Fender Blues Jr noise ?

[gtrplayr1976]

I ran into this with the original boards. I had replaced the OT with an Allen Amps TO22 ,and changed some components recommended by a gentlemen on another forum to help clean it up some. I had this nasty distorted tone as the sound fades. Kind of like when you turn the amp off while strumming to discharge the caps. After trying some things I decided to gut it ,and use Doug Hoffmans turret boards he designed to replace the circuit boards. So everything replaced (except tubes). Still have that nasty distortion. Using the NFB (meat control) makes it more pronounced as you increase the pot setting.
This is not the stock cabinet. The chassis is in a 2x12 ,and the speakers are wired 4 ohm. That's why I had to add the new xfmr. All of the voltages seem in line.
There was also a 100pf cap added across R30 to help prevent oscillation with the new xfmr according to Bill M.
I have tried another cab, cable, tubes. Forgive the work.

[Chuck H]

What is the bias current?

[gtrplayr1976]

What is the bias current?

Biased at 22mA

[KilgoreTrout]

On the sixth picture, is that a 4.7 uf cap instead of a 47?

-Kilgore

[gtrplayr1976]

On the sixth picture, is that a 4.7 uf cap instead of a 47?

-Kilgore

Yes it is. It was one of the components I changed that was recommended. There are some other components that have been changed. I have changed most back to what the schematic shows with the exception of the 4.7 cathode bypass , a .047 bass tone cap , 3.3K grid resistors ,and 820R screens..I think.

[gwell1]

Sounds like a current issue. As if low current (low volume input) is not enough to drive the pre amp. Good question by Chuck H. Bias current sounds like a good starting point. You may be close to cutoff

[gtrplayr1976]

Sounds like a current issue. As if low current (low volume input) is not enough to drive the pre amp. Good question by Chuck H. Bias current sounds like a good starting point. You may be close to cutoff

At 22mA it should be 60% ?

[Chuck H]

Try biasing hotter. It won't hurt the amp and there's nothing to lose. I suspect grid conduction is skewing the bias and causing crossover distortion due to the already cool-ish bias. el84's are especially susceptible to that. Try, maybe, 28 mA. I think stock is about 25mA and some players have similar complaints to this one. Hotter is safe. At 320Vp you have a lot of leeway on bias voltage.

[gtrplayr1976]

Try biasing hotter. It won't hurt the amp and there's nothing to lose. I suspect grid conduction is skewing the bias and causing crossover distortion due to the already cool-ish bias. el84's are especially susceptible to that. Try, maybe, 28 mA. I think stock is about 25mA and some players have similar complaints to this one. Hotter is safe. At 320Vp you have a lot of leeway on bias voltage.

I was under the assumption that these were biased hot from the factory (maybe 90%) ? I can try hotter to see what happens. I'm just trying to clean up the sound some. More Fendery.

[Leo_Gnardo]

I was under the assumption that these were biased hot from the factory (maybe 90%) ? I can try hotter to see what happens.

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.

[gwell1]

I guess the question is: is the problem in the pre amp section or the main output tubes?
What you could do is turn up your gain really high but keep your master volume low. If you still hear the problem then it's in the main output tube area. If you set the gain really low and crank you master volume really loud, and still hear it then it's in the pre amp section. Either way it sounds like a tube is close to cutoff. Think of a tube as a transistor.

[mozz]

On the last picture, all 3 of those 100k resistors are crossing a wire underneath, sure they aren't touching?

[Chuck H]

On the last picture, all 3 of those 100k resistors are crossing a wire underneath, sure they aren't touching?

I agree that the design is questionable on many levels. There's an uninsulated HV rail buss (dangerous) with uninsulated leads crossing 0V in close proximity. These are the highest voltage differential circuits in the amp. It's pretty shameful really, but... If any of the leads on those 100k resistors was touching the result would be a dead short of the HV rail to 0V. That would demonstrate far worse than the symptoms given

[gtrplayr1976]

On the last picture, all 3 of those 100k resistors are crossing a wire underneath, sure they aren't touching?

No the resistors are on top of the turrets ,and those jumpers are against the board.

[gtrplayr1976]

I agree that the design is questionable on many levels. There's an uninsulated HV rail buss (dangerous) with uninsulated leads crossing 0V in close proximity. These are the highest voltage differential circuits in the amp. It's pretty shameful really, but... If any of the leads on those 100k resistors was touching the result would be a dead short of the HV rail to 0V. That would demonstrate far worse than the symptoms given

The resistors are on top of the turrets ,and the jumpers are against the board. I didn't design it, I just tried to build it. It seems dead quiet other than that decaying distortion.