Troubleshooting SE homebuilt amp

[Twangmaster]

Hey All,

I built a 5C1 champ style amp set-up to run an EL34 with 330ohm cathode resistor or 6V6 with a 500ohm. It recently started overheating the power tube and cathode resistor. It does not matter if I have it set-up for the EL34 or 6V6, either will become red hot. It did not do this when I first built the amp a year ago,so something has went bad but I can't seem to find it. Any Ideas or input will be greatly appreciated.

Thanks, Twang 

[bigdaddy]

What do you mean overheating?

What are your voltages on the power tube, plate, screen, bias....???? What is your B+ voltage?

[Twangmaster]

What do you mean overheating?

Like red hot colateral damage overheating, the cathode resistor and power tube are about to flame out but I shut the amp down. When I first noticed the problem the volume started dropping and I could smell the cathode resistor cooking.
 
What are your voltages on the power tube, plate, screen, bias....???? What is your B+ voltage?

With the EL34 in B+=292v, Pin3=272v, pin5=1.5v, pin8=64.3v

Thanks for the reply!

[sluckey]

Pin 5 should be zero volts in a 5C1 amp. +1.5v will cause the tube to run hot. Disconnect the end of the .02µF coupling cap that is connected to the volume control. Does pin 5 now read zero volts? If so, replace that cap.

[bigdaddy]

I remember another guy with a Champ having the same problem and the 220K resistor to ground off that cap was not securely grounded. So it could be a cold solder joint, bad cap or resistor.

[Twangmaster]

With 1 end of the .02 coupling cap lifted, pin 3 of the power tube now measures 0.9v. That's not right is it?

[Twangmaster]

EL34 330R cathode
B+=292v
Pin1:=64.3 
Pin3:=272v,
Pin4:=252v
Pin5:=1.5v,
pin8=64.3v

Isn't the voltage on the tube cathode way off? Would a bad filter cap or ot cause this?
Thanks!

[zendragon63]

If what you have is basically the same circuit as the 5C1--and it was working before--you might check the health of or replace that 1 meg pot on the grid of the EL34 for starters. The short explanation is that if the resistance of the grid leak climbs too high, it does allow a positive grid current condition and the tube to start running away thermally. Regards

dennis

[Twangmaster]

Thanks for the reply zendragon, I'm also a Dennis by the way

I have the grid wire pin 5 disconnected at the EL34 tube socket and I still have 1.5 volts to ground off the tube grid. I have tested my power tube and have tried a different EL34 that tests good also with the same results. Measures 64v on the cathode and 1.5v on the grid!?!

[sluckey]

I have the grid wire pin 5 disconnected at the EL34 tube socket and I still have 1.5 volts to ground off the tube grid.

Don't do that. The tube grid must have a dc path to ground.

Connect a jumper between pin 5 and chassis. Does the cathode voltage get happy now? If so, then you have a bad connection between pin 5 and the pot, or the ground connection to the pot is bad, or the pot is bad. Sounds like you already eliminated the coupling cap.

If the cathode voltage does not get happy with the grid grounded, the socket is likely bad.

[zendragon63]

Twangmaster, if you got what you got--by lifting the leak from ground--you probably zeroed right in on the problem and the fix. Socket notwithstanding. Hopefully no tube or other damage done. Heck if it were me, I would have already turned that EL34 into an ash tray. 

If it turns out to be the pot, you might consider sizing the replacement to include a fixed grid leak resistor to ground--like is routinely done on most PPIMV. Regards

dennis

[Twangmaster]

Thanks everybody for the help. The amp is now a lot happier as well as the owner! It seems I added and extra .022 blocking cap after the volume pot on the EL34 control gid. :sign5: Oh well, you learn something new everyday!
I now have:
  B+=367v
Pin3=350v
Pin4=340v
Pin5=0v
Pin8=22
Cathode R330
According to my bias calculator I need about a 250 ohm cathode resistor to get the EL34 into the 25watt dissipation range. Again, thanks everyone. :icon_thumleft: