How hot is too hot for AC15-ish build?

[dbroekema]

Hello forum members,
I've just completed my first new amp after taking a multi year break (fatherhood + grad school), and I'm feeling pretty newbie-like. The amp is a half-power (2x EL84) AC30 built on a Hoffman board, in a Hammond chassis that is similar size to a Fender tweed chassis, and the amp is in a 5E3 cab. After chasing some hum and oscillation issues, I'm pretty happy with the amp and the voltages are in the ballpark. The only thing is, this sucker is HOT after playing for 30+ minutes, and I'm a little worried that it's too hot.

I've built a number of these as heads, but this is my first in a cabinet, so I realize that this could be contributing to the heat. But I wanted to ask you guys who hot you run EL 84's in such an amp. According to this website, https://robrobinette.com/Tube_Bias_Calculator.htm, I'm now running them at 115% plate dissipation. Is this crazy or within acceptable limits?

Details of the build: Herboer 18 watt PT and OT from the Hoffman store, NOS 5AR4, JJ EL-84's, 100 common cathode resistor, 307 V plates, 9 V cathodes. One weird thing is that the screens (100 ohm resistors) are about 10 volts higher than the plates. I've seen this in other builds before- is that an issue?

Would appreciate any thoughts or suggestions, Thanks!
Dave

[sluckey]

I have several 2xEL84 cathode biased amps. The EL84s run hot enough to blister your fingers. I use 130 or 150Ω cathode resistors. I recommend increasing your 100Ω resistor too. Don't worry about the screens being a little higher than the plates. Good air circulation in a combo amp is important. If you are concerned, put a small fan inside. I don't have a fan in any of my combos but I'd probably add one if I was gonna play it for several hours.

[dbroekema]

Thanks for the reassurance Stuckey.
I started with a 110, went up to 120, and then put a 100 in there as it seemed like the lowest dissipation (both plate and cathode dropped ~5 volts). Since there is a 5AR4 in there now, I'm thinking to stick a 5Y3 or 5R4 in there and see what that gets me. Would like to ideally see 100% or slightly less, but it seems like my stage amp was biased similarly to this one (115%) for years with zero issues. But I'm nervous about such things.
Dave

[HotBluePlates]

... I'm now running [EL84's] at 115% plate dissipation. Is this crazy or within acceptable limits?

... 100 common cathode resistor, 307 V plates, 9 V cathodes. One weird thing is that the screens (100 ohm resistors) are about 10 volts higher than the plates. ...

Plate Dissipation = Plate current * Plate-to-Cathode Voltage
Plate Current = Cathode Current - Screen Current
Screen Current = Voltage (across legs of the Screen resistor) / Measured Screen Resistance
Plate-to-Cathode Voltage = Measured Plate Voltage (to ground) - Measured Cathode Voltage (to ground)
Cathode Current = Measured Cathode Voltage (to ground) / Measured Cathode Resistance

You have Plate-to-Cathode Voltage of 307v - 9v = 298v, and a Cathode Current of 9v/100Ω = 90mA (assuming the cathode resistor measures exactly 100Ω). That's for 2 tubes sharing a cathode resistor, so we'll assume 45mA per tube (you could insert a 1Ω resistor between each tube's cathode and the top of the 100Ω resistor to know individual cathode currents). 298v * 0.045A = 13.41 watts.

But we didn't subtract out screen current, because those numbers were unavailable. So plate dissipation is less than the calculated 13.41w/12w = ~112%.

... I started with a 110, went up to 120, and then put a 100 in there as it seemed like the lowest dissipation (both plate and cathode dropped ~5 volts). ...

Dissipation went up with the 100Ω resistor, though it may have seemed to drop.

1. Plate voltage dropped because plate current increased. The output transformer winding has resistance, so when plate current increased then the voltage drop across the OT winding increased, leaving less at the EL84 plate itself.
2. Voltage drop across the cathode resistor dropped because the resistance itself dropped. 80mA (40mA per tube) through 120Ω give a voltage drop of 9.6v, but 80mA through 100Ω gives a voltage drop of 8v.
3. Plate current always increases if you reduce the bias voltage, all else being equal. If the cathode voltage dropped 5v (this amount seems unlikely) from ~14vdc to ~9vdc when using a 100Ω resistor, then bias voltage was reduced and plate current increased. It is likely that plate dissipation went up with the 100Ω resistor because the increase of plate current (as a percentage) is likely much greater than the drop in plate-to-cathode voltage (as a percentage).

