FX Loop Problem

[chunkitup]

Attached is my fx loop and voltages. When switched out of the circuit the amp is perfect. When switched in it crack and pops when I hit the stand by and I immediately switch the sb off. As I right this thread I am looking at the buffer and notice it has 380v on the anode and the JJ is only rated for 300v. Perhaps I need a dropping resistor(or another B+ RC node) to get the voltage down. Or perhaps the triode is bad. What am I doing wrong? The circuit is wired correctly into the amp.

[Enzo]

So slap a different tube in the socket and find out if it is a bad tube.

[chunkitup]

Well the only reason I didnt put another tube in is that I didnt want to ruin another triode if the anode voltage is too high. If 380 is ok then I will try another tube.

[pdf64]

The max plate voltage spec is with respect to the cathode. In the light of that, are you sure it's an issue here?
The bigger problem is generally the heater to cathode voltage (going over spec can cause the insulation to break down).

[chunkitup]

I am using elevated heaters. Well, with 80v on the cathode resistors the tube is dropping 300v. The guy that I built the amp for took it home for a spin without the loop and now he loves it so much he might not want the loop. But this is a learning thing for me so I have to get it right. I am trying to learn as much as possible from Merlins Book, which is awesome!

[Chuck H]

Also, not sure what else may be wired funny but without switching jacks in place the loop, as you have it wired, will not work. It will also likely raise the gain of the preamp considerably causing instability problems. How much this could be effecting your problem is hard to say. I do agree that your cathode to heater voltage is likely too high and I've read that JJ's are tender about that.

[chunkitup]

Chuck you are most correct about the jacks. I did not have a switching jack in my library for Tinycad so i just put the wire there. It is on the switch though. Also, reading thru Merlins Book he says that with elevated heaters the heater to cathode voltage should be ok. I have about 50v elevation going on.

[Chuck H]

With the heaters elevated I think your cathodes will be fine. As Enzo suggested you should try another tube. If the other tube behaves poorly I'll guess it's instability. You could also try plugging a volume pedal or gain variable EQ into the effects loop. Turn the volume all the way down on the device and see if things stabilize.

[chunkitup]

Update. I changed the buffer to a fixed bias arrangement and the signal in is identical to the signal out, buffer achieved. The problem now is the volume is alot louder with the loop in and I get a pop when I switch off the stand by. How should I remedy this? Perhaps a resistor to ground after the buffer coupling cap?