Fender Blues Jr noise ?

[gtrplayr1976]

Yes, I often see Blues Jr's biased in the 35-40 mA range, ow! I find that 26-29 is plenty enough. 22 mA is a bit on the cool side.

I'm going to try to bump it up. I've been biasing using the OT xfmr resistance ,and the voltage differential between the center tap ,and plates. Not sure I remember how to do it using the 1ohm resistor.

[Chuck H]

The 1R resistor goes between the cathode (pin 8) and 0V (ground). One thing about the OT resistance method is that you are indeed measuring plate current and not total tube current. The 1R method measures total tube current and it's then ideal to find the screen current (by measuring across the screen grid resistors) and then subtract that from your total to identify the plate current. But I seldom do that. It's enough to just be aware that the screens are using a little current. Knowing you're not biased too hot is where it matters and that's usually closer to class A.

[Pedro Vecino]

If the problem is the same now as before the transformation, look at the common elements. If you have used the new output transformer before transforming the board, it would be advisable to disconnect the feedback line and invert the wires to the EL84 plates.

[gtrplayr1976]

If the problem is the same now as before the transformation, look at the common elements. If you have used the new output transformer before transforming the board, it would be advisable to disconnect the feedback line and invert the wires to the EL84 plates.

I have had experience with reversed plates. It doesn't sound like that.

[g1]

There are many different ways reversed plates can sound. There is not one single symptom.
Don't be fooled by a previous experience, it has happened to others many times here for the exact same reversed phase issue.

[Chuck H]

There are many different ways reversed plates can sound. There is not one single symptom.
Don't be fooled by a previous experience, it has happened to others many times here for the exact same reversed phase issue.

Since I build mostly "prototypes" I can back this up. One amp may seem "ok" until you get the volume up a bit and then it just starts to get quieter (though maybe I heard a virtually ultrasonic tone too). Another will just break into a loud howl the moment you flip the standby switch and you'll be slapping at it to shut it off. And all points in between.

[frus]

The 1R resistor goes between the cathode (pin 8) and 0V (ground).

These are EL84, so cathode is pin 3

[Chuck H]

These are EL84, so cathode is pin 3

Woah! Good catch. Thank you.

Fortunately it looks like nothing is connected to pin 8 on the sockets now. So the worst that should happen is nothing.

[diydidi]

Add a 100pf across both plate resistors of the PI, not just R30. See if this helps...
Some people have also found that putting the shielded back panel back on the amp, cures the oscillation too.

[Chuck H]

Add a 100pf across both plate resistors of the PI, not just R30. See if this helps...
Some people have also found that putting the shielded back panel back on the amp, cures the oscillation too.

Wait. What? Is the problem described in the opening post common to this model?

[Justin Thomas]

There have been more than a few reports that the "problem" (whatever it may be, seems to take many forms) is "cured" by replacing the <shielded> back panel and securing it, yes... Not a rule, but worth checking in these amps...

Justin

[gtrplayr1976]

There have been more than a few reports that the "problem" (whatever it may be, seems to take many forms) is "cured" by replacing the <shielded> back panel and securing it, yes... Not a rule, but worth checking in these amps...

Justin

This one is in a custom 2x12 cab. I don't think the back panel has shielding ,but I can add it.

[gtrplayr1976]

Changing the bias didn't help. I did notice the B+ was changing it goes from 346 to 336 +/- 2V. I have this plugged into a regulated supply so the input AC is not changing. The AC at the rectifier Is changing by a few volts also though. I took all new voltage readings a bit ago. Here's what I have.
V1 Plates 161 , grids 0, cathodes 1.34 , V2 Plates 147, grids 0, cathodes 2 , V3 Plates 189, 181 , grids 30(loud noise when I touch the lead to it), cathodes 52 , V4 , V5 pin 1 -12 , pin 3 30mA , pin 7 335 ,pin 9 320.

[gtrplayr1976]

Since I build mostly "prototypes" I can back this up. One amp may seem "ok" until you get the volume up a bit and then it just starts to get quieter (though maybe I heard a virtually ultrasonic tone too). Another will just break into a loud howl the moment you flip the standby switch and you'll be slapping at it to shut it off. And all points in between.

Yes that howl is what I'm familiar with. I assumed since I didn't have that they were wired correctly.

[gtrplayr1976]

Add a 100pf across both plate resistors of the PI, not just R30. See if this helps...
Some people have also found that putting the shielded back panel back on the amp, cures the oscillation too.

You mean add one to EACH resistor , or one between the two ?