Wiring 6L6's

[Zippy]

Zippy,
Why would you rectify your heater voltage?? I mean DC heaters on PReamp tubes make little to no difference in the noise floor, and are TOTALLY un necessary on output tubes.

You are absolutely right. Mea culpa. I woke up about 5 this morning and realized what I had typed. My bad. Not appropriate to these amps.

[Tdale]

I was going to use 1K, but I only had the in 3W, so I paralelled two, to get 6W, and settled for 500 ohm for now.. I might replace them with some 5W 1K later, but I noticed that many amps use 470K for the screens, so I guess my resistors will work fine for now..

Tommy

[heisthl]

Next time you are fooling with a 6l6 amp measure the voltage drop across the screen resistor, it is not much and a 2 watt is more than adequate. As someone else mentioned the main thing is the heat from the tube. I have never noticed a tone difference between a 1K and a 500 on the screens - I usually use the 1K and tie pins 1 and 8 together so I have the option of using EL34s with a bias change. After reading Funk's comment on "a cloud of smoke" with EL34s I'm going to have to measure my Marshall 50watter and see how much those Screen resisters are dropping.

EDIT: I just looked up the tube data and at Max signal the screen current is 22MA on the 6L6 and the EL34 is double that at 44MA. Marshall must be using 5 watters to build in a large safety factor. At 44MA the 1K is dropping 44 volts and power is less than 2 watts The 6L6 with a 1K is dropping 22 volts and power is less than 1/2 watt.

[skyboltone]

Next time you are fooling with a 6l6 amp measure the voltage drop across the screen resistor, it is not much and a 2 watt is more than adequate. As someone else mentioned the main thing is the heat from the tube. I have never noticed a tone difference between a 1K and a 500 on the screens - I usually use the 1K and tie pins 1 and 8 together so I have the option of using EL34s with a bias change. After reading Funk's comment on "a cloud of smoke" with EL34s I'm going to have to measure my Marshall 50watter and see how much those Screen resisters are dropping.

EDIT: I just looked up the tube data and at Max signal the screen current is 22MA on the 6L6 and the EL34 is double that at 44MA. Marshall must be using 5 watters to build in a large safety factor. At 44MA the 1K is dropping 44 volts and power is less than 2 watts The 6L6 with a 1K is dropping 22 volts and power is less than 1/2 watt.

Jeepers, I thought I was loosing my mind the way folks were jumping all over this in this and another thread. Heishl's numbers square with my arithmetic. While it is true that the screen resistor is there to limit screen current the only time it will be required to dissipate more than that small amount of power (watts) is when the tube takes off due to a parasitic. Nearly every beam power tube out there cautions that a screen resistor is required primarily to head off parasitic oscillations. I'm not making that up. Look at the tube data sheets. When a parasitic occurs the screen turns into a plate and draws current like mad. If the amp is unstable generally, it may go into oscillation right away, but it can also wait until the right combination of conditions occur for the oscillation to begin. This may explain why some have had smoked resistors after turning up at a gig.

In no way will having larger (wattage) screen resistors hurt anything. They will work just fine. I think BobI uses three watt jobs because that's the largest size Mouser has in a metal film. That's why I chose them as well. If you want to use wirewound 20 watt resistors that's good too. Scott L has said he can hear the difference between a 470 and a 1K in the Dumble 6L6 screen circuit. The KT66 tube data sheet suggests a 300 ohm but the Marshall crowd seems to like the 1K when modding the Bluesbreaker reissue. I have absolutely no experience with this but I tend to listen when the Dog speaks. I'm putting in 470 ohm 3 watt metal films.

The Dunce