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Bottlehead Forum Welcome! Need support, you got it. Or share you ideas and experiences. |
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Bottlehead Forum Welcome! Need support, you got it. Or share you ideas and experiences. |
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In Reply to: Re: Paramour mods! posted by Paul Joppa on November 26, 2000 at 17:02:47:
I've got one amp modified. 2 mV RMS output hum with the input plugged with a dummy 2K RCA plug. I’ll modify the other amp tomorrow.This is my first cut at the mod. Don't be upset if I or someone else finds a better way in the near future.
Unplug the unit and wait for the capacitors to bleed down. You may want to remove the tubes too.
Step 1: Cuts the hum from 22 mV down to 11 mV.
A) Disconnect (cut) the wire from 15 to B1 at 15.
B) Put a 22uF 500V from 20 (+) to 16 (-). (I used an axial leaded capacitor; it was all I had in my junk box. The capacitor was originally purchased from www.Tubesandmore.com.)
C) Place a 5.1K 1W resistor from 15 to 20. Solder to the lead of the 5W resistor going to 15, not to 15 itself. Solder to lead of capacitor on 20, not to 20 itself. Be careful not to short anything out.
D) Route a wire from 20 (the - side of the capacitor) along the body of the capacitor, make a right hand turn at terminal 15 and solder it to the lead of the resistor going to terminal 12.
E) Connect the end of the wire that used to go to terminal 15 to terminal 20.Now the hum is lower but the waveform looks nasty.
Step 2: Cuts the hum from 11 mV down to 2 mV
A) Solder three resistors in a straight line:
Solder a 200K 1/4W to a 100K 1/4W. Take the free end of the 100K 1/4W and solder it to another 100K 1/4W. Across this last 100K, solder a 0.1 uF >200V (I used a 1kV ceramic. ½ W resistors would have been a better choice.)
B) Solder the free end of the 200K goes to terminal 20 (don’t melt the cap.) Do not let any of the resistors touch any of the nearby capacitors. The resistors will be running a little warm.
C) Solder the end with the 0.1 uF/100K goes to terminal 16 (don’t melt the cap.)
D) On the end of the 0.1 uF not going to terminal 16, solder a wire from this point to B5. (Don't melt the capacitor.) Route the wire under all the other wires on the board and route it on the power transformer side of both the RCA jack and the tube socket.This biases the filaments to approximately 90V and provides an AC bypass to signal ground for the capacitively coupled noise from the power transformer. I choose to route this to signal ground not chassis ground to avoid RF ground loops through chassis ground. I may try a large cap across the bottom 100K resistor at a later date.
The capacitor from B5 to ground did not need a DC bias to work. But the capacitor stored a nasty amount of energy and makes a spark when discharged. Remember I like to have two discharge paths to ground for capacitors. The bias network solved the OUCH hazard.
I will be trying a 1 uF across the filaments soon. But I want to hear a stereo pair of amps with this mod before I do more bypassing.
I’ll have a web page up for this soon. If you can get a 47 uF 450v capacitor, use it instead of the 22 uF 500V. There won’t be any pictures for a while. May be Santa will be nice to me and leave a Digital camera under the tree.
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Follow Ups
- First cut try at the mod: 2 mV Hum - VoltSecond 20:26:37 11/26/00 (0)
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