Blues Junior Low Output

[acorkos]

Speaker is OK, changed all tubes, cleaned input and speaker jacks, correct high voltages present at power supply nodes/preamp tube plates/power tube plates and screens. negative bias voltage present at power tubes.

as i turn up the master volume and volume pots, the low hum gets louder, and a low level guitar signal can be heard, but that's it. higher pitch hum gets introduced as the reverb pot is raised. the amplifier appears to be amplifying, but the input signal seems to be getting squashed somewhere.

thanks for your help in troubleshooting this one.

http://www.schematicheaven.com/fenderamps/blues_jr.pdf

[Jazz P Bass]

That schematic is chock full of labeled voltages.
Time to troubleshoot.
Start testing the amp according to the test points.
The problem voltage should show itself.
That will indicate which stage is having issues.

[52 Bill]

It sounds like there is a break in the signal path. Start at the back of the amp and inject a signal into the driver stage at TP11 and work your way to the front of the amp. Look for broken wires, loose parts, unsoldered input jack, etc.

What color are the tube sockets? There have been a few bad runs of these in the past where the contacts were loosening and not making good contact. The bad one were black, the new replacements are brownish gray.

[acorkos]

the tube sockets are black...i'll tighten 'em up and see what happens. thanks!

[Mark Black]

This might sound silly, but are you doing your testing with the back cover off the amp? Reason I ask is those amps tend toward high frequency oscillation due to the physical layout of the output transformer wiring in relation to the driver tube wiring, and sometimes it's worse with the rear shield not in place.

If you have a scope that would quickly reveal whether there is oscillation (at too high a frequency to hear) present on the output. That kind of thing can cause the symptoms you report.

[Billy R aka DynaFreak]

Never run in the high freq oscillation, but I've replaced blown output transformers in few of those. small amps tend to get run wide open a lot. besides the low output, the hum is a clue - inbalance in the output stage . Not that this is a definitive test, but have you checked the resistance of the OT primary from each side to center tap? The other thing i would double check is the phase inverter.

[Enzo]

There are no FX loop jacks, but turn the reverb up and shake the pan. Does the spring crash come out loud and clear or is it troubled? That tests the power amp.

I think Bill is on the right trail. The fact you can turn the hum up and down with the controls makes me think the signal is getting lost early.

[acorkos]

...makes me think the signal is getting lost early.

very early...'twas the input jack. bad innards