Blues Junior sounds muffled

[opauly]

Hello! My Blues Junior (green board) sounds muffled/distorted even with low gain. Tubes are OK (tested with a tube tester and swapped with good ones). Bias seems to be fine. Checked resistors, caps, speaker, and seems to be everything fine. I am getting crazy! Any ideas?

 

[turk 182]

these amps have input jacks that are directly soldered to the pc board, if there has been any mechanical stress from a jack cord pull or it's been knocked over on it's face with a cord plugged in it's quite likely it's developed cracked solder joints.

 

[JonSnell Electronic]

Check the Anode load resistors on the ECC83s. R3 (100k), R10 (100k), R17 (91k) and R19 (100k).

 

[JonSnell Electronic]

They are off pins 1 & 6 of the ECC83s. The new model uses different indent numbers but the values are the same.

 

[opauly]

I already tested R3 (100k), R10 (100k), R17 (91k) and R19 (100k) but they seem to be OK. Also tried replacing input and output 1/4 jacks, but the same result.

 

[Malcolm Irving]

I wonder if the speaker could be rubbing or something like that. Can you try into a different speaker?

 

[opauly]

Yes, already tested a different speaker and the ugly distortion is still there.

 

[dotneck335]

Try this:
Billm Audio >> Why mod your Blues Junior?

 

[alkuz1961]

Recently I fixed a similar issue. Just check the resistors of the second grids of the output tubes. They have very little power and often fail.

 

[opauly]

Thanks for the hint. How do you check them? I measure continuity between each resistor terminals to check if the resistor has blown, but I am not sure if it would be better to unsolder them and measure ohms as single components. What do you think?

 

[alkuz1961]

In my case, it was just a contact break inside 100 ohm resistors. The second common problem with these amplifiers is the leakage of electrolytic capacitors in the power filter. IC is not the most reliable capacitors.

 

[dotneck335]

IC is not the most reliable capacitors.

Illinois Capacitor? That's the second time I've heard that.

 

[alkuz1961]

Illinois Capacitor? That's the second time I've heard that.

Very often it looks like this:
Billm Audio >> Bad Filter Caps in New Blues Juniors

 

[opauly]

Bingo!! The R35 and R36 resistors were damaged. No visibles signs of that, just used the multimeter to measure ohms and both were in open circuit. Besides, the previous owner had installed 1/4 W resistors, and the schematic requires at least 1/2 ones. Installed new R120 1 W resistors and the amp almost blew up the windows! Thanks everyone for your help!!