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Bringing this thread back up as I've been working towards completing my first tube amp and was looking to get some input from those with more experience.
My amp design is two 12AT7 triode stages for voltage gain driving a 12DQ6B pentode for the output, class A. I am currently cathode biasing the pentode with around 440 ohms at 120v B+/ 770 ohms for 240v B+ and a 100v 470uF bypass capacitor. This gives me about 50-75mA average current on the pentode. (more on the dual voltages below)
Trying to keep the budget minimal and the circuitry simple, I want to keep all stages running on the same B+ supply. I originally wanted everything to run on rectified mains via an isolation transformer (cheap), but I ran into issues with the output transformer.
I had hoped to use a 15W 70v line matching transformer for PA systems but in my testing the impedance is too low, low-end frequency response is meh and I can't seem to drive it any harder than about 70vac on the primary (makes sense right
)
So I went to option #2, I had salvaged the guts of an all tube reel-to-reel tape recorder at the junkyard so I tried the output transformers from that. Audio performance was much better but power output still left a fair bit to be desired at 120v B+.
So using a bench supply I bumped up the plates to 240v and the cathode resistors to 770 ohms. Now I'm getting a bit over 2.5 watts RMS into a 4 ohm load and I'm thinking I've about hit my limit for this design. Any more B+ and I'll be hitting the plate dissipation limit for my pentode (18W).
So I suppose my main question is do these numbers sound reasonable for my junkpile amp? If so I will start removing my ********* mess of prototyping and get it finished
Teaser:
My amp design is two 12AT7 triode stages for voltage gain driving a 12DQ6B pentode for the output, class A. I am currently cathode biasing the pentode with around 440 ohms at 120v B+/ 770 ohms for 240v B+ and a 100v 470uF bypass capacitor. This gives me about 50-75mA average current on the pentode. (more on the dual voltages below)
Trying to keep the budget minimal and the circuitry simple, I want to keep all stages running on the same B+ supply. I originally wanted everything to run on rectified mains via an isolation transformer (cheap), but I ran into issues with the output transformer.
I had hoped to use a 15W 70v line matching transformer for PA systems but in my testing the impedance is too low, low-end frequency response is meh and I can't seem to drive it any harder than about 70vac on the primary (makes sense right
)So I went to option #2, I had salvaged the guts of an all tube reel-to-reel tape recorder at the junkyard so I tried the output transformers from that. Audio performance was much better but power output still left a fair bit to be desired at 120v B+.
So using a bench supply I bumped up the plates to 240v and the cathode resistors to 770 ohms. Now I'm getting a bit over 2.5 watts RMS into a 4 ohm load and I'm thinking I've about hit my limit for this design. Any more B+ and I'll be hitting the plate dissipation limit for my pentode (18W).
So I suppose my main question is do these numbers sound reasonable for my junkpile amp? If so I will start removing my ********* mess of prototyping and get it finished
Teaser:
