EL84EH screen current?

[krash]

Hey all-

long time no post.

Anyway, recent amp build I did (still on the bench) is using Electro-Harmonix (so they say) EL84EH tubes. Actually the tubes are labeled SOVTEK, so who knows, but according to AES, they are EH.

Short story, the amp has a seriously bad distortion sound that I have previously found associated with screens too hot. When I went about calculating for this amp's voltages, I got an alarming number.

I have a 2.2K screen resistor per tube, 1K dropping resistor to the screens from B+ (and 22uF cap). B+ is 318V, screen voltage (top of the 2.2K resistor) is 304V, screens are sitting at 292V, cathode is 9.44V (130 ohm resistor), my math says I have plate dissipation in the 11W range and right at 5.5mA of screen current.

that's:

(304V - 292V) / 2200 = 0.005454 A

I can't find a Sovtek datasheet for their EH tube but most datasheets show screen current for EL84s @ 5.5mA max.

So, is this too close for comfort? I don't think I've seen it this high in my other amps but this is the first time I am using EH EL84s. I have had problems with screens cherry red and also redplating with non-EH Sovtek (worn) EL84s in test amps with B+ ~350V.

[TheGimp]

I have not found an EH data sheet. However, The Tube Store states the EH is a faithfull Mullard reproduction with increased dissipation.

The standard EL84 is spec'd at 2W screen dissipation and you are below 1.6W.

Therefore I could conclude that you are in the SOA (Safe Operating Area) of the tube, although the 91% plate dissipation is a bit above the recommended 75% for long tube life. Guitar Amps tend to run the tubes hot so this is probably normal for a guitar amp.

[jjman]

5.5ma screens at idle sound way to high to me for an EL84. Especially with dedicated 2.2k's on them. However I don't have actual EL84 examples and am not easily finding schematics that would quantify.

My 4 6v6 amps idle 1-3ma on their screens.

[Phil_S]

According to Duncan's TDSL, AB1, 1.6-22mA for Ig2, depending on operating voltages and whether it is fixed or cathode biased.

[krash]

22mA?!?!?

Anyway, I have some other EL84s kicking around, gonna try them and measure the draw.

[Gaz]

Just a thought: Have you observed the screens in a dark room while driving the power tubes? Abused screen grid will undoubtedly glow orange to a hot white under overdriven conditions. This is the worst with pentodes (in guitar amps) including EL84s, as they have a much higher screen draw than tetrodes. if they don't glow, with signal, you're probably cool

[krash]

Well, I don't think my problem is the screen grids, unfortunately.

I put in different tubes, one old Sovtek, one old JJ, and the screen draw is in the same ballpark and the bad sound is absolutely the same.

It's a harsh sudden brittle distortion like it's clipping somehow. Sounds like arcing to me. I have been over the amp with a fine tooth comb and can't find a single problem with it.

In fact when I remove the PI tube and jumper from the output of the first gain stage straight to the EL84 grid coupling cap, it produces the very same distortion sound. Maybe it's a bad OT? Tried different power tubes, no dice. Added 130 more ohms cathode resistance, same problem. Pulled the cathode cap on the power tubes, same sound.

??

any clues?