Ampeg V4-b, bad 6k11 stage...

[parentheticalfact]

I've been working for a while trying to find the issue with an ampeg v4-b amplifier a customer of mine brought in for repair.

Now bear with me, this is kind of a wordy and involved question.

This amplifier has a constant pink noise type sound with crackling and the signal is very weak and distorted.
I signal traced the bad sound to the pin 2 of the 6k11 (the output of the first gain stage the 6k11 produces).
With the tube removed pins 2, 5, and 10 (where b+ is being fed to the different sections of the tube) show high voltages that coincide with the original ampeg schematic. However when I reinstall the tube pins 2 and 5 (which are fed by resistors) drop dramatically to about 40 volts on pin 2, and 8 volts on pin 5. Pin 10 remains high.
I tried replacing the 220k and 470k resistors that feed b+ to pins 2 and 5 respectivly, but the same problem. I ordered a 6k11 (philco) tube from CE distributions and had the same problem when installing this tube. Is this an issue of 2 shorted tubes? Is there something else I should be checking for?
Here is the schematic for anyone to look at.
http://www.schematicheaven.com/ampegamps/v4bpreamp.pdf
If anyone has any insight as to what might be going on that would be great!

[Gtr_tech]

Check for DC on the grids?

[jrfrond]

Well, pin 10 can ONLY remain high, as it is a cathode follower, so that is no indication of anything. I really don't think you have a shorted tube. Personally, I've never seen a shorted NOS 6K11, only ones that got noisy or plain tired.

You MIGHT have a bad cathode reistor, but my FIRST suspect would be the .01 coupling cap from section 1-2. If it were shorted, it would drive section 2 into cutoff and upset the DC balance. Lift one end of the cap, plug the tube in, and see if you have your proper DC voltages. You only have a handful of components there, but I'd start with that.

You might also have a carbonized and/or contaminated tube socket. Take a look at the socket and see if it looks burnt and/or brittle in any areas.

[parentheticalfact]

Right you are about the last stage. I forgot it was a cathode follower when I was writing out all of my voltage findings and trying to visualize what was going wrong.

The .01 coupling cap was the culprit the whole time! I de-soldered the cap and everything looked peachy, so I put in a brand new cap and the amp sounds wonderful.
Thank you for your expertise. You must be a valuable tech in your area. And what a learning experience.



Well, pin 10 can ONLY remain high, as it is a cathode follower, so that is no indication of anything. I really don't think you have a shorted tube. Personally, I've never seen a shorted NOS 6K11, only ones that got noisy or plain tired.

You MIGHT have a bad cathode reistor, but my FIRST suspect would be the .01 coupling cap from section 1-2. If it were shorted, it would drive section 2 into cutoff and upset the DC balance. Lift one end of the cap, plug the tube in, and see if you have your proper DC voltages. You only have a handful of components there, but I'd start with that.

And to the other reply, there was some dc on the grids, but it was a very small voltage and I didn't know it would throw things off as badly as it did. I will check the rest of them and replace any of the remaining coupling caps that are passing dc.
Thank you for your help!

You might also have a carbonized and/or contaminated tube socket. Take a look at the socket and see if it looks burnt and/or brittle in any areas.

[jrfrond]

Glad I could help. Wanna e-mail my boss and tell him that? ;)