Bias Sampling Resistor 1 Ohm 2 Watt 1% Tolerance Blown KT66
Posted: 06 Jul 2010, 21:26
For those who blow a KT66, if your 1k/5W screen grid resistor is ok and all your tubes light up after replacing the bad tube but you are unable to get a bias reading and your amp sounds like doggie doo, check R83 and R84. It should read 1 Ohm if you touch both sides of it with your test leads (amp on or off).
If one or both are blown, replace with 1 Ohm, 2 watt 1% tolerance resistor ike this one. In fact, probably a good idea to keep a few of these on hand as Radio Shack doesn't carry anything with a 1% tolerance and Steve stressed that this is critical to getting an accurate bias reading. Otherwise, your bias reading could be 5-10% off if that's what the tolerance is on the Radio Shack part. I bought a 1 Ohm 10 watt 10% tolerance resistor at Radio Shack to hold me over till the real ones arrive but that's all they have http://www.radioshack.com/product/index ... Id=2062290
What you want is this one below...
http://www.newark.com/jsp/search/produc ... R0000FKE14
DISCLAIMER: If you are not a qualified service technician, do not attempt to do this unless you know what you are doing. The high voltages inside an amp can kill you dead.
Here's what it looks like replaced on the PCB. The original was smaller and blue with colored bands. It must have been the .75 watt one instead of the 2 watt one Steve recommends to use now.
If one or both are blown, replace with 1 Ohm, 2 watt 1% tolerance resistor ike this one. In fact, probably a good idea to keep a few of these on hand as Radio Shack doesn't carry anything with a 1% tolerance and Steve stressed that this is critical to getting an accurate bias reading. Otherwise, your bias reading could be 5-10% off if that's what the tolerance is on the Radio Shack part. I bought a 1 Ohm 10 watt 10% tolerance resistor at Radio Shack to hold me over till the real ones arrive but that's all they have http://www.radioshack.com/product/index ... Id=2062290
What you want is this one below...
http://www.newark.com/jsp/search/produc ... R0000FKE14
DISCLAIMER: If you are not a qualified service technician, do not attempt to do this unless you know what you are doing. The high voltages inside an amp can kill you dead.
Here's what it looks like replaced on the PCB. The original was smaller and blue with colored bands. It must have been the .75 watt one instead of the 2 watt one Steve recommends to use now.