Change cap, or even better, use a power supply board ( like the first one $35.5 ) :
Power Supply Kits
A can can makes ground contact by bending the tabs onto the chassies, as the can is made of aluminium it won't take solder and the contact is often bad. The above board solves this by using soldered caps and real ground cables.
Power Supply Kits
A can can makes ground contact by bending the tabs onto the chassies, as the can is made of aluminium it won't take solder and the contact is often bad. The above board solves this by using soldered caps and real ground cables.
Rayma, The "bad" one is already on an RA back to the vendor. Even thought the new one was 30-30-30-10. The previous one was 30-30-30-30. It is hum free on every selector switch setting and sounds great. In Michigan our power dipped to below 113 volts today, so every voltage reading was way down. Still the PAS and St70 sounded great.
I've never seen that happen before. It must be an unusual manufacturing defect.
Was there a short jumper from the can capacitor to ground terminal on the chassis? Unless the can is "floated" for some reason, some of those "twist lok" mechanical connections to the chassis weren't good electrical connections for grounding. A bad ground on the can will put 120 Hz hum through the entire preamp.
RMB, Yes, there are actually four grounds that attach to the can. There is a picture of this on page 2 of this post. This is my actual PAS I rebuilt. The new can at 30-30-30-10 works like a gem. When I was troubleshooting the bad can I used a 20uf that I jumpered each section to try and determine if the can was bad. This did not originally work and the hum remained. One of the recommendations here was to purchase another can and try it, which I did and it worked perfectly, no hum...gone. The PAS is not a perfect preamp. I can say that it sounds 100% better with new boards and matched components channel to channel. I matched everything within 1%. Maybe overkill. It is feeding a Dynaco ST70 I also rebuilt. Lastly, I have a line on some almost as new DCM Time Window 1A's that ought to make this set up sound spacious. Thanks for your feedback, comments and any more help.
I have new update a request to this original post. My PAS 3 hum is very low now. I did a few more things to reduce it even more. I found a ground loop in the interconnects. I was using some very inexpensive cable between the PAS and ST-70. I bought new gold pated RCA jacks for the ST70 and found two sets of as new Monster cable interconnects from our local Goodwill store. i tried my old interconnects with the new jacks and as my hand would approach the interconnects i would introduce hum like I was a transmitter. I then installed the Monster cables and that problem disappeared. This is 60HZ hum as i was able to verify this part. Now the next piece...On the PAS I installed the power supply board as shown in this post. With the cover off i noticed a slight increase in hum as i touched the two filament supply capacitors. When i used two fingers and touched the capacitors the hum went away. So, i took a solid wire and literally taped the stripped ends to the top of the two cans and guess what hum is almost zero in the right channel.
New question. i keep reading about lifting the heater voltage. I cannot understand how lifting the heater voltage on a heater circuit does not burn out the filaments. At 12.6 volts and you lift the voltage say 50 volts why do the heaters not go to 62.6 volts? Why do the two DC voltages not become additive? I am just trying to understand the concept of lifting heaters. Lastly, the one thing i have read is that you cannot ground the heaters. Why?
Thanks to all on this post for advice.
New question. i keep reading about lifting the heater voltage. I cannot understand how lifting the heater voltage on a heater circuit does not burn out the filaments. At 12.6 volts and you lift the voltage say 50 volts why do the heaters not go to 62.6 volts? Why do the two DC voltages not become additive? I am just trying to understand the concept of lifting heaters. Lastly, the one thing i have read is that you cannot ground the heaters. Why?
Thanks to all on this post for advice.
You "unground" the filament supply, and connect that node of the supply to a resistor divider
with decoupling to ground. The voltages across the filaments remain the same, only the voltage
wrt the ground will change. That is, the whole filament supply floats at some DC voltage. Usually
this DC voltage is set to somewhere from +25V to +75V.
This doesn't seem very necessary to do in the PAS, so I wouldn't bother. This is most commonly done
when a cathode follower is added to the circuit, so that tube has a cathode significantly above ground.
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Same reason why a bird can sit on a high tension wire all day long. Because the bird is not referenced to (touching) ground. The heater will burn, if you lift one side of the filament and ground the other side with a small enough resistor.
Rayma & Allen, so basically there is a high DC voltage, say 50 VDC, that has no ground, it just exists at some elevated level and the 12.6 volt heater supply rides in or on that 50 VDC elevated ungrounded voltage like Allen says the birds do on a high tension wire?? The 12.6 voltage has to return to some negative potential like a diode doubler circuit my PAS has right now, but has no earth ground except the number 9 pins on one of the circuit boards? If i removed the number 9 pin grounds i could elevate the heater voltages and no filament burn out occurs?? I understand the birds in the wire. I am trying to visualize how I can just superimpose the 12.6 volts on the elevated voltage and how the elevated voltage just exists on a wire. My apologies of being so dense about this. Rayma, I agree i do not want to do this on my PAS. I was just "reading" again trying to understand another concept. You have all been great help.
Imagine the PAS filament supply without the ground wire from the phono filaments'
center tap connection to ground. Then this whole circuit floats without a particular
voltage potential with respect to the chassis ground.
Now make a two resistor voltage divider from the HV to ground, giving say 50VDC.
Filter it by adding a capacitor from the junction of the two resistors to ground.
Connect this 50VDC point to the former "ground" of the filament supply.
This imposes a "common mode" 50VDC voltage on the supply and the filaments.
Relative to the near-ground 12AX7 cathodes, their filaments are at 50VDC.
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It's mostly used for circuits that include a cathode follower, where one or more cathodes
are substantially above ground potential, 75-150VDC. Thus can reduce hum in some circuits,
just not for the PAS, where the phono hum is normally below the hiss level.
New update. I have eliminated the PAS3 hum. The right channel bass control wire runs from the font panel runs right by the power switch and ceramic capacitor. I took my needle nose pliers and kept bending the wire away and closer to the circuit board and the hum is gone. The noise level was minimal, but noticeable. The change was so great that the whole presence of the PAS 3 changed. I recommend anyone chasing ground loops to move this wire away from the power switch and capacitor location. Again thanks to everyone that has been helping keep looking for the simpler solutions.
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