Nasty distortion on Blues Jr. (Fixed)

[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. (like striking a chord ,or note ,and letting it fade) After trying some things I decided to gut it ,and use Dougs boards. So everything replaced (except tubes). Still have that nasty distortion. Using the NFB (meat) makes it more pronounced as you increase the pot setting.
 Maybe I should replace the components back to what Doug has on his schematic ,and go from there ?

[Ed_Chambley]

Does it sound like when you turn off an amp while playing and the you still hear the amp while the caps drain?

You can certainly keep replacing parts, but try getting voltage readings of each tube and post them along with photos, lots if you can, of the chassis inside.

I would guess you simply have a component going south.

Also, have you tried known good tubes, especially the rectifier tube a different speaker, guitar, cord and plugged into a different outlet?  These are easy things to rule out first.

[gtrplayr1976]

Does it sound like when you turn off an amp while playing and the you still hear the amp while the caps drain?

You can certainly keep replacing parts, but try getting voltage readings of each tube and post them along with photos, lots if you can, of the chassis inside.

I would guess you simply have a component going south.

Also, have you tried known good tubes, especially the rectifier tube a different speaker, guitar, cord and plugged into a different outlet?  These are easy things to rule out first.

That's a good comparison. It does sound very similar to that. I have tried good tubes, cab, cord. Outlet not yet ,but I will. I have some pictures I can post. SS rectifier. Voltages are all good. V1 , V2 plates 160 , V3 plates 198 ,182 , grids 30 ,cathodes 51, B+ is 340 , Z 320, Y 278, X 249, I'll have to check power tubes again ,and the cathodes ,and grids on V1 ,and V2. They were good when I checked them , but I didn't write them down.

Pictures won't load. I get errors.

[jecars]

Food for thought:

Blues Jr, like many other LTP phase inverter Class AB amps, can get a nasty under-damped oscillation in the PI output. This part of the circuit can behave like an oscillation tank, and give a raspy, fizzy, rattle-like tail on notes (for about 500 milliseconds) as you describe.  The following might help, and are easily undone,

* Use a wooden chop stick to touch the phase inverter wire leads while someone else plays the guitar. Sometime, just slightly moving these wire positions relative to each other will stop oscillation by changing any induced cross-talk. Look up "lead dressing" if you want to know more.

* Electrical option #1: Place a 100 pF cap across the PI output. On the Hoffman board, PI pins 1 and 6 have wires that run to the turret board.  Using the 100 pF cap and alligator clips to jump across these turret points will temporarily do it. Give it a listen for any improvement.

* Electrical option #2: Place a 100 pf cap as a jumper across either of the PI load resistors (R29 or R30), it doesn't matter which.  Alligator clips make it fast and removable. Again, give a listen.

Option #1 and #2 are referred to as "snub" or "snubber" caps, if you want to look up more info.  Marshall, Vox and Fender use them in several PI circuits. Option #1 changes the phasing of the offending high frequencies which adds damping. Option #2 gives the offending frequencies a way out of the "tank", which adds damping.

[gtrplayr1976]

Food for thought:

Blues Jr, like many other LTP phase inverter Class AB amps, can get a nasty under-damped oscillation in the PI output. This part of the circuit can behave like an oscillation tank, and give a raspy, fizzy, rattle-like tail on notes (for about 500 milliseconds) as you describe.  The following might help, and are easily undone,

* Use a wooden chop stick to touch the phase inverter wire leads while someone else plays the guitar. Sometime, just slightly moving these wire positions relative to each other will stop oscillation by changing any induced cross-talk. Look up "lead dressing" if you want to know more.

* Electrical option #1: Place a 100 pF cap across the PI output. On the Hoffman board, PI pins 1 and 6 have wires that run to the turret board.  Using the 100 pF cap and alligator clips to jump across these turret points will temporarily do it. Give it a listen for any improvement.

* Electrical option #2: Place a 100 pf cap as a jumper across either of the PI load resistors (R29 or R30), it doesn't matter which.  Alligator clips make it fast and removable. Again, give a listen.

Option #1 and #2 are referred to as "snub" or "snubber" caps, if you want to look up more info.  Marshall, Vox and Fender use them in several PI circuits. Option #1 changes the phasing of the offending high frequencies which adds damping. Option #2 gives the offending frequencies a way out of the "tank", which adds damping.

I had tried option 2 already. Since I used a TO22 OT it was recommended. I have it removed now.  I will try the other two options as well.
Thank you

[Ambugaton]

I had similar problems with an 18w. It wasn't my main problem but in the attempt to solve other issues I installed a NFB pot to fine tune where I wanted it. I felt like it helped with this bit of sizzle that comes towards the end of a note that I was hearing. Like very thin distortion that seems to just accompany each strum as it dies?

