Good afternoon Analog-forum,
just to present my tube preamp.
It's a mu-stage/Kimmel-stage based on a Philips MRO PT8 triode as the voltage amplifier and a Amperex 8608 penthode as the "current source".
These chassis started their life as EL82-monoblocks which were good but not great. So one monoblock became the 6C45 PP-SPUD amp (powering the midrange in my tri-amped system) and one monoblock became the new preamp. The metalwork and power-supply part was simply too good to through away. These are proper combined/seperated audio-components: the power supply is in a separate housing but the connections are not channeled through connectors but through hollow metal casing-parts. very rigid and no chance of external influence; let alone the possibility of corroding contacts....
Pictures of the 6C45 poweramp included next to the PT8 preamp.
These Philips MRO PT8 are "repeater-triodes"; these items were made/used throughout the world to make international communication possible. Nearly every country/manufacturer made these triodes (repeatibility; stabilty, trustworthyness, etc) were key. There are many many great choises to choose from and besides the filament amperage (as they are nearly all 4 volt) only the sockets can be different. But with some covertor sockets i can make use of UV4 options as well as UX4, Europe B4/5, P-sidecontacts, etc.
Of course there are differences between the tubes and i do have my favourites but currently i can pick-'n-choose the best deal for the money. Not bad for 80 years old triodes....
Of note is the volume control. This is not the standard (acceptable) blue ALPS but a combination of stepped attenuator sourced from (Rohde & Schwarz) measuring equipment. These are 50 ohm affair so the audio-signal has a somewhat different path to follow: input transformer - Kimmel-stage (PT8 and the like) - step down interstage to 50 ohm - stepper 1 (9 x 1 dB + 5 x 0,2 dB) - stepper 2 (6 x 10 dB) - output transformer.
So my output impendance will be stable.
Regards, Reinout