Let's isolate the possible hum source..Your B+ PS is separate from the filament supply....two different issues. In the SE, recall your running your B+ at half power,all the time, irregardless of output. The push-pull will only sap power as levels increase. Two different animals. If an SE is going to hum, say from poor design with a bad ripple content.......it will show up at low levels.
The filament PS, usually is AC driven & can leak thru some hum component.
Using DC regulators for Filaments is a down & dirty problem-solver. Easy......a little "cheating" as your using 'sand' components to quiet things down.
If your B+ supply is 'clean' and your filaments are DC regulated, you probably have grounding issues.
_________________________________________________________Rick..........
The filament PS, usually is AC driven & can leak thru some hum component.
Using DC regulators for Filaments is a down & dirty problem-solver. Easy......a little "cheating" as your using 'sand' components to quiet things down.
If your B+ supply is 'clean' and your filaments are DC regulated, you probably have grounding issues.
_________________________________________________________Rick..........
Filament hum is seldom a problem, unless you're using DH types. I did a project with AC on all the heaters, with the heater winding left floating (to avoid a potential problem of excessive Vhk during power up). The only hum is a 6.0mVp-p, 60Hz, sine wave at the output: barely audible at the woofers unless you stick your ear right up against the speeks. Otherwise, the orange glow from a voltage regulator VT is about the only indication you're powered up.
Filament DC might be a good idea for very low level stages (mV or uV). Otherwise, why bother?
Filament DC might be a good idea for very low level stages (mV or uV). Otherwise, why bother?
Another source of hum is magnetic radiation from transformers.
I changed the heaters to DC and used a dropper resistor on the B+ to a smoothing capacitor and still got hum. I then realised the valve was sat right above a mains transformer. I moved the transformer to the other end of the box and the hum went away.
On the journey I did find an interested fault on the B+ with thediodes giving off switching pulses. I fixed this with a 100nf across the output of the diodes. Realised later i could have used fast switching diodes that recover much better to switching.
I changed the heaters to DC and used a dropper resistor on the B+ to a smoothing capacitor and still got hum. I then realised the valve was sat right above a mains transformer. I moved the transformer to the other end of the box and the hum went away.
On the journey I did find an interested fault on the B+ with thediodes giving off switching pulses. I fixed this with a 100nf across the output of the diodes. Realised later i could have used fast switching diodes that recover much better to switching.
6mVpp? (2mVrms) In my system with the Onkens this would be on the verge of tolerable, so one of the other questions that the OP needs to answer is what sort of efficiency his speakers have and their LF extension. (My system residual noise level is below 500uVrms and I can still hear the hum - enough that I will eventually fix the problem..)
The ac voltage on the heater leads/resistance couples in to the cathode circuit as a function of the voltage difference between a section of cathode and its adjacent heater section. This coupling is minimised if the nominal 'centre-point' of the heater is at 0V (relative to the heater voltage). The geometry and coupling between heater and cathode is not always symmetric, which is why you will typically get a better hum null if you use a tuned humdinger (ie. pot).
Using a DC offset voltage between heater and cathode moves the coupling in to a higher resistance region to the induced noise current flow. This coupling resistance is lowest when both are at 0V difference (a type of anti-diode characteristic). Positive DC offset is typically used, as this benefits heater-cathode voltage limits in most circuits, but negative offset is often equally beneficial to hum reduction.
Ciao, Tim
Using a DC offset voltage between heater and cathode moves the coupling in to a higher resistance region to the induced noise current flow. This coupling resistance is lowest when both are at 0V difference (a type of anti-diode characteristic). Positive DC offset is typically used, as this benefits heater-cathode voltage limits in most circuits, but negative offset is often equally beneficial to hum reduction.
Ciao, Tim
That does nothing to change the fact that it's barely audible, and then only if you put your ear right next to the speaker. Even a couple of feet away, and you can't hear it at all. Good enough.
I am mainly using headphones (HD650 and DT880). Have built the Elekit TU882R as is, and find a level of hum that is audible, not very loud, but once you have it, I can't get it off my mind! Very annoying. I have built a Mapletree Audio Ear+ and I don't notice a hum there. But then, this question is generically a moving forward question. I was looking into building the Raven line/headphone amp, and that is a balanced Class A design, and was wondering would it be worthwhile in that instance, since there is only one tube, and would remove hum from at least 1 source (of many...).
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