NewYorkDave
Well-known member
My on-again, off-again attempt to build a tube mixer continues to yield some useful (to me, anyway) bits. I had some rare free time the other day, so I breadboarded a line amp circuit.
schematic (10kB GIF)
This can put out a pretty hefty signal, and can even drive 600 ohms directly with reasonably good performance. (Here's a good use for all those UTC 600:600 +30dBM transformers you have laying around :wink: ).
Into 600 ohms, distortion is pretty low right up to maximum output, at which point the output stage goes into cutoff and the THD soars. Into a higher-impedance "bridging" load, it can swing up to +35dBU before the output even starts to flatten. Through a 2:1 stepdown transformer, it can pump +27dBM into 600 before clipping.
Open-loop gain is about 40dB and there's 20dB of negative feedback as shown, giving a closed-loop gain of 20dB. The 22k feedback resistor could be replaced with 56k to give a loop gain of 26dB, but the THD figure suffers somewhat. I would not recommend using less feedback than this.
The THD at all levels is mostly second harmonic. When working into a bridging load, at +30dBU, there were no measurable harmonics past 5th. I'm defining "measurable" as being above the noise floor, which on this breadboard was about -88dB (unshielded wiring floating all over the place). Higher-order harmonics appear at measurable levels when working near +25dBM into 600 ohms. Interestingly, when I added the grid stopper to the lower half of the 12BH7, the maximum output increased, but so did the proportion of odd harmonics.
These measurements were made with resistive loads. Through a transformer, the percentage of odd harmonics increased somewhat (probably because I was using a cheap transformer). The figures include whatever noise and distortion my old HP 200 tube oscillator was contributing. Individual harmonics were measured with an HP wave analyzer, from the fundamental all the way up to the highest harmonic that would produce any sort of reading above the noise floor.
schematic (10kB GIF)
This can put out a pretty hefty signal, and can even drive 600 ohms directly with reasonably good performance. (Here's a good use for all those UTC 600:600 +30dBM transformers you have laying around :wink: ).
Into 600 ohms, distortion is pretty low right up to maximum output, at which point the output stage goes into cutoff and the THD soars. Into a higher-impedance "bridging" load, it can swing up to +35dBU before the output even starts to flatten. Through a 2:1 stepdown transformer, it can pump +27dBM into 600 before clipping.
Open-loop gain is about 40dB and there's 20dB of negative feedback as shown, giving a closed-loop gain of 20dB. The 22k feedback resistor could be replaced with 56k to give a loop gain of 26dB, but the THD figure suffers somewhat. I would not recommend using less feedback than this.
The THD at all levels is mostly second harmonic. When working into a bridging load, at +30dBU, there were no measurable harmonics past 5th. I'm defining "measurable" as being above the noise floor, which on this breadboard was about -88dB (unshielded wiring floating all over the place). Higher-order harmonics appear at measurable levels when working near +25dBM into 600 ohms. Interestingly, when I added the grid stopper to the lower half of the 12BH7, the maximum output increased, but so did the proportion of odd harmonics.
These measurements were made with resistive loads. Through a transformer, the percentage of odd harmonics increased somewhat (probably because I was using a cheap transformer). The figures include whatever noise and distortion my old HP 200 tube oscillator was contributing. Individual harmonics were measured with an HP wave analyzer, from the fundamental all the way up to the highest harmonic that would produce any sort of reading above the noise floor.