Page 1 of 1
Screen Grid Resistors Getting Cooked slowly
Posted: Tue Jan 31, 2017 2:17 pm
by matttornado
Hi everyone. been a while since posted....
I have an issue with my 1974 100 watt superlead. All four screen grid resistors show signs of being over heated. even the yellow wires going from pin 4 to the circuit board turned brown around the tube socket. The amp works and sound great though.
Plate voltage is 490V & I have it biased to 36mA.
I'm in the process of a new cap job now....
Can anyone steer me in the right direction?
Re: Screen Grid Resistors Getting Cooked slowly
Posted: Tue Feb 07, 2017 12:12 pm
by Haze13
Here is the deal: the current that goes through the resistor is strong (on peaks) that resistor can't deal with it. It was working more than 40 years, so may be it's time to replace it with a better one. Every part has his life span, even the chassis
There are resistors that even though they can handle the power, but cannot handle the voltage.
Example: Ohmite 50W resistor at 1000v will dissipate 20W of power, which is fine according to the ohms law, but it's rated only at 600V, so he will heat up very quickly, to more than 200Deg. According to the de-rating graph from the same datasheet it can't be used at full 50W, so check the voltage also when you will be buying new resistors
5W %5 cement wire wound is good enough for that application.
More closer screens to the B+, the more current they will draw. Problem of every GUITAR amp.
Re: Screen Grid Resistors Getting Cooked slowly
Posted: Tue Feb 07, 2017 2:20 pm
by matttornado
Thanks for the reply!
I replaced the screen resistors about a month ago with 7 watt 1K Welwyn type & they are already showing signs of stress.
Just finished the cap job last night.....
Also, my PT gets really hot! I wonder if the two are related?
Re: Screen Grid Resistors Getting Cooked slowly
Posted: Wed Feb 08, 2017 1:00 pm
by Haze13
WELWYN W22-1KJI Through Hole Resistor, 1 kohm, 200 V, Axial Leaded, 7 W, ± 5%, W22 Series
These?
Re: Screen Grid Resistors Getting Cooked slowly
Posted: Wed Feb 08, 2017 2:35 pm
by matttornado
Yes I believe they are ones. I got them from valvestorm.
Re: Screen Grid Resistors Getting Cooked slowly
Posted: Wed Feb 08, 2017 11:54 pm
by Haze13
You have to be sure, becauze these rated for 200V only. I would sugest not to use these in this application. Current through this reistor is not static. It is changing during tube apperation. Better to use high-voltage rated resistors. Even at Aikenamps it is written so, if my word is in question...
Best regards
Re: Screen Grid Resistors Getting Cooked slowly
Posted: Thu Feb 09, 2017 4:24 am
by Roe
try colder bias
Re: Screen Grid Resistors Getting Cooked slowly
Posted: Thu Feb 09, 2017 6:19 am
by neikeel
What tubes are you running that are drawing that much current?
I suspect that is as much the issue, particularly if the wires to the screens are going brown (the wire will be 600v rated). Is the 200v rating of the Welwyns ac or dc?
kevin 'connor wrote:Hi Guys
The screen signal current is typically about one-tenth of the plate signal current. In a 50W amp, the peak plate current might be 300mA, so peak screen current is then around 30mA. That causes a 30V drop across 1k and a dissipation of just under 1W.
The important characteristics about the screen resistor are that it should be 1k minimum and that it be flame-proof. Using a 5W wire-wounmd assures the latter. You could just as easily use a 2W metal-oxide type that is also flame-proof. DO NOT use those red-lacquer-covered Rs as they burn.
The important facet here is not the voltage drop but the increase in the screen circuit impedance. This is "reflected" through the tube and keeps fault currents to a safe value. TUT showed this in 1995. We advocated the 1k-5W usage simply to assure that the new screen-stops would be flame-proof.
Have fun
Re: Screen Grid Resistors Getting Cooked slowly
Posted: Thu Feb 09, 2017 10:31 am
by matttornado
Thanks....
I'm running a quad of Ruby BSTRs at ~35mA per tube. B+ voltage is ~480 -490 depending on what time of day it is.
I'm not sure what the specs are for the resistors I'm using,. I'll have to take a look at the data sheet if I can find one.
Thanks for bringing that up!
I recently took of 1K 5 watt white block ceramic wire wound resistors as they were showing burn marks as well.
Re: Screen Grid Resistors Getting Cooked slowly
Posted: Tue May 09, 2017 6:34 am
by FourT6and2
Yeah, you gotta make sure whatever component you're using is rated for a high enough voltage. Not just wattage. You can have a 10W resistor that's only rated for 200V. In a part of the circuit that sees 500V, it won't last long. Then you can have a 1/2W resistor that's actually so stout it's uprated to 1W and handles 500V, like the Dale/Vishay CCF60. Not saying you should use that for this spot (they are metal film). But just an example.
Re: Screen Grid Resistors Getting Cooked slowly
Posted: Mon Jan 29, 2018 6:45 pm
by rip
The 200v is the element limited voltage or the voltage across the resistor...they should be fine as the voltage drop shouldn’t be that much. Using max screen current (50mA) for a EL34 and 1K resistors:
0.050A x 1000ohms = 50V
That said the spec doesn’t specify AC or DC.
I’d wonder how your OT or tubes are doing?