Hello everyone..
I recently acquired two mono-block tube amplifiers. When these were manufactured, they were purported to be very high-end amplifiers. However there are some issues with line noise (120cps full-wave rectifier hum) which ruins any other benefits that this amp might have had for being audiophile quality, and I am trying to address this... I have replaced all electrolytics and so we know it's not a bad cap, so we won't waste our time on that theory..
I see three major factors that seem to be at the root cause, each factor compounding the other.
First, the manufacturer should have used a good CLC pi filter, but instead used a single 1650uf capacitor @800v (actually two 3300uf caps in series, rated @ 400v each, with 220k bleeder resistors) which serves as plate supply for the 807 output tubes.
That B+ rail feeds into additional resistors to drop voltage for feeding the 807 screen grids, and has 330uf/400v caps on the low side of the resistor, which theoretically should further smooth this lower voltage rail, but it's the lack of a pi filter preceeding the 807 B+ supply that I believe to be our first contributing factor.
The second contributing factor is that this amp has four solid state rectifiers (discrete 1n4007 junction diodes for B+, a separate rectifier for bias supply, a separate transformer and rectifier for 12AT7 B+ supply, and then a large square rectifier for 6.3v filiment supply.) .. These all share the same ground rail, and none of them have snubbers, nor are they followed by CLC pi filters (again just single caps with nothing for the ripple voltage to fall across).
The third contributing factor and the reason I am reaching out to this forum for help, is that someone who owned these amps, at some point removed the original supply transformers and substituted them with much heavier-duty supply transformers with a lot more available current, which means that they have a considerably lower secondary winding impedance, and this will factor into the re-design of the power supply filters and I want your input.
My multi-pronged attack for eliminating the hum, is 1) Place 0.22ohm 1 watt resistors in series with each 1n4007, and then install 0.01uf snubbers across each diode/resistor pair.
2) Remove the two 3300uf/400v series connected electrolytics, and replace them with two 1500uf/600v electrolytics, but this time create a CLC pi filter with one of the 1500uf/600v caps preceeding the inductor and the other 1500uf/600v cap following the inductor.
I did some calculations and the needed value I came up with for my inductor is 8.5mh. However I looked at some on-line calculators and when I plugged in the variable, I see that the impedance of the supply transformer is a fairly critical factor, or so it would seem.
3) I am contemplating the removal of the rectifier from the filiment supply and connect unrectified filiment secondary directly to the tube filiments, as this will probably present less noise than the unsurpressed ringing of the diode junctions.
With these details in mind, I would like your input for choosing the inductor value, including if necessary, figuring the impedance of the B+ supply secondary of the supply transformer...
Also if anyone thinks I may be missing the boat somewhere for cause/solution to this line-noise problem, please speak up. I want the help.
Regards,
Jim Butler
I recently acquired two mono-block tube amplifiers. When these were manufactured, they were purported to be very high-end amplifiers. However there are some issues with line noise (120cps full-wave rectifier hum) which ruins any other benefits that this amp might have had for being audiophile quality, and I am trying to address this... I have replaced all electrolytics and so we know it's not a bad cap, so we won't waste our time on that theory..
I see three major factors that seem to be at the root cause, each factor compounding the other.
First, the manufacturer should have used a good CLC pi filter, but instead used a single 1650uf capacitor @800v (actually two 3300uf caps in series, rated @ 400v each, with 220k bleeder resistors) which serves as plate supply for the 807 output tubes.
That B+ rail feeds into additional resistors to drop voltage for feeding the 807 screen grids, and has 330uf/400v caps on the low side of the resistor, which theoretically should further smooth this lower voltage rail, but it's the lack of a pi filter preceeding the 807 B+ supply that I believe to be our first contributing factor.
The second contributing factor is that this amp has four solid state rectifiers (discrete 1n4007 junction diodes for B+, a separate rectifier for bias supply, a separate transformer and rectifier for 12AT7 B+ supply, and then a large square rectifier for 6.3v filiment supply.) .. These all share the same ground rail, and none of them have snubbers, nor are they followed by CLC pi filters (again just single caps with nothing for the ripple voltage to fall across).
The third contributing factor and the reason I am reaching out to this forum for help, is that someone who owned these amps, at some point removed the original supply transformers and substituted them with much heavier-duty supply transformers with a lot more available current, which means that they have a considerably lower secondary winding impedance, and this will factor into the re-design of the power supply filters and I want your input.
My multi-pronged attack for eliminating the hum, is 1) Place 0.22ohm 1 watt resistors in series with each 1n4007, and then install 0.01uf snubbers across each diode/resistor pair.
2) Remove the two 3300uf/400v series connected electrolytics, and replace them with two 1500uf/600v electrolytics, but this time create a CLC pi filter with one of the 1500uf/600v caps preceeding the inductor and the other 1500uf/600v cap following the inductor.
I did some calculations and the needed value I came up with for my inductor is 8.5mh. However I looked at some on-line calculators and when I plugged in the variable, I see that the impedance of the supply transformer is a fairly critical factor, or so it would seem.
3) I am contemplating the removal of the rectifier from the filiment supply and connect unrectified filiment secondary directly to the tube filiments, as this will probably present less noise than the unsurpressed ringing of the diode junctions.
With these details in mind, I would like your input for choosing the inductor value, including if necessary, figuring the impedance of the B+ supply secondary of the supply transformer...
Also if anyone thinks I may be missing the boat somewhere for cause/solution to this line-noise problem, please speak up. I want the help.
Regards,
Jim Butler