Blues Junior - screams from hell!

[redneckgeek]

I've been beat up by this Blues Junior. Please help. It plays good and everything works, but, When you turn the volume pot up to say 90% and above there is a severe scream. It is 19.2 kHz. It is present on the three stages of preamp with the phase inverter and the EL84s out. The signal is strongest on pin 6 of the second 12AX7. With a Fluke DMM set on AC Volts, here is what happens in the following locations: Pin 1 of 1st tube = voltage goes from 0 to 15 VAC. On pin 7 of the 2nd tube, the voltage goes from 0.3 VAC to 3.5 VAC. On pin 6 of the 2nd tube the voltage goes from 0 to 65 VAC.

I also observed the plate voltage on the DC Volts function at pin 6 of 2nd tube. With the volume pot down the Plate V = 195 VDC as the pot is turned up the voltage increases from 195VDC to 216VDC.

One of the obstacles I have is that the schematic I have obtained from Fender has different numbers on most of the parts. The schematic seems to be good but R 9 could be R32 on the printed schematic.

I have removed one lead from all components associated with both pre amp tubes, tested and reinstalled them. All values were good. I have replaced all tubes twice. I have replaced the transistor (J111), The 1N4448 diode, The op amp, the volume pot and the 100 pF caps next to the pots.

Other observations are. The feedback is present on the preamp tubes with or without the p/i or EL84s plugged in. The noise of course is in the speaker with all tubes operating but not when they are removed. The tone control pots will turn the noise off independently. ie. you can turn the master to any position and the volume to max and turn the noise on or off by turning any of the low mid and treble pots up or down respectively. However each pot will produce a different frequency. The treble pot being the one that produces the 19.2 kHz signal. The base pot produces a much lower frequency signal.

This amp has a serial number of B-250852.

[Enzo]

There are a bazillion revisions of that model amp, so ther is no one schematic. You need the one for your version. The amp file contains a bunch of schematics, not just one. See attached.


What is happening is the amp is oscillating - feeding back within itself when the gain is up. Take off the rear cover and look at the circuit boards. The main board is vertical in teh chassis. Then there are gray ribbon cables from that down to the tube socket boards, which as horizontal in the bottom. The ribbons usually are bent in an S shape.

The ribbons over the preamps tubes - scrunch them together so they don't bow out more than necesary.

Near the power tube sockets are the blue and brown wires from the output transformer - they plug onto push-on posts on either side of the power tube ribbons. The blue wire crosses the ribbons to get to its post.

Make sure that blue wire is dressed - situated - as close to a right angle as possible with the ribbons. Wires crossing at right angles don't link very well.

[Prairie Dawg]

Check out the billm audio website-it is a wealth of information on these amps.

Billm Audio » Cream Board Phase Inverter Oscillation

Welcome back from the edge, Enzo.

[redneckgeek]

Well it's July 6th. I had another Blues Jr come in the shop so I did some comparison checks on it with the one that is the subject of this thread. Thanks Enzo. I got the right schematic. Thanks Prairie Dawg I went to the site you sent. The manipulation of the blue plate voltage wire didn't do anything. I removed the ribbon wires and replaced them with individual wires. I used shielded wire on the grids. I covered all joints with silicone rubber. In short. It made no difference at all. I also reversed the two speaker wires. That made it worse.

Here is what I found to be the difference between the two amps. Amp 1 (the bad one) has a 40+kHz oscillation all the time. As I increase the volume pot and/or the treb mid or bass pots this changes the frequency. At about 8 on the volume pot the oscillation is decreased in it's frequency to the point that it becomes audible. On amp 2, the good one, the 40+ kHz oscillation is always there (on the blue wire) but the changing of any pot does not alter the frequency, thus, the signal never becomes audible and the amp seems normal (and maybe it is for a Blues Jr) . By the way, the PCB's are both the same cream color.

Is there possibly a single component (or components) that could go bad allowing the volume pot to become the variable part of an oscillating network? I have checked most of them in all preamp and PI circuits. I have also looked at the grounds throughout the preamp
circuits. Any ideas.

[phydauex]

hey Redneck -
I went thru something similar a while back after doing some of the Bill M mods to a blues Jr and ended up with an oscillation that no amount of cable positioning would qwell. I'd bet a large sum that the oscillation is coming from the ribbon cable of the first preamp tube. What I did to stop it, was to make a shield out of some adhesive backed copper foil that I wrapped around the ribbon, for just a couple inches. Then I soldered a short jumper to that I mounted on the ground strap just to the right of the board there. not only took care of the offensive screaming, but made the amp way quieter than it ever was. This oscillation gets really bad there, and I did everything that I could find about dealing with that problem to no avail. I also resoldered the ribbon cable, then used some hot glue at the base to keep it from moving or vibrating.

Seriously, even a piece of stranded wire (22-24 ga) wrapped around that first ribbon and connected to the ground strap will fix it too. I ried that first and while it stopped the offending squeel, I decided to use some shock & awe and really shield it.
Give it a try before you make yourself crazy like I did.
Matt

[Chuck H]

Have you tried any tube swaps in case it's a microphonic tube? Even if it's not a microphonic tube there are some preamp tubes that have much more high end than others. Try a Sovtek 12ax7WA in the first preamp socket.

[Ronsonic]

Check filter decoupling caps, especially for the preamp stage.

[jimmymack]

Can I jump in on the tail of this thread?

I have a Blues Junior that I am very happy with, as far as tone. I have played a few, and none of them does for me what this one does. So it's worth saving!

It does not belong to me, but to the club I gig at regularly. Owner will not pay for repairs! So I want to fix it myself with my soldering iron!!!!

No sound from the amp. The first pre-amp would crackle upon moving it. Took it out and out came a small piece of metal attached to one of the tube's pins.

Needs to be put back in.

But before that happened, every now and then, the amp would begin to blast a single loud note, like a tugboat horn. From memory, it sounded approximately like a middle A (4th string, 7th fret) brought on sporadically when different people played the amp.

Does this sound like an easy fix, probably not - right? But money is tight, and it's not even my amp, I'm just very fond of it. Help????

[pontiacpete]

I would start with changing that socket.