Try raising your cathode resistor if you're concerned about tube dissipation, and go through each of the steps outlined earlier. If you want to be conservative (or don't want to take the extra steps), you can skip calculating the screen current and assume cathode current equals plate current. The tube will actually be running cooler than you calculate.

[dbroekema]

Great, thanks guys! Sounds like it's not in imminent explosion mode, I'll probably just leave it as-is.

[hesamadman]

One thing to take into consideration is cabinet design. If your amp is built like a fender style combo and your tubes are mounted on the bottom side of the chassis, heat rises. It needs to escape somewhere. As much as you can anyway. Things like an aluminum chassis or fender style switches will absorb heat. The switches can get hot to the touch. A vent in the rear of the cabinet and top of the cabinet make a huge difference and help airflow.

[PRR]

> running them at 115% plate dissipation. Is this crazy or within acceptable limits?

For "best service", never go over rated Pdiss.

> my stage amp was biased similarly to this one (115%) for years with zero issues.

Yes, guitar amps often do. Tube abuse is cheaper than a bigger amp. Also I think the "12W" Pdiss of the EL84 was conservative, specced low to avoid eating sales of larger tubes; and that all modern EL84 makers know their bottles will be cooked and build to reduce early failure.

> Plate-to-Cathode Voltage of ...298v, and a Cathode Current of 9v/100 = 90mA...

For the typical 8K loading, this should be more like 75mA.

This leads to a 130-150 Ohm cathode resistor. And probably get you in the Pdiss rating, near-enuff.

Screens can be higher than plates, often normal. However I think a HARD-worked tube can feature 500-1K screen resistors so current peaks (in clipping) drop G2 voltage and dissipation.

[dbroekema]

Thanks, PRR. I'll stick a 130 in there asap.

Problem is, I know this will raise global voltages on V1-3, and they are already higher than I'd like- I generally like V1 plate around 130-140v, and right now it's at 145ish.. Would raising all three of the power rail resistors remedy this? Right now I believe they are respectively 22K, 10k, and 22k, would raising these to say, 27K, 15K, and 27K work? Not sure how big the effect of +5K will be.

Thanks!

[tubeswell]

Increasing the resistance of the HT supply-dropping resistors for the pre-amp stages with lower the preamp HT voltages.


The amount of voltage drop desired can be calculated using Ohm's Law: V = I x R.


You will have an idea of the voltage you are starting with for the 1st node in the dropper chain, and you will know that each 12AX7 triode gobbles up about 1mA on average, and you can work out the screen current for each EL84. so you can work out the current being drawn through each supply resistor (from the sum of the current for the screens and all the triodes being supplied through first resistor, and then the sum of all the triode current being drawn through the 2nd resistor, and then the sum of the triode current being drawn through the 3rd resistor etc). Therefore you can work out the voltage dropped by any given resistance within the R/C supply-dropping chain.

[smackoj]

I felt like adding my .2 cents worth on this thread because the subject matter intrigues me. I don't need to add any of my 'shade tree' math to the mix since PRR and Sluckey know more math than I will ever know. But I do have a general sense of what's going on here so here goes. Sluckey says that two EL84s will fry your fingers and senor hesamadman added that "heat rises". That all makes perfect sense when factored in with the original design produced by Vox which used either two or four EL84s. Now Vox is known for 'hot' running amps and their Big Kahuna model that they produced to compete with Marshall back in the Beatles days, was reported to actually catch fire. They have made a lot of AC15 and 30 amps and they all have one thing in common, a nice plastic grate on top of each amp to let the tube heat get the heck outta Dodge. So keep the air freely flowing around those red hot finger stingers or add the already suggested aux. fan unit and play until the cows come home. If making tube amps sound 'exceptional' was simply a function of mathematics, then we'd all be playing the best amps on the planet. But other more earthy things are happening inside that 'black box' that can't always be explained with Ohm's Law. Like Joe Dirt's pappy said in the cult classic movie, "Joe Dirt" with David Spade, Dennis Miller and Kid Rock, ".....how does the posi-traction work on a '69 Dodge Charger? It just does."