Okay, new circle startup... so a new amp project aimed at those single driver systems.
So, who doesn't want something bigger than 2-watts using a real directly-heated triode output stage, has very wide power bandwidth and doesn't require a second mortgage for NOS DHTs?
Okay, for the one or two left... it's a 4-watt design, runs in Class A2 mode and at reasonable plate supply voltages. I just recently completed this design and there are plans to implement a pair in mono-block format.
The output tube is a 3C24 which can be purchased NOS for under $20 each. It uses a common 4-pin base like a 45, 2A3, 300B, etc. but also requires heatsink/contacts for the plate and grid connections which are brought out thru the glass envelope via tungsten wires. A 6FQ7 (similar to a 6SN7 in a tall 9-pin package) acts as the input amplifier and dynamic follower. The circuit is straightforward and is shown below:
Overall gain results in 750mv input for 1-watt output. A simple DC filament supply for the 3C24 yields a signal-to-noise better than 80dB (referenced to 1-watt) and the power bandwidth at 4-watts is within 1dB from 50Hz to 50KHz using a Hashimoto H-20-3.5U output. Distortion is about 1.2% at 1-watt output and increases as power output increases up to 3.8% distortion at 4-watts output.
Here's a pic of the early prototype:
Regards, KM
