Screen current on EL34

[cryptozoo]

Messing with an amp that runs a single EL34 in the power section and am getting around 12.6ma across the 1k screen resistor. I believe I've read somewhere that you want to stay at 10ma or under, but I've been unable to locate the reference again and haven't found it in the data sheets I've reviewed.

Can anyone chime in on why 10ma would be the max? Am I in danger of screen meltdown?

[NateS]

Messing with an amp that runs a single EL34 in the power section and am getting around 12.6ma across the 1k screen resistor. I believe I've read somewhere that you want to stay at 10ma or under, but I've been unable to locate the reference again and haven't found it in the data sheets I've reviewed.

Can anyone chime in on why 10ma would be the max? Am I in danger of screen meltdown?

EL34 screen is rated for 8W. At typical usage in guitar amps, you should probably be safe under 17mA or so. That's MAX though, not idle. Idling it should probably be more like 3mA (going from memory). What's your screen voltage and plate cathode current?

[Mick Bailey]

In an SE amp you'd expect a little more idle current than in AB. Mullard spec sheets show typical values as 15ma screen for class A with a screen & plate voltage of 250v and 13ma for screen & plates at 300v. 10ma max would depend on the screen and plate voltage.

But spec sheets are conservative to ensure long tube life. Guitar amps run tubes harder.

[big_teee]

The Valve wizard has a good read on this.
The Valve Wizard -Single Ended
Also I've read on here where some techs use 1Ks for 6L6s, and use 2Ks for EL34s.
That is to prevent screen failure.
T

[cryptozoo]

Thanks for the replies, fellas. I figure I'm probably okay at the current current (get it?), and so far, so good. One thing that has puzzled me a bit is the data sheet plate voltage limitations for a single EL34 power section. Most state 250–265v max for class A operation, but I regularly see schematics online and know of a few production SE amps purporting to use an El34 at much higher plate voltage (350-400v).

[big_teee]

Thanks for the replies, fellas. I figure I'm probably okay at the current current (get it?), and so far, so good. One thing that has puzzled me a bit is the data sheet plate voltage limitations for a single EL34 power section. Most state 250–265v max for class A operation, but I regularly see schematics online and know of a few production SE amps purporting to use an El34 at much higher plate voltage (350-400v).

Per Valve Wizard:

"Referring to the data sheet, we can see that an EL34 can be operated from a very wide range of HT potentials. Other single ended EL34 designs commonly use between 250V and 400V. The lower end of the range will suit lower transformer impedances, and vice versa. In the following example we'll use 300V.
Incidentally, although the El34 data sheet claims the maximum quiescent anode voltage is 800V, this applied to current production in the 1960's. Modern EL34's have questionable reliability at such high voltages, so it would be wise to restrict ourselves to an HT of less than 500V."

So it sounds like the data sheets are for NOS tubes, and data may not apply exactly to modern day tubes.

[mhuss]

So it sounds like the data sheets are for NOS tubes, and data may not apply exactly to modern day tubes.

Certainly the specs have gotten a lot looser since the golden age of tubes, but this is particularly true of high voltages. I generally recommend people stick to the 500vdc and under range, everything is much easier (and safer).