Red plating el84

[hesamadman]

One of my 30watt (based off hoffman ac30 poweramp) has a red plating EL84. Switched tube and it still does it. Since all four tubes share cathode cap and resistor, all that it leaves is screen grid and control grid. Is there anything else it could possibly be?

[tubenit]

Switched tube

Does this mean that you switched out the tube for a completely different tube ............. OR  does it mean that you switched the positions of the 4 tubes and it red plated in a different position?

If it is the latter, the problem could be that you don't have a matched quad set of tubes.

And if you switched out the tube completely removing from the amp and used another one, .......... you could still have a quad set that are not matched.

I bought a "matched" set of 6V6's once that were not even close to being matched.  And one of them would quickly red plate.  I had built Doug's bias checker and it showed they were waaayyy off from being a matched pair even though they were sold that way.

with respect, Tubenit

[hesamadman]

Well I tried multiple things. I switched around existing tubes (just changed placement) and the same POSITION not the same tube red plated. I mean it glowed very bright. Now it could be an unmatched set. It could be something else. But In the case of this build.....it could be that I'm an idiot and did not space my circuit board far enough from the PT that's under it and the bottom side of a turret that's associoated with that tube socket is in contact with something it shouldn't be. I just wanted to get opinions first but I'm gonna dive into it shortly and see. 

[John]

If it stayed with the socket, not the tube, it's not the tube. You might very well have a solder hair or something similar that is grounding that socket's cathode pin to ground, which would make that tube a real conductor! You'll get 'er figured out, it's probably something very simple. At least, all of my mis-wires and mis-takes have been. 

[tubenit]

If it stayed with the socket, not the tube, it's not the tube. You might very well have a solder hair or something similar that is grounding that socket's cathode pin to ground, which would make that tube a real conductor!

+1 !!

You'll get it figured out.  Test voltages.  Quadruple check every connection around that socket. I've had something like the smallest wire fragment goof something up like this. 

Look for traces of arching on the tube socket, for the socket pins to make good contact,  for cracks in the tube socket, for touching wires (like a heater wire barely touching the plate or grid, make sure the cathode is making excellent contact to cathode resistor/cap, & make sure the screen has resistance to ground........... etc.......

With respect, Tubenit

[hesamadman]

Yep. Silly mistake. No tellin how long its been like this either. I didnt notice the red plating for a while because the amp worked it just wasnt as loud as my other 30 watter. It has a different OT so i thought maybe thats what it was. It has done it for so long that the red JJ written on the tubes is now a faded gray JJ. Oh man.


So the problem was the 1.5 control grid resistor wasnt soldered on. The solder missed the lead of the resistor. (that is the control grid right? bottom grid on an el84 is control and one above it is screen??)


But now I have another issue. I think I know what it is its just a matter of finding it. But i figured you could tell me what you think. Plugged in the amp after the red plate repair and no sound. Jiggled the input and it had sound. Oh no...brand new cable is going bad. Well that wasnt it. Changed the input....still only worked if I moved the input. Heres the kicker.......if i touched the chassis with my finger....the sound came back. (not holding the guitar). If i let go.....it went away. My thought is something is grounded out or a very very bad connection to ground somewhere. This is possible because I had to rais my board to clear the PT that is a through mount style.

[John]

So the problem was the 1.5 control grid resistor wasnt soldered on. The solder missed the lead of the resistor. (that is the control grid right? bottom grid on an el84 is control and one above it is screen??)

If it was a 1.5Kohm resistor, that was probably going to the screen.


For your next issue, "jiggling the input" to make sound. Are you saying the input jack itself is loose, or were you jiggling the guitar cord?.... And by sound, I assume you mean music. Anyway, check the wiring on the input jack. Make sure there's not a missed or broken solder joint on there. Without your guitar cord plugged in, all 3 lugs should show zero ohms to ground. With it plugged in, your tip lug should show 1Meg ohms to ground.


Remember Hoffman's Law: if it's wired right, it will work.