How closely should the HV supplies be timed?
Obviously, I don't want to leave the voltage sitting on a cold tube too long, but...
26 pre hum 120Hz in Sic cathode bias
Hi guys,this is my problem with new 26 pre with clasic cathode bias but instead of resistor I have used 10 pieces Sic diode in series.I have before the battery grid bias with 2032 cell battery 3 pieces on the grid with 10nF cap silver mica and 10Mohm resistor parallel to battery.I have from start on no hum no hiss nothing,just totaly silent pre with Rod Colemans regs for filament heating.Now I have buy like I say Sic schotky diodes to try clasic bias with this like bartola with 01a tube and I become verry strong 120hz hum when I conect the string of 10 diodes from positive side of Rods regs on tube socket to ground.I have try to rewire the grounding in this pre but no luck with this.What have I missed by using Sic in clasic bias why there is so loud 120hz hum.Iv used Sic before and have never some problems in cathode,but this was always in Idht tubes.I try to put Sic diodes on negative side from Rod regs to ground and the problem is the same.Have anyone some idea what could be wrong??��
The preamp design is with Ale Moglias Mu folower aka gyrator boards,psu is clclcrc with all clarity tc mkp caps,output taken from MU on Mu folower thru 0,47uf ft3 cap.Grounding is from Dhtrob with signal ground and piwer ground conected on star ground with chasis and earth ground.
Hi guys,this is my problem with new 26 pre with clasic cathode bias but instead of resistor I have used 10 pieces Sic diode in series.I have before the battery grid bias with 2032 cell battery 3 pieces on the grid with 10nF cap silver mica and 10Mohm resistor parallel to battery.I have from start on no hum no hiss nothing,just totaly silent pre with Rod Colemans regs for filament heating.Now I have buy like I say Sic schotky diodes to try clasic bias with this like bartola with 01a tube and I become verry strong 120hz hum when I conect the string of 10 diodes from positive side of Rods regs on tube socket to ground.I have try to rewire the grounding in this pre but no luck with this.What have I missed by using Sic in clasic bias why there is so loud 120hz hum.Iv used Sic before and have never some problems in cathode,but this was always in Idht tubes.I try to put Sic diodes on negative side from Rod regs to ground and the problem is the same.Have anyone some idea what could be wrong??��
The preamp design is with Ale Moglias Mu folower aka gyrator boards,psu is clclcrc with all clarity tc mkp caps,output taken from MU on Mu folower thru 0,47uf ft3 cap.Grounding is from Dhtrob with signal ground and piwer ground conected on star ground with chasis and earth ground.
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Should be OK so long as the preamp HV is not later than the power amp's HV.
The grid voltage (ECC83) should not be above the normal working voltage, or the startup will be more stressful for it.
What's the voltage across the diode-string ?
And what's the input voltage to the filament regulator ?
Hi Rod,
....the voltage around Sic diode string is 8.05V and the input voltage from your raw dc supply is 7.22V on heater I have 1.495V.Did I make something wrong??Iv trayed before to put 1.5Kohm resistor with cap for bias in there and that worked fine.��
Like I say this is not filament bias but clasic bias but instead of resistor I would try Sic diode in there.
....the voltage around Sic diode string is 8.05V and the input voltage from your raw dc supply is 7.22V on heater I have 1.495V.Did I make something wrong??Iv trayed before to put 1.5Kohm resistor with cap for bias in there and that worked fine.��
Like I say this is not filament bias but clasic bias but instead of resistor I would try Sic diode in there.
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Hi Rod,must try the cap across I just now again rewired to battery grid bias and the pre totaly silence without hum or hiss.I have put the 10 diodes on some testing boards 10 pieces in series.I like the sound with diodes verry much,only the 120hz hum is the problem.I can hear this hum about 3meter from my speakers thats what is the problem.
Have you not changed anything else other than the SiC diodes instead of the cathode resistor? Very strange.
I don’t use more than 6 diodes myself as never liked the sound of too many of them in the array. Andy Evans have experimented with many diodes in the array with similar results.
However, diodes are very quiet and can’t see why they will get you that noise themselves. Never had to bypass them, but would be a good test to make to see where the noise is coming from.
Can you take some pictures of your layout and attach a diagram?
Thanks
Ale
I don’t use more than 6 diodes myself as never liked the sound of too many of them in the array. Andy Evans have experimented with many diodes in the array with similar results.
However, diodes are very quiet and can’t see why they will get you that noise themselves. Never had to bypass them, but would be a good test to make to see where the noise is coming from.
Can you take some pictures of your layout and attach a diagram?
Thanks
Ale
Hi Ale,like I say before this pre was made with your gyrators on anode and Rods regs for filament,for some time now Iv used battery bias on grid and the Rod reg was used for heating filament and negative side of this was conected directly to signal ground.This worked fine from first moment on.Than I have idea to tray Sic for bias and so the problem was this hum.I will try to put some capacitor across diodes to see if it will be better.
I didn't have a problem with using 10 or even 12 of the SIC diodes. Not sure what the issue is. Have you also tried filament bias instead of SIC diodes?
I went over to using a 4P1L in the end when I changed my output stage from PSE4P1L to SE 300b. I needed more gain so I've been using a 5x opamp stage in front of the 4P1L, and getting good results with OPA134s and NE5534s. Since the 4P1L can be used at 30mA it has good drive. I don't use a preamp - my AK4490 DAC goes straight into the amp. The 26 is nice for preamps, an old favourite.
I went over to using a 4P1L in the end when I changed my output stage from PSE4P1L to SE 300b. I needed more gain so I've been using a 5x opamp stage in front of the 4P1L, and getting good results with OPA134s and NE5534s. Since the 4P1L can be used at 30mA it has good drive. I don't use a preamp - my AK4490 DAC goes straight into the amp. The 26 is nice for preamps, an old favourite.
So Rod and Ale,first schematic is with no hum and hiss,second with Sic diodes with hum 120hz but no hiss.I must buy some good resistor that can handle some wattage and I will try filament bias in the future also.But for now I would try this Sic option.Ale plese be so nice and tell me how you made grounding in your oreamps.You mentioned some 3 point grounds like input stage,filament return and output stage.To have this 3 points separate that hum is not come in the output stage from filament how you make this ground scheme??
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Hi Andy,thanks for answer.Im using my 26 pre with Pass aleph 5 diy power amp and it is nice combination.For my speaker I dont need many power so with this setup Im happy.My speakers are alpair 10m3 with Scott Moose and David Lugos plans and for now in some mdf prototyp encolure this are truly superb speakers.
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Hi,
IMHO the problem is the lack of correct (DC:0) grid potential.
I never use any tube input without resistor from grid to tube cathode "ground" point.
Input potentiometer is the volume control, not biasing element.
Yes.
Try to use 100k-200k.
The signal wire between pot and preamp input is the potential hum source, if your grounding scheme is not correct.
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