty mixed range of music all the time.
It's built up from previous work & amps plus what other people have built that sounded good. I know 45's can sound very special, clean and pure, if not over-loaded by speakers - hence prime choice for a low-power HF amp. The transformers are specially built for HF. The ECC40 , I first heard in a lovely 3-stage 45 amp . I've since learned that it has low levels of higher harmonic distortions ( 3rd and above ) compared to other valves, and I used it in a 3-stage 300B amp for a while. I want to hear it again , and use with a 45 , to see how good the top-end sounds. Other than this, the decision was based on getting just enough gain, plus using items I had around.Wolfgang wrote: ↑Mon Jun 18, 2018 4:04 pm When you design a circuit like this do you start from a specific "sound" you want to achieve which would be the reason for setting the operating point of the tubes (considering the harmonic/distortions)and adjust the rest around this or do you simply define the output gain and work from there backwards?
But mostly I just work from the datasheet charts to get the basics set up. I believe the choice of valve determines the 4th/5th etc harmonics.