Screen grid resistors - do they have to be 5W

[Tdale]

I'm about to put things together, and discovered that I don't have any 1K 5W resistors for the screen grids. Do they have to be 5W, or will 3 do? I might have some 3W, but I don't want to use them if 5W is really needed.

Also, the 1ohm resistors I plan to use for the cathodes on the output tubes ( to measure bias voltage) measures about 1.4 ohm. Is that ok, as long as I include it in the equation when I measure the current and calculate the voltage?

Tommy

[Normster]

I haven't done any calcs, but judging by how hot the 5W resistors get, I'd say you'd be pushing it using 3W. As for the 1Ohm resistors, I use Ohmite 2W 1% resistors. I suppose you could just compensate with the math for now, but you might consider replacing them in the near future.

BTW, if you're running 6L6's, you could run two 3W/1k resistors in parallel which would give you 6W/500R...maybe a better fit for the 6L6. (Of course, then you're screwed if you want to run EL34's.)

[Tdale]

Does that mean that 500 ohm is ok for the screens, in stead of 1K?

I might have some 2K or 2.2K 3W, which I could parallell, to get 1K 6W, I'll check that out.

I'll find some accurate 1 ohm resistors later I think.

Can the 6L6 be replaced with EL34, without doing anything with the circuit?

Tommy

[Elcabong]

If I remember correctly, old Bassman where using a common 470 ohm resistor for the screens and early Marshall copied that too.

I do not think there is a problem to use a 3 watt resistor there. The higher value, like 1K also means less current through it. I suspect the 5 watt is used to insure a higher working voltage.

[Tdale]

I found some 2W resistors

I have 2.2K 2.7K and 4.7K

I guess three 2.7K i paralell would work.. right under 1K total, and 6W I might go for that until I get some nice 5W's

Four 4.7K in parallell will work too, but 4 resistors for each tube is a bit too much I think

Tommy

[Bob-I]

I'm about to put things together, and discovered that I don't have any 1K 5W resistors for the screen grids. Do they have to be 5W, or will 3 do? I might have some 3W, but I don't want to use them if 5W is really needed.

3 watt is fine. The issue is one of heat, not current. Even 1 watt is enough but they'll give out from the heat since they'll be mounted directly on the tube sockets.

Also, the 1ohm resistors I plan to use for the cathodes on the output tubes ( to measure bias voltage) measures about 1.4 ohm. Is that ok, as long as I include it in the equation when I measure the current and calculate the voltage?

Tommy

Yes, that's fine. Next time you place an order get some 1% 2 watt 1 ohm resistors. It just makes life easier.

[ApexJr.]

If you want overkill, I stock 500 ohm 10W and 1K 10W
wirewound axial silicone axial resistors, there $1.00ea
other values are also avilable.
www.apexjr.com/miscellaneous.html 1/2 way down the page

Steve @ Apex Jr.

[skyboltone]

Tommy:
I think the brain trust around here has been using 470R screen resistors for better tone. In my opinion the only thing that makes the 6CA7 and 6L6 interchangeable is that the pin function is identical. A pair of 6L6 at (say 470 Volts) has a load impedance of around 4200 ohms. A pair of EL34/6CA7 under the same conditions will have a load impedance of 6500 or so. It will work but it won't be in the sweet spot of the EL34.

[drz400]

I'm about to put things together, and discovered that I don't have any 1K 5W resistors for the screen grids. Do they have to be 5W, or will 3 do? I might have some 3W, but I don't want to use them if 5W is really needed.

Also, the 1ohm resistors I plan to use for the cathodes on the output tubes ( to measure bias voltage) measures about 1.4 ohm. Is that ok, as long as I include it in the equation when I measure the current and calculate the voltage?

Tommy

How are you measuring those resistors? did you 0 out your probes to relative?
1% 1ohm 3W are good on the cathodes unless you are looking for a fuse.
Screens you can use 1K for both tubes to make them interchangeable but EL34 will not make it sound like a Marshall with out the transformer.
Wattage you will need a larger wattage resistor for EL34. You dont want it so big they wont blow under a tube fault. You can measure across the screen resistor and see where you are at in W when slaming into a load
Since 5W in a Marshall will still fry I'd say they did their homework so for a design such as their I'd stick with those values, you can get them from Mouser you know as well as the 1ohm 1% 3W

[skyboltone]

I'd stick with those values, you can get them from Mouser you know as well as the 1ohm 1% 3W

Greetings Lads:
I'm trying to imagine a scenario where you would need a 3 watt 1 ohm resistor at the cathode ground. Let's see. We bias a pair of 6L6s at what, maybe 55-60ma per tube? This should be at a bias voltage in the neighborhood of 30-40 volts. This means we drop approximately 55 to 60 millivolts across the 1 ohm resistor. P=IxE or .06 amps x .06 volts = .0036 watts. Seems like a quarter watt precision resistor ought to take care of it.

