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'Course, they're 100 ohm, not 100K ohm resistors

9/19/2004 12:19 AM
Scott W
'Course, they're 100 ohm, not 100K ohm resistors
Just to make sure it was just a typo....
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9/19/2004 1:37 AM

Bruce is on vacation. And God bless him! He got two 5E3 kits out to me right before he left. It's awesome to see someone treat customers so well. And, on those 100 ohm resistors, be sure they're actually 100 ohms not 100K.  
9/19/2004 5:52 AM
Carl Gigun

i wonder if it would make much difference?  
9/19/2004 10:24 AM
The 100 ohm resistors are there to form an artificial center tap for the filament winding and supposedly to act as fuses that would protect the PT's filament winding in the event of a tube short and a resultant over current situation that would burn it out.  
This is normally done with the twisted end of the two resistors going directly to ground. With the two ends going to pin 8 instead, this adds another 270 ohms (the cathode resistor)between the heater taps and ground. This 470 ohms is still a long way from 100k ohms and I would think any expected fuse like burn out of one of these 1/2 watt resistors would not happen with 100k rating before the winding were damaged. Connecting to pin 8 floats the filament ground reference above ground by 20 to 40 volts and helps to reduce 60 cycle hum in the amp.  
9/19/2004 11:49 AM

Nice desription Barry. This center tap works! I had a guy bring in a Bassman head that his buddy borrowed and connected the speaker out directly to the input of a mixer (to get amp distortion without a speaker! Yikes!) and promptly blew up the output transformer, and these little 100 ohm resistors saved the filament winding on the PT.  
9/20/2004 6:10 PM
Carl Gigun

I was actually wonder if 100k resistors would work the same for hum canceling.  
On the fuse thing, how does this work exactly? The resistors aren't in series with the heater winding current like a fuse would be, so you could easily draw way too much filament current and fry the power transformer without the resistors knowing a thing about it. Does it protect against some other failure mode?  
9/20/2004 8:01 PM
here is an explanation from Doug Hoffman's website:  
Adding 2 100 ohm resistors to the heater system  
If your power transformer has a green wire with a yellow stripe and it is soldered to the chassis ground you already have a heater supply center tap and you do not have to install the 100 ohm resistors.  
I actually prefer the two 100 ohm resistor method because they act like fuses if a power tube plate wire ever shorts out from pin 3 over to pin 2. Pin 2 is one of the heater wires and is a direct short to ground for the high voltage that is found on pin 3. This happens all the time and you can save your power transformer by having the pair of 100 ohm resistors instead of a transformer center tap. The 100 ohm resistors will melt in half if the plate wires ever short out to pin 2. This is way cheaper than replacing a power transformer.  
If you want to get rid of your heater supply center tap and replace it with two 100 ohm resistors, all you have to do is unsolder or clip the green/yellow center tap wire and heat shrink or tape it off. You must make sure this wire is tapped off and cannot touch or make contact with the chassis or any other wires. It will just be coiled up inside the amp and not used any more. Add the two resistors by following the instructions below.  
How to add an artificial heater supply center tap.  
The two 100 ohm 1/2 watt resistors are connected to the lamp assembly and then to ground. Solder a 100 ohm 1/2 watt resistor from each green winding on the bulb holder to the chassis ground lug next to the pilot light and you are done. In other words, you will have a 100 ohm resistor soldered to each green heater wire and the other end of each resistor is connected to the chassis ground.  
The 100 ohm resistors create an artificial center tap for the heater system. If you do not have a heater supply center tap, you will get a 120 cycle hum. Most people confuse a 120 cycle hum with a 60 cycle hum. You must listen carefully. A 120 cycle hum sounds more like a buzz than a low pitched hum.

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