Cathode resistors running hot?

[wiseowl]

My EL84 cathode resistors are getting hot and I.m confused as to why. The schematic shows 130r 5W and I have 130r 7W wire wound fitted.

My voltages seem OK to me, all tubes being within a few points of each other at:-

Pin 3 = 10.8
Pin 7 = 323
Pin 9 = 308

If I calculate the power dissipated across each of the cathode resistors then I get 0.9 W, (P = V*V / R) well within the 7W rating. Any thoughts as to why they would be getting hot, even if the amp is kept on but in a quiescent state? Should I invest in even higher wattage resistors, or is there something else at fault which I'm not seeing.

I notice that on this design the pin 9 resistors(I forget it's name.) are 100r, where as on the 18 watt schematic has 100r resistors plus a 1K in series with them. Would this be a worthwhile mod?

Thanks

Martin

[zaphod_phil]

Those resistors still have to burn off that 0.9W somehow, so they will inevitably get hot. Just not as hot as if they were dissipating 5W, say.

Also, personally I don't use anything less than 150 ohms on the cathodes these days. Likewise, the 1k in series with the individual 100 ohm resistors is also good insurance to help protect the screen grids. This is a recent amendment to the original 18W power amp design.

[wiseowl]

Those resistors still have to burn off that 0.9W somehow, so they will inevitably get hot. Just not as hot as if they were dissipating 5W, say.

Fair enough, but they're getting hot enough to smell as if they're burning and too hot to touch, after only 5 minutes or so without any signal. It really feels wrong to me.

I'll get some 150r's to try and add a 1K to the screen grids.

Thanks for the reply.

Martin

[s2]

Your cathode voltage tells me everything is pretty OK. You should be seeing about 11V at idle.

It is perfectly normal for the cathode resistor to get very hot. 180+ degrees is nothing to worry about. New resistors will often emit an odor until they are broken in for a few hours. New tubes do the same. Is your 1.5K dropping resistor between the plates and screens equally as hot?

I'm going to have to go on record disagreeing with Sir Phil on the screen resistor thing (off with my 'ead!). Part of the 18W sound is the shared 100R screen resistor. This yields 200R per tube since it is shared. Going above 470R per tube chokes the tone of the power amp IMHO. Limiting the screen current seems to have detrimental effects on the tone of EL84s.

What we are really talking about here is tube life. Would you rather have an amp that sounds better but needs new tubes every 6 months or have an amp that sounds almost as good and needs tubes every 2 years? I know which way I would go...

[Plexi]

What we are really talking about here is tube life. Would you rather have an amp that sounds better but needs new tubes every 6 months or have an amp that sounds almost as good and needs tubes every 2 years? I know which way I would go...

i have to agree.. also on the resistor.. some of the blue colored oxide jobbies.. they can give off a burning electrical smell..more than any other resistors out there.. I try to stay away from those. You can use a higher wattage resistor,it will run cooler than a smaller wattage one.

But yeah, the cathode resistors get hot..so remember to leave some room between it and your bypass cap..

[zaphod_phil]

YI'm going to have to go on record disagreeing with Sir Phil on the screen resistor thing (off with my 'ead!). Part of the 18W sound is the shared 100R screen resistor. This yields 200R per tube since it is shared. Going above 470R per tube chokes the tone of the power amp IMHO. Limiting the screen current seems to have detrimental effects on the tone of EL84s.

Heh I'm actually pretty middle of the road on this one, as there's also folks around who swear that an 18W doesn't sound nearly as good with less than 2.7k screen grid resistors. It seems they like the increased compression that gives. So I think it's more about subjective differences in taste. As I tend to use NOS tubes a lot for my own amps, I'm pretty keen for them to last, whereas someone using current production needn't be quite so cautious. There's also been a lot of 18W Lite IIbs and SuperLite TMBs built with the 1k resistors, and the people who built them rave about the tone. I don't really mind either way, as long as people enjoy the sound of what they've built.

Edit: BTW thinking about it, I have a Lite IIb which uses a pair of individual 1k3 screen grid resistors which I happened t have at the time, and I swear it's one of the sweetest sounding 18Ws ever, although I think a lot of that comes from the NOS tubes.

[mcrracer]

Those resistors still have to burn off that 0.9W somehow, so they will inevitably get hot. Just not as hot as if they were dissipating 5W, say.

Also, personally I don't use anything less than 150 ohms on the cathodes these days. Likewise, the 1k in series with the individual 100 ohm resistors is also good insurance to help protect the screen grids. This is a recent amendment.

Please excuse the hijack...
ZP, I realize you guys are discussing the EL84 version but I am building a 6V6 EF86/12AX7 version. Do the updates on the latest 18watt Lite IIb schematic pertain to the 6V6 version also? There is now a 10 item list that I had not seen before. Need to know before I go any further. Thanks

[wiseowl]

Thanks for the reply, helps to put my mind at rest, just as well that I have them mounted above the tag strip in plenty of air.

Is your 1.5K dropping resistor between the plates and screens equally as hot?

I have a 1K 10W in there and that runs much cooler.

Cheers

Martin

[zaphod_phil]

Do the updates on the latest 18watt Lite IIb schematic pertain to the 6V6 version also? There is now a 10 item list that I had not seen before.

The Lite IIb schematic hasn't actually been changed over the last few years, as it seems to work pretty well. They're only recent relative to the original 18W design, with the screen supply changes having been proposed as part of "Jason's Mods" back in 2003. So I'm a little surprised you say you haven't seen these changes before A number of folks have built 6V6 versions successfully with the Lite IIb schematic as is, just changing the value of the power tubes' cathode resistor (270 ohms) and the PI tail resistor (20k-ish). There are other optional tweaks you can do such as change the cathode resistor in the PI to 470 ohms to get a bit more Plexi-style crunch, and some people use individual 470 ohm screen resistors, like in some Fender amps, instead of the regular 18W or Lite IIb scheme. There aren't too many hard and fast rules.

HTH

[mcrracer]

Do the updates on the latest 18watt Lite IIb schematic pertain to the 6V6 version also? There is now a 10 item list that I had not seen before.

The Lite IIb schematic hasn't actually been changed over the last few years, as it seems to work pretty well. They're only recent relative to the original 18W design, with the screen supply changes having been proposed as part of "Jason's Mods" back in 2003. So I'm a little surprised you say you haven't seen these changes before A number of folks have built 6V6 versions successfully with the Lite IIb schematic as is, just changing the value of the power tubes' cathode resistor (270 ohms) and the PI tail resistor (20k-ish). There are other optional tweaks you can do such as change the cathode resistor in the PI to 470 ohms to get a bit more Plexi-style crunch, and some people use individual 470 ohm screen resistors, like in some Fender amps, instead of the regular 18W or Lite IIb scheme. There aren't too many hard and fast rules.

HTH

Thanks ZP, I guess I have been looking at so many schematics and layouts that I just didn't pay attention to that list of tweaks. I have one Lite IIb almost completed (EL84) and I am almost sure I did not use that schematic. or implement those twesks. I will go back and look. Thanks again for your expert, timely advice.

[zaphod_phil]

FYI the stock Lite IIb hasn't changed. Anything different would have been the original Lite II or Lite IIa. But that's going quite a long way back....