Blues Junior distortion problem

[jpcar]

I'm having a problem with distortion on a Blues Junior. It happens at pretty much all volumes, low and high. It's not very severe, just noticeable when I play two notes or more. I do have another Blues Junior to compare it to also.
I have tried changing all the tubes. I have tried connecting a different speaker.
I changed the bias resistor (R52 in the power supply section) 22K to a higher resistance. I'm using a variable resistor and now it's closer to 27K. This is changing the C- voltage to about -12vdc (instead of stock -10.6vdc) and also lowering the current draw of the power tubes to about 25mA. I got info on this from Billm website.
The bias change doesn't seem to affect the distortion problem. It still happens if I run the tubes hot or cold.
I have tested the voltages at indicated points on the schematic such as B+, Z, Y, X and the cathodes in the pre-amp section. Everything seems pretty close to the schematic voltages. The B+ goes up by about 20v when I lower the bias.

I have attached a schematic.

Any ideas on what I should do next?
Could bad filter capacitors - C25, C26, C27, C28 cause a distortion problem like this? Or bad coupling capacitors between the stages?


Thanks.

[g1]

Could bad filter capacitors - C25, C26, C27, C28 cause a distortion problem like this? Or bad coupling capacitors between the stages?

Maybe, the decouplers (filter caps) seem a bit prone to issues in this series. Especially when you mention it being more noticeable with 2 notes played. Do you have a 450V cap (10 or 20uF) that you can clip across C28 ?

[Sowhat]

Any ideas on what I should do next? Could bad filter capacitors - C25, C26, C27, C28 cause a distortion problem like this? Or bad coupling capacitors between the stages? Thanks.

Hunting and pecking is probably the most unsatisfying way to track down something like this but I understand that sometimes you have to deal with the cards you're dealt. At this point I would start looking for the distortion with a scope, short of that, it's a total guess as to what could be causing this effect. It could be coupling, bypass or even filter caps, or then again, it could be micro-cracks inside of a resistor, or not. it could be a slightly corroded input jack contact, or tube socket or something not getting good ground, it could be oscillation and then again, maybe it's not, as I say it could be anything at this point and without some hard evidence of the fault, and unfortunately the "it's something I hear" is not hard evidence, it will be very hard to track down.

Do you have any electronic measurements that are out of the ordinary and not expected?

[Pedro Vecino]

Maybe, the decouplers (filter caps) seem a bit prone to issues in this series. Especially when you mention it being more noticeable with 2 notes played. Do you have a 450V cap (10 or 20uF) that you can clip across C28 ?

Very possible.

Any traces of this kind in the filter capacitors?

[jpcar]

Very possible.

Any traces of this kind in the filter capacitors?

The filter caps look ok. Not leaky or bulging.

[jpcar]

Maybe, the decouplers (filter caps) seem a bit prone to issues in this series. Especially when you mention it being more noticeable with 2 notes played. Do you have a 450V cap (10 or 20uF) that you can clip across C28 ?

Nice! That's a clever to test the caps without having to remove them.
So, I just tried this with 10uF 450V cap. Unfortunately the distortion remains. I also tried those other other filter caps - C25, C26, C27.

[The Dude]

Have you checked the amp with a known good speaker? It could be voice coil rubbing.

[jpcar]

Hunting and pecking is probably the most unsatisfying way to track down something like this but I understand that sometimes you have to deal with the cards you're dealt. At this point I would start looking for the distortion with a scope, short of that, it's a total guess as to what could be causing this effect. It could be coupling, bypass or even filter caps, or then again, it could be micro-cracks inside of a resistor, or not. it could be a slightly corroded input jack contact, or tube socket or something not getting good ground, it could be oscillation and then again, maybe it's not, as I say it could be anything at this point and without some hard evidence of the fault, and unfortunately the "it's something I hear" is not hard evidence, it will be very hard to track down.

Do you have any electronic measurements that are out of the ordinary and not expected?

That's a good point. I need some evidence to narrow it down. I don't think I'll see the distortion by sending a sine wave through and testing it on the scope. But I could try a few experiments what the signal is and I have the other working Blues Junior.

Could I use the two Blues Junior together? jumper from the master volume pot across to the other, and then see if it's happening in the preamp or power amp. Maybe that's a dangerous thing to do? Both amps would need the same ground reference.

I'm trying to find measurements that are out of line according to the schematic or the other BJ. But haven't found anything yet.

[jpcar]

Have you checked the amp with a known good speaker? It could be voice coil rubbing.

I have checked it with another speaker (from another Blues Junior) and also and 4X10" cab which is also 8 ohms. I hear the same crunchy distortion on each.

[Sowhat]

Could I use the two Blues Junior together? jumper from the master volume pot across to the other, and then see if it's happening in the preamp or power amp. Maybe that's a dangerous thing to do? Both amps would need the same ground reference.

Wow... interesting concept! I never tried that but if you put a .02 cap in the signal path between the two to block any DC, no special grounding should be needed however those two masters are going to conflict with one another impedance-wise, I don't know exactly what will happen but it should work... theoretically, maybe.

[dstrat]

desolder D11 and D12 and check the sound.

[Pedro Vecino]

Before more complex experiments, try the simplest:

- Check for high voltage (around 180/200VDC) on pins 1 and 6 of the third 12AX7.
- The same on pin 9 of each EL84 (around 300VDC)
- Measure the value of the cathode resistor in the third 12AX7. R24 (820 Ohms).

[jpcar]

desolder D11 and D12 and check the sound.

Thanks. I tried this. Lifted a leg from both of these diodes. Sadly no change.

[jpcar]

Before more complex experiments, try the simplest:

- Check for high voltage (around 180/200VDC) on pins 1 and 6 of the third 12AX7.
- The same on pin 9 of each EL84 (around 300VDC)
- Measure the value of the cathode resistor in the third 12AX7. R24 (820 Ohms).

Thanks, I tried these.
The voltages at the plates of the phase inverter tube (V3 pins 1,6) are not the same. About 10v difference.
Pin 1: 204vdc
Pin 6: 215vdc
But then I tested the other Blues Junior and it’s about the same difference. 220vdc and 226vdc.
I also tested all the resistors around here. Plate resistors, resistors between the grids. And that cathode resistor. All seem fine.

Also tested pin 9 of EL84s, seems fine.

[jpcar]

So I tried to identify the distortion with the scope. Sine looks pretty normal, as expected. Tried a triangle wave and the output definitely looks pretty dis-shaped. I’m not sure if this the distortion I’m hearing but hey, I’ll go with it for now.
Scoped the input R2, looks fine. Scope at R7 looks fine. Scope at R15 looks bad. Weird shape. And then after R20 looks bad. So it seems to be between 2nd half of V1 and that tone stack. Checked R8, R9, R10. R11. Changed C5, C6, C7. Weird remains.
Maybe I should change out those resistors even though they’re measuring ok. Not much else left between these two points.