Well, I've finally finished my CK2III and it's now time to turn my attention to my valve project that started life as a starving student. (Today it runs off dual linear power supplies 24V and 52V, with a CCS plate load 1mA per dual triode). However I get the feeling that it isn't running at it's full potential. I don't listen to music particularly loud so I don't really need the gobs of current that MOSFET output stage on the SSMH provides. I've been toying with the idea of taking this amp full valve and over the past few weeks I've been simulating different topologies (Cathode and White cathode follower output stage, Bijou style futterman etc.) The one that seems to come out on top is the Bijou, ok, I could make a point to point Bijou like the original design but here comes the hard part: I want to keep my budget to 50€ before postage.
This therefore necessitates trying to reuse as many components as possible: In my starving student I currently have:
- 2*12V 15VA toroid
- 2*22V 4VA potted pcb transformer (currently providing the 52V to the valves)
- A mix of switching power supplies from 5 -19 V (possible heater supply)
- A few decent 63V caps (1000uF, 680uF etc. (totals to about 10mF))
- A 2.2mF 100V cap
- Resistors, assorted small caps
- 4 250V 220nF wima coupling caps
- 3 12AU7s
- Small signal transistors
- Hardware : Pot, jacks, heatsinks etc.
I eventually came up with the following design using the the secondaries of the toroid in series and a voltage quadrupler:
This is perfectly capable of outputing 110dB (25mW) into my DT880 250ohm, which is my target (considerably higher volume than I ever listen at). This certainly comes with a price though as even the simulations happy little naive world reports 0.56% THD... The preamp 12AU7 is being run at 4mA with Vgc=2V so that end is happy, but the output stage runs at about 17mA to put the 25mW out. Whilst this is quite high current for 12AUs the plate dissipation is at safe levels due to the low voltage that each tube sees.
If one increases B+ to 250V then simulator THD drops to 0.02% which is rather attractive. There are however a couple of major issues here:
- How do I obtain this voltage (power transformers cost a lot of money and my little toroid can't sustain this circuit with a voltage octupler between them).
- The 12AUs will end up running way too hot in the output stage as far as I can tell (I'd like to keep plate dissipation below 2.2W if possible).
I would therefore like to ask you folks for a bit of advice one what the best way to proceed with my budget are.
Ideas I have considered (to get the ball rolling):
- Buying a bigger transformer (they seem so expensive, so this will be a large chunk of the budget). Also one can't forget the cost of filtering a bigger higher voltage transformer...
- Getting some 6n6Pis for the output (they can be run hotter but they might really need more voltage so the transformer becomes an issue again)
- Giving up, since I've gone round in circles so many times
I'd love to hear any ideas or suggestions that you may have, no matter how odd, feel free to chuck them out here so that I can consider them.
I'm certainly not anti-solid-state as the phase splitter above will attest, I just want a nice tubey sound from this amp.