my se el84 running at 370 vdc ?

[john_t]

My GA-5 build with EL84 output tube is running at 383 vdc  on the plate - about 13vdc  on the resister. The amp sounds pretty good. Real tight over drive, but I am concerned about the voltages. If correct I,am way over the operating design of this tube. Any advice on this. I guess I could put in a 6v6. or even change out the transformer. Or should I just runthe amp hard and see how long the tubes last.

[John]

Or should I just run the amp hard and see how long the tubes last.

Well, don't know if you should or not, but that's what I'm doing. 

Actually, this winter I want to dig into mine again and experiment with dropping resistors, etc. and see if I can't get the voltage closer to 300 instead of 400. I think the overdrive would be much nicer, less "crunchy" like it is now. But it does have all kinds of headroom right up to about 7 on the volume.

[tubesornothing]

No problem - I have a number of EL84 amps running at that voltage.  When it gets about 400, then use a variac or something.

[john_t]

I  did hook mine up to a variac  and try a few different voltages. At a little under 300, 270 or so  on the plate voltage I thought the bottom end was a little sloppy or loose sounding. at around 330 or so I thought it sounded pretty good. In fact hard tell tell the difference from running at the 370 mark. and I thought the amp would run a little cooler.

[phsyconoodler]

Just regulate the screen voltage so it doesn't go much over 320v and the current so it's not over 14 watts dissipation and your will be just fine.
  I run lots of amps with EL84's at 420v.I just throttle the screens and current back and they live.

[tubesornothing]

Yeah, 330 would add a lot to the tubes life.  DR Z Maz 18 run at 360 B+ with  those voltages.  They fry tubes real nice!
 

...but sounds great.

[john_t]

Phsyconoodler, thanks for the help. Now I need a little advice on knocking back the screen currant and wattage. Do you have advive on resisters and values to replace. Thanks all for the replies.

[phsyconoodler]

Increase the screen resistor until it's at 300v or close.Same with the cathode resistor.then go to webers' bias calculator and plug in some numbers to get your dissipation.

[john_t]

phsyconoodler, I.am using 250 ohm 5 watt cathode resister and 1k 3 watt on the screen. Can you recommend a few different values that may be appropriate. I am going o order a bunch. There cheap enough to experiment with.

Thanks John t

[phsyconoodler]

you didn't mention what the screen was at now.That will be your starting point.

[john_t]

I,am reading 13.2 on pin #3 and 386 on pin 9 of my output tube

[phsyconoodler]

Increase the screen resistor to 2k 5 watt and re-measure the voltage at pin 9.Also you need at least a 390 -470 ohm cathode resistor.
You are at about 19 watts dissipation right now.That EL84 will last a very short time at those current levels.

[john_t]

I may have some of the resisters I need. I have a few extra 5 watters from some other builds. Would I be better off to change out the transformer for one with a lower secondary. I appreciate the help

[phsyconoodler]

Resistors are cheap.Change em out and be happy.

[john_t]

ok my resisters came in. I replaced the 250 cathode with a 360 5 watt and the 1k screen with a 5 watt 2k. the voltage went up on the plate a bit. Up to 393. I have 14.2 across the cathode. My measurement from pin 3 to the plate direct is 379. If my calculations are correct that puts me at about 14 watts plate dissipation. Should I try to get the screen down lower.   I have a 500 ohm and a 2.7 watt if I need them.

[jojokeo]

You could also install a 470 to 1k 5w - 10w resistor btwn B+ and the first main/resivoir filter cap and lower all of the voltages a bit. You lower this enough and it simulates a tiny bit sag. I don't notice it enough though really in SE situations w/ these values and are fine to try or use. See what you think, it's easy and fast to do or put back if you don't like it.
There's the trial and error way to sub various resistors and then you can calculate a better educated guess for both the screen and power dissipation values if you are okay w/ a little math?

[john_t]

ok I must be doing something wrong. I have changed the screen resister on my GA-5 / schedule forty build from the original 1K to a 2k and now a 2.7 K 5 watt resister and the screen voltage has not gone down at all. In fact now that I changed out the cathode R from  250 ohm to a 500 ohm the screen voltage is higher. Am I missing something. I was trying to get the screen down around 300

[jojokeo]

Screen Max dissipation = 2w, so @ 300v this will be 6.7mA
If you still have ~386v and want 300 you'll have an 86v drop needed.
86v/.0067=12.8k resistor needed. Use nearest value
For power rating of resistor, the formula is VdropxVdrop/R's value x 2
so, 86x86=7396/12.8k=0.58x2(for safety)=1.16 watt resistor needed. You could get by w/ 1 watt for dialing in but for long term and peace of mind I would go w/a 2watt.

This condition is if the tube draws max dissipation and won't likely happen when you're measuring. So the value could go higher to drop where you need it.

[john_t]

JoJOkeo, thanks I would never had tried such a large jump. Iam up tp a 27 K 3 watt, screen is now at 345. Iam using a 500 ohm cathode resister. my plate is at 396 actual voltage. I might go down to a 470 ohm. I notice when I went up from the 250 ohm on  the cathode so did my plate voltage. Il keep you posted. thanks for the help.

[phsyconoodler]

As you lower the current,the plate voltage will rise.That's normal with less load on the PT.
  Are you measuring your actual plate voltage from the cathode to the plate?Not to ground,that's not the actual plate voltage.
you have lowered the screen but a bit more wouldn't hurt.Don't worry about a little more plate voltage as long as the dissipation is lower and the screen is lower than the plate.345 is not killer hot.

[john_t]

phsyconoodler, Yes the 396 measurement was cathode to plate. IT is 409 or so pin 7 to ground. The 27 K is the last 3 watter I have laying around here. I will have to get a few more values to experiment. Thanks so much for the help. I think Iam getting close to where This amp needs to be. I had concerns based On what I had read To big a value on the screen has ill effects on on amp. IT still seems ok At this point. Definitively less volume and headroom though. But tolerable

[john_t]

My new batch of resisters came in.  I have the voltages about where the recommendations suggested.

397 V pin 3 to 7
12 V p3 across a 470- ohm 5 watt resister. I think that puts at about 10 watts.
308 V 9 pin 9

I measured 105 v across the now 47K 3 watt resister.

Will a 3 watt resister handle that kind a drop. Iam not sure.

 

[Boots Deville]

I measured 105 v across the now 47K 3 watt resister.

Will a 3 watt resister handle that kind a drop. Iam not sure.

W=V^2/R = 1/4W in this example. 
Doubling for safety would be 1/2W
3W should be fine.

[john_t]

Ok I have been running the amp a fare amount. So far so good. It is no where near as hot as it was before. If there are any other schedule 40 builders out there I would still like to see your voltages and how your projects came out.

Mean while thank you everyone for all the help.

I am already onto my next project