[jecars]

I had tried option 2 already. Since I used a TO22 OT it was recommended. I have it removed now.  I will try the other two options as well.
Thank you

Good luck. Best case it helps, or at minimum, it should rule out the PI.

[gtrplayr1976]

I had similar problems with an 18w. It wasn't my main problem but in the attempt to solve other issues I installed a NFB pot to fine tune where I wanted it. I felt like it helped with this bit of sizzle that comes towards the end of a note that I was hearing. Like very thin distortion that seems to just accompany each strum as it dies?

I have a pot installed. It seems to get more pronounced as I increase the pot.

[gtrplayr1976]

I had tried option 2 already. Since I used a TO22 OT it was recommended. I have it removed now.  I will try the other two options as well.
Thank you

Good luck. Best case it helps, or at minimum, it should rule out the PI.

Thank you.

[gtrplayr1976]

Food for thought:

Blues Jr, like many other LTP phase inverter Class AB amps, can get a nasty under-damped oscillation in the PI output. This part of the circuit can behave like an oscillation tank, and give a raspy, fizzy, rattle-like tail on notes (for about 500 milliseconds) as you describe.  The following might help, and are easily undone,

* Use a wooden chop stick to touch the phase inverter wire leads while someone else plays the guitar. Sometime, just slightly moving these wire positions relative to each other will stop oscillation by changing any induced cross-talk. Look up "lead dressing" if you want to know more.

* Electrical option #1: Place a 100 pF cap across the PI output. On the Hoffman board, PI pins 1 and 6 have wires that run to the turret board.  Using the 100 pF cap and alligator clips to jump across these turret points will temporarily do it. Give it a listen for any improvement.

* Electrical option #2: Place a 100 pf cap as a jumper across either of the PI load resistors (R29 or R30), it doesn't matter which.  Alligator clips make it fast and removable. Again, give a listen.

Option #1 and #2 are referred to as "snub" or "snubber" caps, if you want to look up more info.  Marshall, Vox and Fender use them in several PI circuits. Option #1 changes the phasing of the offending high frequencies which adds damping. Option #2 gives the offending frequencies a way out of the "tank", which adds damping.

Nothing seemed to help although I have little lead dress to play with ,so I can't tell any difference moving what I have. I try to keep them separate, or at 45° crossing angles to avoid noise. The amp is noise free other than when I strum ,or pluck.  If anyone is familiar with the Hoffman board would it matter if the jumpers on the bottom were crossing ? I know he show them separate from each other. (not crossing)

[sluckey]

If anyone is familiar with the Hoffman board would it matter if the jumpers on the bottom were crossing ?

Does not matter on this board. All those jumpers are either grounds or power supply and they are kept well away from any sensitive areas.

[gtrplayr1976]

If anyone is familiar with the Hoffman board would it matter if the jumpers on the bottom were crossing ?

Does not matter on this board. All those jumpers are either grounds or power supply and they are kept well away from any sensitive areas.

Good to know. I sure didn't want to pull that board unless I had too. Is the PT on the Blues Jr the same as on the Pro Jr ?

[sluckey]

Probably not. You could compare the schematics to know for sure. You can find the schematics by following the link at the bottom of this page.

[gtrplayr1976]

Probably not. You could compare the schematics to know for sure. You can find the schematics by following the link at the bottom of this page.

Yes they're different. Dang it.

[gtrplayr1976]

Turns out it WAS a bad speaker. At least the biggest part of it. Apparently the other cabinet I used to test the amp also has a bad speaker.  It is a new cab with speakers I bought used , but were suppose to be like new.  I have played that cab with my JTM45 ,and didn't notice any noise. (except my awful playing)   Live ,and learn.

Thanks to everyone that tried to help.

[jamaio]

Glad you found the problem!! I have been trying to get my Blues Junior to sound better only to find out the problem I had was with the volume pot on my guitar. It sounded very muddy and just had an awful tone, a friend stopped by and started playing, he adjusted the volume on my guitar and it sounded like my amp just quit. I plugged in a different guitar and the problem went away.

[gtrplayr1976]

Glad you found the problem!! I have been trying to get my Blues Junior to sound better only to find out the problem I had was with the volume pot on my guitar. It sounded very muddy and just had an awful tone, a friend stopped by and started playing, he adjusted the volume on my guitar and it sounded like my amp just quit. I plugged in a different guitar and the problem went away.

I must say this amp sounds pretty good now. Sounds more full with some of the component changes. Easier to work on now with the turret board. :)