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EL34 glowing internally with signal...
Hi all, I have a project where I'm using EL34s (new Mullards) and I noticed that as I played I could see an orange glow through the hole in the plates that would pulse with the music. Is that normal? This amp has a b+ around 400-410 at idle and I have 1k 10W resistors on the screens (drawing ~5mA at idle), and I'm biased rather cool at 32mA. I don't really think I'm pushing it, but the screen resistors are getting pretty darn hot. I had another EL34 design with a higher B+ and 1k 7W screen resistors that I remember being able to touch after an hour of playing with the EL34's at max. Is there something I should check or am I just being a worry wart? Thank you all!!

(Sorry if this is in the wrong place, I didn't see a "Power Amp" section of the forum!)
Hi liquidair

It is normal to see the changes in the electron flow within the tube as you describe. If you see the plates turn orange then things are not quite right - possible bias failure, so turn off the power immediately.

We assume yor amp has typical voltage ratios?

In normal operation of 99% of tetrode / pentode push-pull output stages, Va is higher than Vs at idle. During the signal cycle, each plate voltage swings much lower than Vs getting withing a few tens of volts of ground. You will see a sine wave output for a sine wave input. If idle Va is made far lower than idle Vs, the input sine wave will distort into a triange and screen dissipation will be excessive.

Have fun
Hi Kevin! Thank you for your fast response!

Just for clarification, what do you mean by "typical voltage ratios"? I assume you mean that Vs is near but less than Vs, and that is the case. Good to see that this is normal, and I didn't see any signs of heating on the plates.

How hot can the screen resistors safely be? I tend to follow a rule of thumb that if it is too hot to touch, it's too hot. But I also know parts are designed to run much hotter than we can stand. It's weird because, I used some 5W resistors on the screens and they got hot, I upped them to some Welwyn 7W and those began to cook, I tried other 7W parts and those barely got warm. So I figured with some beefy 10W parts things would stay nice and cool, but these were toasty.
Hi Guys

For a given power rating power resistors can vary quite widely in their physical size and materials. For a given power dissipation, the smaller part will get hotter than a larger part. Data sheets for the components show how hot the manufacturer is comfortable in letting the device be operated, and what the life expectancy is operating continuously at these temperatures, and the actual figures can astound you.

All components with leads will initially try to dissipate their internal heat via the leads to the outside world and to whatever electrical conductors the leads are attached to. This is why some diodes - particularly those 3A singles - have such fat leads. The leads should be left as long as is feasible. Power resistors are the same and I've seen solder pads lifted from PCBs because a 5W wire-wound was subject to excess dissipation. The resistor was not discoloured or off-value, but the solder pads had to be cut off the board.

The screen resistor typically carries about one-tenth of the plate current as a maximum, but this is on an instantaneous peak basis. So, in a 50W amp with 4k-aa, Ia might be 300mApk, suggesting Ig2 peak of 30mA. A 1k resistor dissipates 900mW peak in that scenario. Usually the screen resistor is heated more by its proximity to the tube than by its own current / voltage self-heating

Have fun
(03-04-2021, 01:46 AM)K O\Connor Wrote: All components with leads will initially try to dissipate their internal heat via the leads to the outside world and to whatever electrical conductors the leads are attached to. 
Interesting. I guess that makes sense given the plastic body will act as an insulator. In my case I'm using cement resistors with leads that are more plates to stand them off the board.

I think I figured out what I'm observing and why heat is such an issue in this's the lower voltages. The OT is a 4.2k Ra-a which pushes the load line further down the Vg1 = 0 curve than the normal 3.6k used with EL34s, but then with a Va/Vs of around 400 the load line intersects well below the knee (Vsat~50V). This is the region where screen current should skyrocket. At just 420V Va/Vs, the slope of Is is much flatter and further limited by Vg1=0. That should explain why the last build's screens were much cooler.

I turned the MV down just a bit last night and after 30 min or so the screen resistors were barely warm. Opened it up and within about a 1 minute they were too hot to touch. Back down again and they cooled down extremely fast.

It also explains why in the previous build, I didn't feel comfortable with how hot the usual 1k 5W part was getting. I remember thinking "Marshall's use a 5W part here on amps that have been abused for 60 years without issue, why do I feel 5W is not enough??" Well, Va/Vs of 440V and a 3.6k Ra-a OT intersect Vg1=0 where screen current will be much, much lower than where I'm at with 400V and 4.2k Ra-a. 

Thank you Kevin for all your help!
Hi liquidair

The observations regarding MV position versus screen resistor heat reflect the fact that the amp is clipping and the plate cannot go any closer to ground than it already is. meanwhile, if the grid drive is still able to increase, the internal resistance of the tube may in fact decrease but any "excess" current from the cathode has only two places to go: to the screen and to the control grid.So, in a hard clip scenario screen resistor stress is maximised.

Note that there is no "usual" or "optimal" plate resistance per tube type. You may see commonly used values, typically initiated by a manufacturer's data sheet showing a possible application. The example is meant to provide a reference of performance not to denote a limitation or optimal condition of operating the tube.

Using the higher Raa at a lower Va and Vs does not in itself lead to hotter-running screen-stops. Rather, the use of a lower Raa might simply because the current can be higher at peak clean output.

Alsso note that Human skin temperature is around 24C. PTs are rated to be 40C or less at full power, although some smaller units stretch this to 60C. Power resistors can have temps up to 150C and tubes up to 250C. You can see that where the component may be quite happy with a 50C rise there is no way you can touch it even for an instant without it leaving a mark Smile Our discomfort with the hot part is more aesthetic and that it is beyond the Human Scale. Were you to measure Marshall's screen-stop temps at clipping you would be as alarmed as you are with those in your build.

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