Shoot me. Please

[daz]

So as i said in another thread my amp was sounding horrid all of a sudden and I thought I fixed it by cleaning and retentioning the preamp sockets. It was good for a couple days then it started doing similar things so i changed the socket. Now the issue is gone and another one cropped up in it's place. I-am-effing-sick-of-this-amp !!! seriously tired of this. So i'm asking if any of you can figure this out because it;s making zero sense to me. Symptom:low DC on the plate (about 80VDC) of the 1st stage and low dull gain. Stage 2 with the same cathode and plate values is about double the voltage. What i cannot understand is both plates are fed from the same place, yet after changing the plate R and cathode cap and R the voltage is still low. I checked and rechecked the socket to make sure all the connections are correct and reflowed each one again. Change tubes 3 times. What in gods name could account for 80v on V1's plate and double that on V2's? I am seriously ready for the funny farm, and i ain't laughing.

[loudthud]

80V on the plate sounds like grid leak bias. Check the grid leak resistor and it's connections.

[daz]

80V on the plate sounds like grid leak bias. Check the grid leak resistor and it's connections.

It's the input stage so i assume you mean the 1M on the in jack, which i already checked when it first started sounding wrong. Hell, i checked all parts since there are one a handful if that on V1A.

Oh, and another symptom is the guitars pot is hideously scratchy as i turn it and thats on any guitar i use.

[SoulFetish]

It's the input stage so i assume you mean the 1M on the in jack, which i already checked when it first started sounding wrong. Hell, i checked all parts since there are one a handful if that on V1A.

Oh, and another symptom is the guitars pot is hideously scratchy as i turn it and thats on any guitar i use.

Dude thats a little scary, that's usually a symptom of DC on a potentiometer. check the DC voltage on the grid of V1

[g1]

Sounds like DC on the grid which would make the guitar pot scratchy.
Maybe you got a funky tube socket?

[loudthud]

Oh, and another symptom is the guitars pot is hideously scratchy as i turn it and thats on any guitar i use.

That is a symptom of the low plate Voltage. Short the grid to ground. Does the plate Voltage pop up to normal? Ohm out every wire and connector. Take nothing for granted. Plug in a guitar cord and Ohm from the tip at the guitar end to the grid. Ohm from the sleeve at the guitar end to the chassis.

[daz]

Sounds like DC on the grid which would make the guitar pot scratchy.
Maybe you got a funky tube socket?

I just changed it, albeit to a used one but still....2 bad sockets? I suppose it could be but i'm not changing it again unless it becomes the last thing left.

That is a symptom of the low plate Voltage. Short the grid to ground. Does the plate Voltage pop up to normal? Ohm out every wire and connector. Take nothing for granted. Plug in a guitar cord and Ohm from the tip at the guitar end to the grid. Ohm from the sleeve at the guitar end to the chassis.

No, it actually went down another 20v.

[SoulFetish]

I just changed it, albeit to a used one but still....2 bad sockets? I suppose it could be but i'm not changing it again unless it becomes the last thing left.



No, it actually went down another 20v.

Okay my man, we need voltage readings at each node and resistance measurements between them. Take some readings, just like LT says, and report back .

[g1]

You said you changed the socket and another issue cropped up. So I don't want to rule out the replacement socket. I'm thinking arcing/leakage, not intermittent connection like the first socket had.
Can you measure the voltage across the plate resistor, from end to end?

[daz]

Okay my man, we need voltage readings at each node and resistance measurements between them. Take some readings, just like LT says, and report back .

I did, they're all normal except V1 which feeds from the same place as V2, both using 100k plate R. V1A's grid reads 500mV by the way. Resistance between nodes would be the value of the droppers, so i think u r looking for a bad one but i just checked and they're all reading 10k.

[daz]

You said you changed the socket and another issue cropped up. So I don't want to rule out the replacement socket. I'm thinking arcing, not intermittent connection like the first socket had.
Can you measure the voltage across the plate resistor, from end to end?

About 150v or thereabouts. But i don't think it;s the plate resistor because i've tried a couple.

[g1]

150V across a 100K means 1.5mA flowing.
So the cathode should be at around 2V if it's a 1K5. What value is it and what voltage at cathode?
150V across the resistor and 80V at the plate means the supply node is 230V there, is that correct?

[Enzo]

I don't think he is worried about the resistor, the voltage across it tells us about how the circuit is performing.


I one time installed a socket, thought I was doing pretty nice work. In fact, when I was done I stepped back and thought my I didn't scorch any wire insulations, all the joints are shiny, and so on. Quite pleased. Plugged it in, didn;t work. Took a while to figure out I had carefully and professionally wired all the pins one off clockwise.

Maybe I missed the meeting or didn't get the handout, but what the heck are we working on and where is the schematic?


Oops, he beat me to it.

[daz]

Nevermind, i found it. Thanks for all the replies and sorry to have you all running around looking for answers when it was my slop that was to blame. Solder blob from V1A cathode shorted it to ground. Homer moment.....

And i really mean it....thank you all for always chiming in to help when I ask questions here. You guys are

[g1]

I was wondering how you could make the current go up when you grounded the grid.
Glad you got it fixed.