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mwhouston: The heater voltage is 6.6 Volts. I will add some resistance to each leg to get it down to 6.3 Volts and let you know what happens. This may be a contributing factor to the hiss. Thanks!
Matt: I rearranged the circuit and moved the output capacitors away from the area above the tube sockets. They are now located next the the power supply capacitors. This gave me more room around the tube sockets, and I was able to arrange the wiring so that the wires and component spacing are now closer to what is normal for my builds. It's still on the tight side, but there are gaps between all components and wiring with nothing touching. It's hard to tell from the photo, but things are spaced from top to bottom as well. I tested the preamp in my system, and found the hiss noticeably reduced, but still objectionable, with exaggerated sibilants. Also, the hiss increases as volume is increased and then goes down at full volume. The reduction in hiss caused by the modification tells me that the close proximity of wiring and components was, and still is, one of the causes of the hiss as you suggested in your last post. Do you feel that given the required tight layout, it would be prudent to add a grid resistor to the 12AU7 cathode follower tube in order to help get rid of oscillation? If so, what value would be advisable for that resistor? I have attached a photo of the modified interior for your consideration.