It is interesting to hear that some boutique caps made it worse some made it better. All in all I guess you ended up spending as much or more on the caps than the original Bottlehead amp. I know the V- caps are very expensive, especially in the large sizes. I bought some for customer at his request. As I said, I don't mind doing it, I don't feel its the best use of money when other more important things can be done in a system.
As to what you heard. The 1 uF output coupling cap is conducting current so that is the one that should make the most difference. The difference in current is more than ten thousand to one vs the interstage cap. The output coupling cap is driving a load of a few thousand ohms, the interstage cap is driving something like 470,000 ohms and at lower 15 times lower signal level (15 is the mu of tube).
I also think the output coupling cap should be a lot bigger. If you are driving a 3000 ohm transformer the -3 dB down point is 53 Hz with 1 uF. This should be a problem, however 32 ohm phones move the -3 point down by 4 to 1 is the inductance of the output transformer is sufficient. Has anyone measured that? I clicked on specs but couldn't find any.
I never use less than 33 uF in my rather similar 6EM7 parafeed amp. Why is that cap so small? These are the things that I think need attention.
Here is a nice calculator for that sort of thing.
http://www.sengpielaudio.com/calculator-XLC.htmI am not surprised to hear the current source mod made a big difference. That is a major circuit change. In my mind it has much more influence over the sound than a cap change. A current source unloads the input triode reduces distortion and raises gain. Did you notice the gain change?
If you had a variety of
output transformers and chokes to try you would hear bigger differences and it's not about the materials. Magnequest promoted some unusual expensive iron some time ago, where is it now? Seems everyone making parts has become a self appointed materials expert.
However who wants to fool around with changing transformers, though yours are pretty easy to get to. Do you know the primary and choke inductance values. They had better be large. Why dont people fool with transformers more than capacitors. The have a bigger chance of making a difference. I will propose that a tranformerm is too difficult and varied to specify. You have to express the impedance ratios, the input inductance, the leakage inductance, the copper loss, the iron loss, the Low frequency saturation point. A cap is easy to specify, capacity and voltage, done.
The rest is going on and on about exotic materials, winding techniques, leads, yada, yada , yada.
How about changing the 6DN7 to a 6EM7 which has the same pinout but lower output impedance. You would have to change the cathode resistors but little else I imagine. The output impedance is 750 vs 2000 ohms, almost 3 times lower. That will make a big difference.
I am simply stating that there are bigger things to do that fooling with caps, however, for some reason fooling with caps has become very popular perhaps only one step below tube rolling. I think it is obvious in any hobby that people change the things that are easiest to change, while leaving the more difficult yet more important things alone.
I could make you some iron that would blow the socks off the Bottlehead stuff. Look at the pricing of the kit. Break it down by the parts you know the cost of and you will see that the two output chokes and two output transformers can only cost about $25- $50 each. If you have $250 to spend on a Vcap how about the same for a transformer or audio choke?
How about replacing that $2 volume control with a Noble or Alps.
I really don't understand how more resolution caused listener fatigue. I find the more detail I get the more I can turn down the volume and that certainly reduces fatigue. Of course the good recording sound better and the bad recordings sound worse. So go play the bad ones in the car.
