Just another wacky Tube issue (challange)

[AstroStrat89]

Fender Pro Junior, been in for filter caps once already.

Had some old RCAs in all four positions (2 x 12ax7 and 2 x el84) The amp started to make a hum noise. It would make the noise with nothing plugged into the amp and the volume had no effect on the hum. Just always there in the background.

Now here is were it gets a little strange. If you think about the volume knob starting at 0 and then going to 1 then 2 and 3 etc. In this case it was like my volume started at 1 then 0 then 1,2,3, etc. So turned all the way down was like having it on volume 1, then as I turned it up the volume would go down for a brief part of the sweep and then begin to go up and behave normally.

So instead of
0123456...

It worked like
10123456...


Now it get even more interesting.

I put all of the stock GT tubes back in and all these issues just go away. Bad tubes right? Well hold on.

I ordered a new pair of GTs el84s and 1 GT 12ax7-c for V1 (figuring it wouldn't matter what was in V2 as long as it worked). Remember at this point the amp is working fine with the stock GTs in all positions, I just want better tubes.

I swap all of the tubes (using one of the stock GTs for V2) and all these weird issue came back.

Come to figure out that its all in the V1 tube. I have 5 12ax7s. Two of them work just fine, the stock GT and a JJ's that I had. Both of the RCAs and the new GT 12ax7c have the issue. It doesn't seem to matter what is in V2 or what power tubes (new GTs, stock GTs, RCAs)

Is it the amp? How could 50 year old tubes have the same issue as a new production GT.

 

[rockon1]

I think its a intermitant amp problem perhaps a bad pot? Can imagine tubes acting like that. Bob

 

[AstroStrat89]

I think its a intermitant amp problem perhaps a bad pot? Can imagine tubes acting like that. Bob

I swear, when I use either of the "good" tubes the volume issue goes away.

I think I have a ghost or something.

 

[rockon1]

Who knows maybe it is something in the V1 circuit. Hopefully somebody will come along with a bit of insight. Bob

 

[trdlasvegas]

I'm not sure if this is it but I've seen the same thing happen on Hod Rod DeVille. I spent a lot of time working on it thinking it was a bad pot or bad ground, but in the end I am sure it was the V1 tube. I felt that the plate of 1/2 of the triode was actually bleeding into the other half of that tube. If you look at the schematic you'll see the volume control is in between each half of the triodes on the SAME tube. When I used a tube that had a metal shield between the plates, like a JJ ECC83S the problem disappeared. But if I used a tube without the shield like a EH 12AX7 I got the volume to bleed even with the Volume control turned all the way down.

Take a look at your tubes and see if this is not the case.

-Tony

 

[andrekp]

What happens if you don't 100% switch the tubes. i.e. just switch one at a time. Is there one tube that seems to control the problem? (can narrown down the tube and the circuit if it's not a tube). Really doesn't seem like a tube problem, but...

 

[phsyconoodler]

It could easily be a tube.I have swapped tubes many times looking for the quietest ones and there are glaring differences between 12AX7's.
Aslo the Pro Jr is plagued with tube socket pin contact problems.The pins don't grip all tubes the same.Some tubes are looser and the contact is poor with a loose-fitting tube.That can cause all kinds of problems especially with power tubes.Retentioning the pins can help.

 

[AstroStrat89]

I'm not sure if this is it but I've seen the same thing happen on Hod Rod DeVille. I spent a lot of time working on it thinking it was a bad pot or bad ground, but in the end I am sure it was the V1 tube. I felt that the plate of 1/2 of the triode was actually bleeding into the other half of that tube. If you look at the schematic you'll see the volume control is in between each half of the triodes on the SAME tube. When I used a tube that had a metal shield between the plates, like a JJ ECC83S the problem disappeared. But if I used a tube without the shield like a EH 12AX7 I got the volume to bleed even with the Volume control turned all the way down.

Take a look at your tubes and see if this is not the case.

-Tony

I was hoping this was it. But two things. The old RCAs (no shield) had been working fine for several months. The GT Chinese tube does have a shield between the triodes.

 

[rockon1]

I'm not sure if this is it but I've seen the same thing happen on Hod Rod DeVille. I spent a lot of time working on it thinking it was a bad pot or bad ground, but in the end I am sure it was the V1 tube. I felt that the plate of 1/2 of the triode was actually bleeding into the other half of that tube. If you look at the schematic you'll see the volume control is in between each half of the triodes on the SAME tube. When I used a tube that had a metal shield between the plates, like a JJ ECC83S the problem disappeared. But if I used a tube without the shield like a EH 12AX7 I got the volume to bleed even with the Volume control turned all the way down.

Take a look at your tubes and see if this is not the case.

-Tony

From what Ive seen placing the volume pot between the 1st and 2cd gain stages ( between each half of the triodes on the SAME tube) is very common. I have literally 100's of old stocck and NP 12AX7 tubes and many amps with this set up. None of my tubes have or cause issues like this. The fact that certain tubes work while others dont still seems indicative of an amp issue thats aggrevated by the design of some tubes over others. Bob

 

[dtube]

My guess would be a cold/bad solder joint somewhere on V1's cathode circuit. Or, intermittent contact on the cathodes in the V1 socket. Why the problem only shows up with certain tubes is beyond me. All the same, re-flow the solder joints on the cathode bypass resistors, caps, and their ground connections. Then, try to retension all the contacts in V1's socket.
-Darren

 

[AstroStrat89]

1st thanks to all for helping. 2nd my old tubes are GEs not RCA, not sure why I keep thinking they are RCAs.

Here is an update. I've had a lot more time to fool with it today and have some new info. I wanted to spend more time swapping out the EL84s to see what effect that had. None from what I can tell. I put the GT 12ax7-c (which has the "issues") back in and when I turned it on it had the issues as I expected. But this time I cranked it up and after about 5 or some minutes of playing it hard the volume issues went away. I still have the hum though.

Next I tried out the GEs one by one in the V1 position and I play them pretty hard but after about 10 minutes each they both still have the volume and hum issues.

So at this point I am just trying to decided if this is just a quirk of the amp to live with or have it serviced again. Its still under warranty but then again they could just tell me to use the tubes that work.

But I will look at the solder joints etc, I had an issue with my Marshall Lead 20 that a quick touch up with an iron cleared it up. But that amp is 25 years old