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After reading what was posted on Tube Lab's webpages about heating DHT filaments with 60 cycle AC, I decided to look at my SE 45 breadboard circuit.
I put a 1KHz sine wave into the input of the amp and looked for 940 and 1060Hz sidebands coming up around the test signal.
The sidebands were there , about -80dB down from the test signal.
Tube Lab said this creates some sort of IMD, even if the sidebands are too low to be heard by the ears.
How this effects the overall subjective music presentation is up to debate as to positive or negative result.
Follow Ups:
I am no EE (obviously!);can you help me interpret the results from what you have provided?What level of 50Hz hum do you currently have on your breadboard? Do/how much the sidebands change in level in when the hum level changes?
Regards,
91.
Edits: 06/18/15
My AC line operates at 60 cycles, not 50.The measurements on the graph were taken at 1.15W power output.
The 60 cycle hum was down -76.6dB from the test tone.
When I changed power output levels to .25W , the 60 cycle hum increased to -70dB down from the test tone.
The 940 and 1060 sidebands went down to -90dB from the test tone.
Why that happened I am not sure, but I can't measure any mVAC at the OPT secondary with the input jacks shorted on my DMM.
I didn't take any IMD measurements.
Edits: 06/18/15
Oh, of course 60 Hz!
What is your AC filament set-up? The amplitude of the side bands should be related to the ac filament noise - it Is important to know the 60 Hz noise level... And maybe how far down the side bands are from a constant signal level for different levels of 60 Hz noise and vice versa.
Regards,
91.
I think the modulating power supply is good for some additional IMD as well.
I recently switched to a C4S 12AX7 driver stage running at 2.25mA.
At 1.15W power output measured across the OPT secondary, the driver stage produces .56% THD.
Voltage gain = 60
Bandwidth -3dB @ 34.2KHz
AC DHT filaments are definitely adding something to the music signal.
I could see lots of sidebands coming up around various harmonics just above the noise floor.
Perhaps there are ways to clean up the AC coming into the filaments I could try before resorting to DC filaments again.
Now perhaps take some measurements with a voltage source and with a current source for the DC.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
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