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Old 22nd May 2011, 04:33 AM   #1
eeyore is offline eeyore  Australia
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Default Reducing Filment Hum in Tubes

Just wondering if a push pull stage will reduce the hum caused by the filament or cathode and the B+? I am finding that hum is quite annoying, and trying to reduce it, source by source.
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Old 22nd May 2011, 04:50 AM   #2
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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.

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Old 22nd May 2011, 09:43 AM   #3
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If you are using DHT tubes, step one would be to start using Coleman type heater supplies.

Then there's of course B+ supplies and grounding as per above.

Right now I'm listening to music with headphones thru DHT tubes, and there is no hum.
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Old 22nd May 2011, 09:48 AM   #4
Gyuri is offline Gyuri  Hungary
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In my Borbely Hybrid Tube-FET Pre caused hum that I was not wrapped in heating power lines.
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Old 22nd May 2011, 01:36 PM   #5
roline is offline roline  United States
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In my latest amp, I raised the heater CT voltage to 50VDC.
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Old 22nd May 2011, 02:22 PM   #6
DF96 is offline DF96  England
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A little more description of what you have, including a circuit diagram, might help. P-P only eliminates hum coming from the P-P stage itself. It can't improve hum from earlier stages.
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Old 22nd May 2011, 10:46 PM   #7
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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?
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Old 22nd May 2011, 11:01 PM   #8
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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.
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Old 23rd May 2011, 12:28 AM   #9
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by Miles Prower View Post
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?
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..)
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Old 23rd May 2011, 11:01 AM   #10
DF96 is offline DF96  England
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Quote:
Originally Posted by Miles Prower
heater winding left floating
This can cause problems, including hum. The heater circuit needs a DC reference; ground is the simplest although not always the best.
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