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|9/1/2004 11:16 PM|
||6L6s and 5U4 Cu cap in a 5E3?|
I built a Tweedy a couple years ago and want to try the Paul C mod and run with 6L6s. I read elsewhere that I could change to 6L6s by simply putting in a 5U4 Coppper Cap retifier. I decided to check here first. I see some mention of modifying to 6L6s, but I've done a search and can't come up with anything definitive, especially with regard to using a 5U4 Copper Cap. I know just a little about this stuff...enough to get in trouble. I know that 6L6s draw double the filament current. The JJ 6L6s that I will use run cooler than some other tubes. Would this help offset the higher filament current? Can the Mission Amp tranny hold up to the higher current? Do I need to change the cathode bias resistor? If this topic has been talked to death already, maybe someone could point me in the right direction so that I can read up on it. Thanks much.
|9/2/2004 4:38 AM|
I think I'll start collecting answers since I don't know if anyone keeps archives here, but many folks don't notice much or any sound change with the Paul C mod in a Cathode biased amp like the deluxe, where you would in say a princeton. While talking to Paul C (he makes a pedal called the TIM), he also couldn't see how this mod, which somehow is wrongly being named after him, affects any tone in a properly designed 5E3.
6L6's should run fine, Bruce often recommends a 10W cathode resistor instead of the stock 5W, and a 5V4 Copper Cap equivalent (WV34 I think it is), yes, 6L6's can draw 0.9 amps vs 0.45 amps for 6V6's, but using a copper cap relieves the PT of taking power from the 5V taps so you should be fine.
Watch the temps, and blow a couple of fans in the back if anything is getting overly hot.
My Cu Cap 5U4s, and 5V4 equivalents are *very* close in B+ measurements, but maybe there is a fluke with them or the differences are in sag at load.
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|9/2/2004 4:59 AM|
Ted - Thanks very much for the reply. Interesting...the so-called "Paul C mod" doesn't affect a 5E3 circuit. I'll do a little more research in the 5V4 vs. 5U4. I already own a 5U4, so I guess I'll give it a try and watch it closely. If I may be so ignorant, can you confirm which resistor in the circuit is the "cathode resistor". Just want to be sure. Thanks for your help.
|9/2/2004 2:41 PM|
The cathode resistor is most likely going to be the sandbox power type -- squarish in it's diameter, and rectangular in length, and white or off-white color, and labelled 250 or 270 ohms / 5W, bypassed with a cap.
|9/2/2004 3:16 PM|
|Bruce /Mission Amps
Yes, the PT can handle a 5U4 rectifier and two cathode biased 6L6s with the two 9 pin tubes as a total load.
I prefer to use a WeberVST Copper Cap WU4 or WV4 but a real glass tube rectifier is OK.
The tranny will get very warm but I've run them on a test bench setup at around 150% of their max rating with temps you'd never see in a real amp and, I've never blown a filament winding.
Use a 10 watt 270 to 300 ohm cathode biasing resistor and lower the 5K1 dropping resistor between the two fist filter caps down to around 2700 to 2200 ohms.
|9/2/2004 11:57 PM|
Thanks for the help, Ted and Bruce. I'd like to measure the current draw when I put in the 6L6s. Is this correct? I'll use the equation I = V/R. To get V, I measure the B+ voltage and subtract the voltage drop across the cathode bias resistor. R = the value of the cathode bias resistor. Not sure at all about this.
|9/3/2004 12:33 AM|
|Bruce /Mission Amps
Current would just be the voltage across the cathode resistor divided by the value of the cathode resistor in ohms.
22v/270ohms = .0815A or, about +40.5ma each tube.
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