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PostPosted: 02 May 2020, 13:53 
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Hi,
Recently I built a phono tube pre amp using a commercially available kit, which did not provide a power supply. Thus, I used an Edcor transformer that I had which provides 6.3VAC for the heaters.
However, the kit requires 12.6VDC for the heaters. Thus, after doing some research, I designed the following voltage doubler circuit to drive three 12AX7 tubes. It provides 12VDC pulling 0.5A. Each heater requires 150ma and I have placed a 0.1UF cap across each heater for additional filtering.
Unfortunately it has a lot of noise/hum.
I do not have an oscilloscope and could not verify, but figured that the problem could be ripple.
After disconnecting the heaters from the supply and connecting an external power supply producing 12VDC the noise/hum is barely audible when you put your hear to the speakers.
Any recommendations on how to fix this?
I thought that the 2200UF caps would provide a clean DC signal.
Any help is appreciated. Thanks,

Danny


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PostPosted: 02 May 2020, 16:44 
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You need energy storage after the 3.3Ω resistor.

I would recommend first about 1000µf after the resistor. This will yield about another 11dB of filtering which you are currently missing. A low voltage 10,000µf cap will yield about 28dB of filtering if more is needed.

Do you know what your heater voltage ripple is currently?

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PostPosted: 03 May 2020, 01:51 
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Hi,

+1 yes more filtering after the 3.3 ohm.
NB you will need to recheck your voltage after fitting more filtering.

Regarding the 2200uF what you are trying to do after the diodes is create voltage the higher the capacitance the greater the charge pulse.
No matter what size capacitors you fit after the diodes you will always get ripple so without a R/C filter or L/C filter you have little chance of removing the ripple.

The higher the cap size after the diodes the higher the charge time, the longer the charge time the longer it takes to get up to supply voltage.
So its not always higher C is better its just finding an optimum value.

NB your DC supply will need to be referenced either to a B+ voltage lift or a false centre tap across the supply with ground ref or voltage lift.
It depends if the tubes are "stacked" or where a cathode is at higher voltage above ground.
If you don't it will hum.

NB regards your 0.1uF across the heater but you haven't snubbed the diodes<<why?

Regards
M. Gregg

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PostPosted: 06 May 2020, 15:13 
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Matt & Gregg,

Thanks for the replies. I think I will place a 10,000uf cap after the 3.3ohm resistor.
Not familiar with how to snub the diodes. I assume this is done to filter transients.
Do I put a 0.1uf ceramic cap across each diode?

Thanks again,

Danny


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PostPosted: 06 May 2020, 21:19 
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M. Gregg wrote:
NB your DC supply will need to be referenced either to a B+ voltage lift or a false centre tap across the supply with ground ref or voltage lift.
It depends if the tubes are "stacked" or where a cathode is at higher voltage above ground.
If you don't it will hum.


This is an important step to removing you hum/buzz, create a "vertual" center tap on the 6.3v winding of the transformer using 2x 100 ohm 0.5watt resistors, one resistor on each side of the winding to ground. I personally have forgotten to do this many times in the past, with AC heaters you will get a lower frequency hum (60hz I believe) with DC heaters it will be a distinctive "buzz" around 120hz? plus harmonics injected into the supply. It can sound like you have a bad ground when you turn your volume up but it is actually noise being injected through the cathode into the grid it seems... some one else could probably elaborate or correct me on that.

Put 0.1uF caps across (parallel) to the diodes like suggested as well as 0.1uF caps across each electrolytic and you should be :up:

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