setting cathode bias?

[chuckb]

Hey guys, I'm trying to set the bias on my cathode biased amp. I have 17v across the cathode of each tube(el84's). I have 411v on the plates of each tube. Can someone tell me what the dissipated power the power tubes draw?

chuck.

[g1]

The missing piece of the puzzle would be the value of the cathode resistor, it is needed to calculate the current.

[chuckb]

The missing piece of the puzzle would be the value of the cathode resistor, it is needed to calculate the current.

hey g-one sorry forgot to add that .it's a 220uf/10w.

Chuck.

[chuckb]

sorry again,it's a 220ohm/10w.

[chuckb]

What's the cathode resistor? V^2/R, that is cathode voltage squared divided by cathode R, gives you plate (plus G2) power for both tubes. Divide that by 2 and that's the plate power for one. FWIW 411V is mighty high for EL84's. I'd like to see the plate power no higher than 12W and often get good results in the 10.5-11.5W region.

Is there a way that I can lower the voltage down from 411v?

[g1]

So 17V divided by 220R gives .077A (or 77mA) for 2 tubes. That's about 38mA per tube. The voltage across the tube is 411V minus 17V = 394V. Voltage x current gives watts, so 394 times .038 = 14.97 watts.
To reduce the idle dissipation, you would increase the value of the cathode resistor.

Edit: I ignored screen (g2) current, it is usually quite small and shouldn't affect the numbers much.

[Leo_Gnardo]

Is there a way that I can lower the voltage down from 411v?

Whups I had a brain poot... Two part calculation. First, find current. That's V/R, in your case 77 milliamps for both tubes, we'll assume they're matched so 38.5 mA each. Second, plate power = plate current X Vpk. Vpk is plate voltage relative to cathode, in your case 411 - 17 = 394v. 38.5 x 394 = 15 watts. Only 25% over spec.

Not so easy to remove excess plate voltage. What's easier is to select low emissivity EL84's. You could increase the cathode resistor value to the point it pinches off enough current to put you in the "safe" 12W or lower range, but I find that's often a losing game. As you raise the Rk, you'll find the Vp also rising. With a lower emissivity pair of EL84 you'll find the Vp also increases but the tubes will "settle" at a lower current and hopefully not exceed 12W.

Of course it's perfectly OK to run 'em at 15W plate but you'll have to accept shorter tube life, and listen to the innards rattle around as they loosen in the heat. Happens a lot in pro amps like Vox AC30's, like that.

[chuckb]

looks like I'll have to find a suitable power tranny? I'll try to lower the values by increasing the cathode resistor values and see where that leaves me?

[Leo_Gnardo]

looks like I'll have to find a suitable power tranny? I'll try to lower the values by increasing the cathode resistor values and see where that leaves me?

Tubes & resistors cheaper than transformers. Just for the record what amp is this? For lowering B+ there is a trick involving placement of a power zener diode in the center tap of a transformer supplying a full-wave rectifier, but I've never done it.

[Tom Phillips]

looks like I'll have to find a suitable power tranny?...

Have you considered converting the amp to fixed bias? Maybe an option.

[chuckb]

Well I changed out the power tranny and now I have 361v on the plates, 15 volts on the cathodes,and I'm now using a 270ohm cathode resistor. So if my calculations are right I'm getting 9.5v on each el84 power tube? Can you confirm my calculations to be sure I'm correct.

[Tom Phillips]

I calculate 9.6 Watts power dissipation in each power tube which is what I think you meant to write.

[chuckb]

Hey tom, yes that's correct. So my tubes are not running to hot now? I believe the max power dissipation for each el84 tube is 12.5 watts?

Chuck.

[Leo_Gnardo]

Hey tom, yes that's correct. So my tubes are not running to hot now? I believe the max power dissipation for each el84 tube is 12.5 watts?.

Tom must have stepped away for an inandout burger. Yes you're way on the safe side now. In fact you might be in crossover-distrortion territory. If you can't get a clean sound out of your amp, and you can see obvious crossover notch on your scope, you can increase your plate dissipation a bit by decreasing your cathode resistor. If you're satisfied with your amp's tone, leave 'er alone.

Nice to have a couple of PT's ready to hand - good for you!

[Tom Phillips]

Tom must have stepped away for an inandout burger...

I hope the amp sounds good to you Chuck. I was fully consumed by house chores for the rest of the day. I did try one of the new Jack-In-The-Box bacon insider burgers this week. I could see the bacon but couldn't taste it. In fact the patty flavor was pretty much like cardboard.