Blues Jr III Universal issue

[Helmholtz]

I would like to add that needle meters may cause oscillation as well. But they are much more robust than DVMs.
Its mainly the radiated field of the hot lead that capacitively couples to a non-inverting input which causes positive feedback.

Also, prolonged high amplitude oscillation stresses the insulation of the OT's primary wire.

[J M Fahey]

Have 3 Blues Jr III's in the shop from a local church. All have that same obscure symptom. All have the same repair as well - open Rscreen. Pulling the PI cures the oscillation/measurement issue! Thanks guys! Awesome thread thanks for the input.

JM, do you think after pulling the PI and mitigating the oscillation, that it still may kill your meter?

No, no oscillation no problem

[diydidi]

I'm only seeing this thread now. There is a fix for this. I have been doing on Blues jnr's for the past two years.
It is due to lead dress. The cream boards where the worst.
Solder a 100p capacitor across one of the 100k PI plate load resistor. This will stabilise everything even for measuring with no change of freq response.

[christarak]

This is such an obscure piece of very useful information for most DIYers (including me) and even some experienced techs that I chose to "sticky" it for a time to keep it visible. Right on guys!

You wrote this comment 2 years ago, Chuck, but I just discovered it in this Corona world gone crazy. I'm right with you, mate. Every single time I get on this site, I leave with new gem of wisdom and insight.

[MichaelNuzum]

I just had this exact problem on my bench last week. To add a bit of perspective, here's what happened to me:

I had a Blues Jr come in with a blown screen resistor. Power tubes tested as one weak and one shorted. I replaced the resistor and installed new power tubes.

I run all diagnostics and signal tracing into a dummy load. During the normal testing procedure, the amp passed all tests. I check voltages at all legs, I inject signal and trace with an o-scope to look for signal degradation or noise. I measure at output and look for max power before distortion. On every test the amp was rock solid. This was all on the bench, chassis out of cabinet, into dummy load. Very stable and consistent readings.

I put the amp back together and fired it up and it sounded glorious. But, I did notice that smell of power tubes running a little hot (which I guess these amps are known for), so I popped the back off to take a few measurements. This was now chassis in the cabinet, back removed, running into onboard speaker. That's when the fun began.

When I would probe the output transformer leads, I would get the left side (brown) reading around 330VDC, and the right side (blue) would flutter between 290-320, mostly hovering around 300VDC. At the same time, I was getting almost 18VAC across the speaker terminals!

I looked around and saw the advice regarding lead dress, and keeping the output transformer leads as perpendicular as possible to the ribbon cables, adding a 100pf cap, etc. Nothing helped.

THE SOLUTION: I found someone saying the amp can be very unstable without the foil-covered back in place. Something about the phase inverter ribbon cable needing to be located next to the plane of the back foil was necessary for the ribbon cable to be properly shielded from oscillations. (?) The suggestion was to mimic that property with a folded piece of aluminum foil, maybe 2x3 inches or so, alligator clipped to the chassis, extending up to just cover the p.i. ribbon cable. It worked!!!!

I ran the rest of my tests and measurements, this time with the chassis in cabinet, plugged into the onboard speaker and back removed, and again the amp passed with flying colors! Both output transformer leads resting stable around 330VDC, with 0VAC at speaker. So, if you are working on one of these on the bench with the back off, it seems you need to either run it into a dummy load, or have that piece of aluminum foil in place, or the amp will go into scary instability.

Hope that helps someone else and saves them from the rabbit hole I went down trying to figure that one out!

[Jon Snell]

Don't forget that for UK users, we have 240volts not 230. Ensure the mains transformer is set accordingly.
Replace the 27k bias resistor with 33k and you will no longer have screen resistor failure and the EL84s will run correctly.
There are also tone stack mods to inprove the sound and get rid of the 'cardboard box being kicked around the room' sound.

[1ampman]

Still get a 100 hz oscillation...When the volume is turned up to 8. Nothing plugged in... Did put a 120 pico across the 100k PI inverter. Suggestions?

[Helmholtz]

Still get a 100 hz oscillation...When the volume is turned up to 8. Nothing plugged in... Did put a 120 pico across the 100k PI inverter. Suggestions?

If it's 100Hz, it's probably power supply ripple - not oscillation.

Better start a new thread for your problem.