Blues Junior - Hi squeal then 100 hz howl WHEN vol turned up

[1ampman]

Regular Blues Junior doesn't like the volume turned up! Even with nothing plugged in...At 8 on volume I get a squeal first then a howl. If you know the part number and what value of the part, please state it.

[Enzo]

Part number? We don't even know if it needs any parts yet.

Disconnect the speaker and connect the amp to some other speaker across the room. Still do it? I want to determine if it is acoustic feedback or not.

Did you ever have the output transformer disconnected? If it was reconnected in the wrong order, you get feedback.

When it is howlwing, look in the rear and grasp each tube one at a time and hold it firmly. Use a rag or gloves if it is too hot. Does gripping any tube stop it?

The output transformer wires hang left/right across a ribbon cable. Try moving the ribbons and those transformer wires. ANy change?

[Jazz P Bass]

The volume control affecting the 'howl' may be a hint.

That means that the issue is most probably before the control.

Not much there.

An input jack with a wonky ground connection might do it.

[vintagekiki]

Fender Blues Junior Schematic

https://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-Blues-Junior-Schematic.pdf
https://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-Blues-Junior-III-Schematic-Rev-D.pdf

Fender Blues Junior Service Manual (21 pages)

Fender_blues_jr.pdf
Fender_blues_junior_rev_a.pdf
Fender_blues_junior_sch_rev_b.pdf
Fender_blues_junior_rev_d.pdf
Fender_blues_junior_sch_rev_d.pdf
Fender_blues_junior_service_manual.pdf

[vintagekiki]

Have you ever replaced some tubes with new since the amp was purchased.

[1ampman]

I've grabbed the tubes when it's isolating (NO difference) , It's NEVER BEEN apart, I've sub tubes, the squeal means some type of oscillation, I'll try moving wires... If someone has already had the fix maybe the part or number would speed up the repair, that's all...

[Jazz P Bass]

Try pulling the first 12AX7/ V1.
If that stops it, then the issue is before it or at it.

[g1]

If you know the part number and what value of the part, please state it.

To post a part number would require the exact schematic. There are many versions, see post #4 and let us know which version you have.

[Helmholtz]

Seems we're discussing the same problem in 2 different threads.

[g1]

Seems we're discussing the same problem in 2 different threads.

I thought there were 3.

But this is the new one, so best to stick with the issue here.

[xtian]

Blues Juniors and Hot Rods are prone to filter cap failure, and that would allow oscillation. You can very quickly clip in a 47uF cap in parallel with each filter cap, one at a time, to see if it changes anything.

[1ampman]

It's a Rev B...

[1ampman]

GOT IT! Clipped a 30uf cap across each filter cap, one at a time...The last cap stopped the whole oscillation. It has the original I C caps . So a cap job is needed. Thx for your suggestions and time...

[Randall]

What is the part number?

[g1]

It's often the 1st preamp node filter cap in the blues and hot rod series amps that is responsible for the oscillation or motorboating.
For the Rev.B that would be node Z and the filter cap is C10.