Tease? I for one, didn't need any more convincing. The DHTs tested were all like microphones. I do suspect the smaller DHTs have these microphonic effects the most, with their hair thin filaments. That "magic" CX340 tube mentioned earlier must be the wimpiest UX base tube ever made with 0.5 mA plate current.
I have some 6C4C tubes now for P-P class A power outputs, so an output minus input test might get done sometime, just for kicks. But I'm more interested now in some other schemes. I'd like to have a switch on the Amp to turn the "magic" sound on or off or adjustable. The acoustic microphone reverb (mentioned above ) could be one way. One could use a feedback tube emulation scheme to make a Sweep tube emulate a DHT ( I like the $3 1E7G DHP, and the $1 21HB5A Sweep in R-Schade mode, then there's 4P1L, 12HL7 and a few more ). Or one could just switch in the DHT for the front end tube.
I have some 6C4C tubes now for P-P class A power outputs, so an output minus input test might get done sometime, just for kicks. But I'm more interested now in some other schemes. I'd like to have a switch on the Amp to turn the "magic" sound on or off or adjustable. The acoustic microphone reverb (mentioned above ) could be one way. One could use a feedback tube emulation scheme to make a Sweep tube emulate a DHT ( I like the $3 1E7G DHP, and the $1 21HB5A Sweep in R-Schade mode, then there's 4P1L, 12HL7 and a few more ). Or one could just switch in the DHT for the front end tube.
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I was looking at a 1J6 dual triode DHT tube here and contemplating some acoustic tests I could try, and realized that shaking the tube at a right angle to the plate could make the filament shake sideways so that it would swing toward the grid twice for every acoustic cycle of drive. Swinging the filament between the opposite sides of the grid, this would make for 2nd harmonic variations in plate current.
In the previous tests from year 2010, a tracking oscillator and analyzer were used ( both a Tek 7L5 spectrum analyzer with Option 25 tracking gen and then using Audio Tester software ). These both gave similar results but the Audio Tester software got the graphs onto the computer easier. But both of them only look at the fundamental generator frequency when scanning. Since I just found a case where 2nd harmonic of the audio would be expected to be generated, I decided to repeat a 1J6 test with a scope and oscillator (and freq. counter ), so I could find harmonics of the audio freq. being generated. To my surprise, 2nd, 3rd and 4th harmonics showed up a various specific freqs. And the fundamental and also some sub-harmonics at certain freqs. These were generally only narrowly around specific freqs for all of them.
The test setup was +230V B+ thru a 15K resistor at 7 mA thru the tube as well. Near 0V on grid1. 2V on the heater. A cap to the scope probe from the plate, scope set for 50mV/div. Top trace is the signal generator sine wave signal (1V/div ) to the small 4" 8 Ohm speaker, and bottom trace is the AC 50mV/div plate voltage thru the 1.5 uF cap. For the 2nd harmonic shown, one side of the tube was at 274 Hz (audio drive) and the other side of the dual triode did similar at 253 Hz. The tube was resting on a cotton ball 1/2 inch above the speaker. I was looking for the 2nd harmonic case, but I was surprised to find all the other assorted harmonics too. All pretty narrow freq.
In the previous tests from year 2010, a tracking oscillator and analyzer were used ( both a Tek 7L5 spectrum analyzer with Option 25 tracking gen and then using Audio Tester software ). These both gave similar results but the Audio Tester software got the graphs onto the computer easier. But both of them only look at the fundamental generator frequency when scanning. Since I just found a case where 2nd harmonic of the audio would be expected to be generated, I decided to repeat a 1J6 test with a scope and oscillator (and freq. counter ), so I could find harmonics of the audio freq. being generated. To my surprise, 2nd, 3rd and 4th harmonics showed up a various specific freqs. And the fundamental and also some sub-harmonics at certain freqs. These were generally only narrowly around specific freqs for all of them.
The test setup was +230V B+ thru a 15K resistor at 7 mA thru the tube as well. Near 0V on grid1. 2V on the heater. A cap to the scope probe from the plate, scope set for 50mV/div. Top trace is the signal generator sine wave signal (1V/div ) to the small 4" 8 Ohm speaker, and bottom trace is the AC 50mV/div plate voltage thru the 1.5 uF cap. For the 2nd harmonic shown, one side of the tube was at 274 Hz (audio drive) and the other side of the dual triode did similar at 253 Hz. The tube was resting on a cotton ball 1/2 inch above the speaker. I was looking for the 2nd harmonic case, but I was surprised to find all the other assorted harmonics too. All pretty narrow freq.
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Oh, I also tried some neg. V on grid1 to see if that had much effect on the second harmonic, but it just attenuated most everything else due to lower plate current. The 2nd harmonic DID peak at -2.5V on the grid, so the grid V and filament proximity to it does activate the 2nd H influence as expected, but drops off with more neg. grid1 V due to decreasing plate current. So as expected.
I also tried +V on grid1 and the 2nd harmonic gets bigger until past the peak at +3.9V where plate saturation was probably setting in.
Hmm, I should measure the freqs. where I saw sub harmonics ( like 1/2, 1/3 ) to see if they are ( at 2x or 3x ) the same freqs. that caused the higher harmonics.
I also tried +V on grid1 and the 2nd harmonic gets bigger until past the peak at +3.9V where plate saturation was probably setting in.
Hmm, I should measure the freqs. where I saw sub harmonics ( like 1/2, 1/3 ) to see if they are ( at 2x or 3x ) the same freqs. that caused the higher harmonics.
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I lost all forum when moved to Texas from California January 1, 2020.
I kept my server in my barn, decided to move to cloud, archived and scp-ed there all data, but the archive got corrupted. 🙁
My bad, I should put that DL-360 with RAID in the container...