Champ Style Amp Control grid hum, whats causing this?

[Michael Inglis]

EDIT: I FIXED IT!!!! LOL, i read a paper about grid stopper resistors and it said they need to be as close to the tube socket as possible. In my previous layout i had one single grid stopper for each tube so the grid stopper was on the EL84 socket not the 6V6 socket. So i figured id rewire it so that theres a 1.5k on the 6V6 control grid pin wired to just before the the EL84 control grid resistor. And wouldnt you know it. IT WORKED! Im thrilled to have this working, its literally kept me up at night for the past week lol. Anyways....that was it. The 6V6 needed its own grid resistor lol.



The amp is a MOD102+ that ive modified to run either a 6V6 or an EL84 (they are on a switch that lifts the cathode of one tube to select on tube or the other). The octal and noval sockets for the 6V6 and EL84 are wired in parallel on every pin except the cathodes. So here is the issue. When i run the amp with only the EL84 installed it works wonderfully, same if i only have the 6V6 installed, also it works wonderfully with both tubes installed with the 6V6 selected. The only time it doesnt work is if i have both tubes installed and select the EL84. If i do that i get a really low frequency rumble unless the volume is turned down to 1 or 2 (out of 10).

My first assumption was that the cathode lift wasnt isolating one side completely but i checked and thats not the issue. Next i got a really good theory from Pete Farrington on here that it could be the insulation in the tube braking down allowing the cathode a path to signal and considering the 6V6 im using is actually a NOS 6N6C i thought that made a lot of sense. So i tried the amp with other tubes in the 6V6 position and at first it worked so i thought it was the 6N6C that was bad only the next day the EL84 wouldnt work with any other tube installed, it would only work if it was by itself again.

Now today ive finally realized that the wire connecting the control grids (running in between the 6v6/6N6C and the EL84) is whats picking up the noise and if i move it away from the other wires its near the problem dramatically lessens but doesnt go away entirely. So now i have to figure out how to route it where it wont interact with the wires around it. But im still not sure which wires its interacting with so i dont know which ones i should be trying to avoid. So im wondering if anyone read this and an obvious solution came to mind. In other words does anyone have any idea what might be going on here? Thanks for taking the time to read this!

 

[Marcomel79]

I would just pull the 6V6 when using the EL84. I did the same mod to a Champ i built and it sounds wonderful with either tube. Im no expert, but the only "problem" i see, is that the cathode of the tube not being used, will be heated for nothing. And over time, that might cause it to get depleated. But then again, dont take my word for it

 

[Michael Inglis]

I would just pull the 6V6 when using the EL84. I did the same mod to a Champ i built and it sounds wonderful with either tube. Im no expert, but the only "problem" i see, is that the cathode of the tube not being used, will be heated for nothing. And over time, that might cause it to get depleated. But then again, dont take my word for it

Your not wrong, it is putting unnecessary wear on the tube not being used. But with that said as long as that doesnt equate to a significant decrease in tube life i can live with that. The switch lifting the cathodes path to ground has been done enough i know its possible. I found that if i also lift the control grid on the 6V6 as well as its cathode when ive got the EL84 selected then everything works as expected. So i could just change the SPDT for a DPDT so i can lift the cathode and the 6V6 control grid. But what i still need to find out is if that is safe and if it will cause any issues.

I wouldnt think it would be a problem but im absolutely not knowledgeable enough on the subject to say with any certainty what so ever. Plus i could always run one tube at a time if i wanted. Id just like to have the switch in place so that nobody else could ever run the amp with both tubes at once. And being able to swap quickly without putting wear on the tube pins and sockets is nice. But again, your not wrong.

 

[FleshOnGear]

I wonder if keeping the unused tube’s cathode connected to ground through a very large resistor would help. I don’t really have any knowledge of any theory to support that, it just seems like it could be problematic to have a tube’s cathode totally disconnected while there’s still HT on the plate.

Like, keep a 100k resistor connecting the cathode to ground, and then when you switch the cathode resistor and bypass capacitor into the “on” position for that tube, the large resistor is simply bypassed. I dunno, just thinking out loud. I wish you luck.

 

[Michael Inglis]

I wonder if keeping the unused tube’s cathode connected to ground through a very large resistor would help. I don’t really have any knowledge of any theory to support that, it just seems like it could be problematic to have a tube’s cathode totally disconnected while there’s still HT on the plate.

Like, keep a 100k resistor connecting the cathode to ground, and then when you switch the cathode resistor and bypass capacitor into the “on” position for that tube, the large resistor is simply bypassed. I dunno, just thinking out loud. I wish you luck.

That is a thought i think everyone has had when looking at this. And it could be that it would be beneficial but in the few production amps like the Mesa Boogie "Blue Angel" for example they arent incorporating a resistor. At least not that i noticed. Ill go look in a minute just to be 100%. And as for my original issue this post was about i just now figured it out lol. The 6V6 needed its own control grid resistor. I made an edit to the post explaining my mistake basically. Such a relief!

 

[Marcomel79]

Your not wrong, it is putting unnecessary wear on the tube not being used. But with that said as long as that doesnt equate to a significant decrease in tube life i can live with that. The switch lifting the cathodes path to ground has been done enough i know its possible. I found that if i also lift the control grid on the 6V6 as well as its cathode when ive got the EL84 selected then everything works as expected. So i could just change the SPDT for a DPDT so i can lift the cathode and the 6V6 control grid. But what i still need to find out is if that is safe and if it will cause any issues.

I wouldnt think it would be a problem but im absolutely not knowledgeable enough on the subject to say with any certainty what so ever. Plus i could always run one tube at a time if i wanted. Id just like to have the switch in place so that nobody else could ever run the amp with both tubes at once. And being able to swap quickly without putting wear on the tube pins and sockets is nice. But again, your not wrong.

Also, in my amp, i have a common bypass cap for the 6V6 and EL84. For the the6V6 i have a 470R cathode resistor, and a 170R cathode resistor for the EL84. Plus i have a 10K screen resistor for the EL84. So the switch switches both cathode resistor and screen resistor.... mine is a head that sits without headshell, so its easier to swap tubes....