sampleaccurate wrote:
Not to interrupt your design discussion but hopefully add to it, one general comment and question about the 300B:
After the cathodes have warmed up and the B+ is applied you can hear the tubes making noise as they thermally expand, and this noise also comes through the speakers. This happens with my Electro-Harmonix 300B Gold tubes at least. My KT88 indirectly heated tubes don't exhibit this behavior.
I'm just wondering if anyone else has the same experience with directly heated power triodes. I'm also trying to figure out what exactly inside the tube it is that's making the noise. Thermal expansion of the plate, of the cathode filament wires rubbing against the supports, etc?
Perhaps better quality tubes don't do this. It stops after 30 seconds or so and is dead silent after that.
sampleaccurate, I would say your assumtion is correct in that you are hearing the expansion of the elements. More specifically, I would suspect the plate. A suggestion I have would be to add a thermal delay-relay to your HV (ie. a 6NO45T - that is the letter O and not a zero). This is a tube that contains a normaly open set of contacts mounted on a bi-metal element and being surrounded by a heating element. Supplied by a 6.3v heater circuit, this tube will have a 45 second delay on closing the contact. It draws a mere 300ma (0.3A) on the heater. By integrating this in to your HV ps section, you can hold off that power for 45 seconds. This should be more than ample to avoid the expansion tings. I would hope that for the lavish cost of the Chinese 300Bs being used in this thread that the manufacturer has taken that consideration. I would hope these tubes would output gold plated sonics at that price.
PS. The delay tube comes in several times and heater voltages. Just like most other VTs, the first number is the heater voltage.The letters are either NO (Normally Open) or NC (Normally Closed) and refer to contact set-up, then the last set of numbers is the amount of delay in seconds. The "T" indicates this is for a 9pin noval socket. No letter means octal pinning. So, the 6NO45T is a 9pin noval tube for 6.3v heaters, Normally Open contacting with 45second delay. Do a Google search and you should be able to find some sources.
http://WWW.VACUUMTUBESINC.COM sells these for about $25.