Rerurn to Romy the Cat's Site

floobydust wrote:

On a second note, you can also modify this for both adjustable AC and DC balance on seperate potentiometers. Both potentiometers would be in parallel with the filament (and padded) but you use the wipers independantly, i.e., one has the bypass cap attached and the other has the cathode bias resistor attached.

I was thinking about making the adjustable AC. I have bought 50V high quality wirewound very sexy mini- rheostat but other advisers of mine were very skeptical about this idea. In fact I have a number of people who question the validity of extra series resistance on the DHT filaments. I can see their point, even though I personally do not make experiments with validating the filaments series resistance’s impact to sound.  However, the “no resistor” position does make a common sense to me.  It is known that I am not treat my amp as some kind of festivity of electrical design – I know very little about it and many of my actions at this point, prior to listening, comes from exercising a common sense.  So, thinking about keeping or dropping the filament fine adjustment with extra series resistance I inclined to feel what Paul expressed when he suggest that the bar has already been set too high (at least what I hope) and saving a few pennies here or there does not make sense. So, I have ordered the “right” dedicated transformer specifically for the requirements that I have and the 5-pin-connectors that would service another run of a separate cable from PS to amp with AC for DH filaments.

floobydust wrote:

No easy drawing program available this second (traveling for 3 weeks) but I can send you a simple schematic in PDF if you like via email. In any case, to adapt what you have already have to the split biasing, just remove the cathode bias resistor from the hum balance wiper and connect it to the filament center-tap. This results in a fixed DC balance against the DHT filament and the balance pot now provides an adjustable AC balance only. In many cases, I've gotten better results in hum nulling this way, but it still requires good tubes.

In my new configuration the filament center-tap will be on another chassis, in PS side. The way how it organized now in my amp is that the negative of the Obbligato Cap on the B+ side of output transformer, the bottom of cathode resistor and the top of the Cube 950 cap from the cathode resistor and the ground of the OPT’s secondary are the same physical point on my ground buss thermal. I kind of like that the output tube is locked within the singular referent to ground. No matter that the “ground” with this type of cathode bias is kind of virtual category. I do not want the cathode restore to “drive” 3 feet of cable unit it “see” the return path (which will be the center-tap in your proposal). I am not sure that I am right but I do see a login in how I see it.

Rerurn to Romy the Cat's Site