Blues Junior distorted and low volume

[jvm]

I have a Blues Junior that sounds low on output and has a raspy distorted signal at R16. I have a clean, strong signal at R15 and also at R17(?) Should I have a audio signal at the cathode resistor? Cathode voltage is 1.4 vdc


Blues_Junior.pdf

[g1]

At the phase splitter cathode (R15) there should be around 54VDC, not 1.4.

[The Dude]

Check the voltages at the plates of your PI tube. My bet is you have an open plate resistor.

[Jazz P Bass]

Here is the schematic.

That link above is a dud.

[jvm]

Sorry about the dead link. Thanks for posting the schematic, JPB. The problem is not with the PI. The proper schematic is for the MIM Revision C. R15 is feeding the grid of the recovery stage after the tone stack. I have a clean signal at the grid, distorted at the plate and also discovered by accident that I have a clean signal at the cathode(?) I have replaced the coupling cap C8 to eliminate it as a potential problem. I have re-tensioned all the tube sockets as this amp has the infamous black plastic sockets

[Jazz P Bass]

Here is a direct link to RevD.
http://support.fender.com/schematics...atic_Rev-D.pdf

Rev C is on page 20 of the SM that I uploaded.

As to your problem.
If you are using a scope, crank up the amplitude on it to make sure the signal is clean, from the input jack all the way to the grid, after R15.

If the scope screen is set too small it may be masking an unseen distortion.

The fact that the plate is distorted & the cathode is not, leads me to this conclusion.

Have you resoldered all of the preamp tube sockets?

[Jazz P Bass]

Another thought: what is your test set up?
These amps do distort & that gain stage after the tone circuit is exactly that: a gain stage.

[jvm]

I'm just signal tracing with a audio amp and probe. I have re-soldered all the tube sockets. I have also subbed a full set of new tubes. Is it normal to have an input signal also present at the cathode?

[gbono]

Check out this discussion - see the end (2nd to last post)......

http://music-electronics-forum.com/t31715/

[Jazz P Bass]

I'm just signal tracing with a audio amp and probe. I have re-soldered all the tube sockets. I have also subbed a full set of new tubes. Is it normal to have an input signal also present at the cathode?

Yes, the signal has to be present at the cathode.

Keep in mind the tube itself is a circuit.

Is there an actual problem with the amp?

If you will take note of the test signal amplitude on the schematic input, it's 10mv's.

Kind of low to me but that is what you need to see at that point if you are going to follow the schematic test points.

The overall rating of the amp is 15 watts/ @8 ohms , 5% distortion, with a 50mv input signal.

[jvm]

Jazz P Bass, I think you are correct that I am just hearing that gain stage do what it is supposed to do. It started seeming to me that it was behaving as if one output tube was not conducting. I first checked PI plate resistors and then verified signal at grid of V4. I then removed each EL84 individually. Works with V5 only, no output with V4 only. I found R35(100ohm) open and replaced it. Amp sounds good for a few minutes then V4 starts to redplate.

[g1]

Likely V4 is bad which is why R35 went bad. Swap V4 &V5, if the problem stays with the tube, replace it.

[jvm]

Bingo, g-one. Red plating followed the tube. I replaced R35 and put in a new set of tubes. Amp sounds great now. Thanks to all for your help. You guys are the best!