Blues Junior distorting and cutting out

[Yoda]

Hello all,

I have a Blues Junior on the bench today and as you turn the volume above 2 or so it creates a sudden, awful distortion that overpowers any other sound. Also, a few seconds after powering on it begins to emit a louder than usual hum. Works ok with a guitar plugged in at extremely low volumes.

A preliminary check of all voltages found 0 volts on pin 7 of V5 and pin 1 of V2.

I believe the V2 triode in question is the one shown all by itself on the schematic as V2-A, which appears to be unused so it may not be a problem after all but I’ve never seen anything like it on a schematic, the grid, plate, and cathode are all tied together to ground.

Best places to start diagnosing? It’s a 2015 made amp I believe, with a green circuit board. Odd because I remember the early BJrs being green and then switching to cream colored but perhaps they’ve switched back to green. But it only says Blues Junior on the back panel, not II or III, etc.

Anyways any and all help is appreciated. Don’t laugh but this is a more complex amplifier than I remember.

[Stevem]

So it’s outputting a sound unrelated to the notes your .playing, besides distorting?

It may be time for all new power supply filters.

I would also check the two plate load resistors on V3, as one may be open or changed in value drastically.

These amps are also known for blown output transformers.

[Yoda]

If I simply turn the volume up but don’t play anything it doesn’t distort like that but if I play anything with the guitar it will distort horribly with the volume above 2-3.

I haven’t worked on it anymore today but yes one of my first suspicions right now is that particular half of the OT primary winding has gone bad.

[chaccmgr]

I had one on my bench recently. A common fault is the carbon film plate resistors of the PI go bad. Check the voltages on the plates before and after the resistors. These Rs are known for intermittent or total failure. If one fails, the signal at the speaker misses one half of the cycle resulting in distorted sound.

This would also explain why there is no problem at low volumes when the power tubes are still working in class A (the remaining tube carries both half cycles)

Other than components on the main board, these are relatively easy to replace without disassembling the entire amp.

[Yoda]

Awesome, thanks for the info, that makes a lot of sense.

[Yoda]

So I swapped in a 2 x EL-84 output transformer and the voltage returned to pin 7 of V5.

I also checked the three phase inverter plate resistors and all checked out fine against the schematic.

I also temporarily alligator-clipped in a 47uF filter cap.

The ugly distortion still seems to persist though.

[thetragichero]

i had similar symptoms from the tubes getting so hot they melted socket solder/lifted traces and only one tube was conducting (also had ribbon cable issues not putting bias on the control grid)
these amps sound great when working but damn if they're not an engineered nightmare

[Stevem]

Can you achieve 10 volts to the speaker or a 8 ohm load like the schematic says should be able to be done?

Do you have a O-scope?

[Yoda]

No I haven’t checked voltage at the speaker output.

I do have an O-scope but it’s down at the moment. Probably staying down since it’s a vintage Tektronix unit, and of course I need a second oscilloscope to repair it which I don’t have.

[Yoda]

I did find the screen resistor to V5 being open circuit as well.

[Stevem]

So how does the amp sound now?

if you put a test tone thru the amp now what output voltage do you see across the speaker or a load?

[Yoda]

The amp is sounding normal now. Normal response from the controls and produces volume as expected. The output transformer doesn’t seem to be bad at all anymore now that the blown screen resistor was replaced all the voltages check out fine. I only replaced the blown resistor with another 100R but I’m thinking of going back and replacing both screen resistors with 1K parts.

The owner also want to run NOS EI power tubes so I will be adding adjustable bias before I return the amp.

I’ll check what voltage I’m getting at the speaker output and report back. This was just a quick play through test to see how it does with the new screen resistor.

I also found the preamp tubes to be microphonic as they cause a screeching oscillation at high volume levels but that problem was mitigated somewhat if I plugged into an external speaker rather than the internal one, and the problem went away completely when I replaced them with new tubes.

[Stevem]

Some of The 8 best sounding NOS preamp tubes I have are a tad microphonic, and 2 of them are only usable in a non combo amp, but I love them!

[Yoda]

As far as cooling the bias is concerned, do you guys have preferences for installing a trimpot and making it adjustable versus leaving it fixed/non-adjustable and just replacing one of the bias resistors to cool it down? The latter option would maintain the plug-and-play aspect of the power tubes.

I personally am inclined towards the trimpot option but replacing the resistor with another allows me to fix the bias now rather than order and wait for parts and return the amp sooner.

[thetragichero]

trimpot is the better option. maybe order 5 of them so next time you don't have to wait