Am I all wet here?

The dunce.

[skyboltone]

Also, the 1ohm resistors I plan to use for the cathodes on the output tubes ( to measure bias voltage) measures about 1.4 ohm. Is that ok, as long as I include it in the equation when I measure the current and calculate the voltage?

Tommy

Hi Tommy:
I had a rather embarassing setback in my build so I suspect you'll be making music ahead of me. Great looking build. Lot's of room.

Anyway, a couple of things cross my mind (what's left of it) regarding this question. Yes, you can use the 1.4 ohm as long as you factor it in your equation. But measuring bias current this way, at the cathode, also includes the screen current. This amounts to about 17 ma on my KT66s. A significant fraction of the idle plate current. I forget what it is on the 6L6s. I think this is why you see so many "Bias Right" devices for sale. With one of these we look at the current through the plate lead alone. Now, some of these guys around here have a bunch of experience with this stuff, and I suspect that they set bias using the cathode current method as a starting point, and then adjust it by ear for the sweet spot.
The point of all that would be that (as drz400 points out) we are limited by the accuracy of our instruments and can only rely on the measurements to the extent that we trust them.

YMMV
The dunce

[Tdale]

Tommy:
I think the brain trust around here has been using 470R screen resistors for better tone. In my opinion the only thing that makes the 6CA7 and 6L6 interchangeable is that the pin function is identical. A pair of 6L6 at (say 470 Volts) has a load impedance of around 4200 ohms. A pair of EL34/6CA7 under the same conditions will have a load impedance of 6500 or so. It will work but it won't be in the sweet spot of the EL34.

Ok.. two 1K in parallell will give me the right value then...

tommy

[drz400]

I'd stick with those values, you can get them from Mouser you know as well as the 1ohm 1% 3W

Greetings Lads:
I'm trying to imagine a scenario where you would need a 3 watt 1 ohm resistor at the cathode ground. Let's see. We bias a pair of 6L6s at what, maybe 55-60ma per tube? This should be at a bias voltage in the neighborhood of 30-40 volts. This means we drop approximately 55 to 60 millivolts across the 1 ohm resistor. P=IxE or .06 amps x .06 volts = .0036 watts. Seems like a quarter watt precision resistor ought to take care of it.

Am I all wet here?

The dunce.

Well I'm assuming you are leaving this resistor in?
Are you considering that resistor needs to handle the current at full throttle not idle???? more like .25v across a 1 ohm even though that is still fine to use a 1/2W.
BUT
You redplate or blow a tube and that 1/2w resistor could blow before your fuse.
It is up to you if you feel like changing an extra component. I have used 1/2W and had them blow when a tube went. This is a pain when you are on stage and are hoping to just change a tube
I've been there and done that, got a very angry call from Europe.
Besides I use the exact same resistors Dumble uses so they sound better
The Screen current is around 1ma on a 6l6 so it can be ignored and around 5ma on EL34 so you should include that unless you like being a little cold when slaming there can easily be 35V across a 1K screen on an EL34 1.2W and again the question is to size it correctly to blow if a major fault happens but you dont want it smokin everytime you pop a tube.

[groovtubin]

I'm about to put things together, and discovered that I don't have any 1K 5W resistors for the screen grids. Do they have to be 5W, or will 3 do? I might have some 3W, but I don't want to use them if 5W is really needed.

Also, the 1ohm resistors I plan to use for the cathodes on the output tubes ( to measure bias voltage) measures about 1.4 ohm. Is that ok, as long as I include it in the equation when I measure the current and calculate the voltage?

Tommy

Tommy, you can use TEN ohm resistors on the cathode and get away with it, won`t affect tone, and your METER reading is MORE accurate, don`t laugh, arguably the best tech in the world ( besides me of course) down at Class A electronics in Fla turned me on to this, it WORKS... jp Use FIVE watts on the screen grids, save yerself time AND trouble i assure you..

[s2]

Fender used 1W screen resistors on all their 6L6GC amps. I imagine those things get warm...