I have seen several low wattage amplifier designs and I am working on one of my own. It is based on the Champ but will use a 6CM6 output tube into a 5500 OPT with a target of 1 watt output. Being mostly familiar with microcontrollers I have never designed a tube amp from the ground up, I do understand the basic concepts though. My question to the experts is how do I change the output power. I know changing the OPT impedance would change the output power. Changing the B+ voltage and bias would as well correct? I am trying to understand the tube plate curves and how to establish this information but could use some guidance. What I came up with for a load line shows a B+ of 180VDC with a bias of 19mA should give me 1W output with a 5.5K OPT.
Simple ballpark guidelines-
higher voltage, lower current, higher load impedance= lower distortion, lower output.
lower voltage, higher current, lower load impedance= higher output, higher distortion.
Making the grid less negative with respect to the cathode will draw more current through the tube.
For SE something in the middle with higher current at moderate voltage is a good compromise, since you cannot ever swing more than the idle current without going into some unusual methods of drive. Just watch the max dissipation.
higher voltage, lower current, higher load impedance= lower distortion, lower output.
lower voltage, higher current, lower load impedance= higher output, higher distortion.
Making the grid less negative with respect to the cathode will draw more current through the tube.
For SE something in the middle with higher current at moderate voltage is a good compromise, since you cannot ever swing more than the idle current without going into some unusual methods of drive. Just watch the max dissipation.
Conceptually:
https://londonpower.com/power-scaling-faq/Lower supply voltage, and trim bias and signal to suit. (You could change impedance but that's too hard, and goes sour if taken too far.)
My 6AU6 amp with 180V supply and 15+k loading makes maybe a third Watt, in a Champ-like topology. (It happened the tube suited an available audio transformer.)
Note that 1 Watt is not much less than a Champ's 6 Watts. Different but you still wake the baby mighty quick.
https://londonpower.com/power-scaling-faq/Lower supply voltage, and trim bias and signal to suit. (You could change impedance but that's too hard, and goes sour if taken too far.)
My 6AU6 amp with 180V supply and 15+k loading makes maybe a third Watt, in a Champ-like topology. (It happened the tube suited an available audio transformer.)
Note that 1 Watt is not much less than a Champ's 6 Watts. Different but you still wake the baby mighty quick.
If you are talking about a Fender Champ, so a guitar amp, and you want just 1 Wrms, instead of the 6CM6 you can use a 12AT(/U)7 with both triodes paralleled. It will bring you there in a cheaper a simpler way. You can use your 5.5 kOhm as a 11 kOhm OPT at around 300-350V B+.
The 6CM6 can be had for only five bucks and data sheet says it’s similar to 6V6. I’m sure that was the motivation. If you’re a real cheapskate there are also plenty of tri/pents in the $3-5 range where the pentode is targeted as a video output with similar current capability to the CM6 or CL6, along with a free 40 mu triode. That gives you the entire power section in one bottle.
Thanks for all of the feedback. It sounds like I can just drop the B+ and everything will lower including the output power. At least according to the power scaling devices. What I really wanted to understand better were the curves for the specific tube and how to calculate the exact B+ voltage, cathode resistor and OPT impedance. I am building a DC-DC supply for this amp, a flyback, which will take 12VDC and provide ~300VDC for the B+. I might actually include a couple of LR8 regulators for the pre-amp and grid voltages. All of the output voltages would be adjustable as well.