Blues Jr 3 Texas Red

[bsco]

Got this amp and whenever I strike the low E note or some other low notes, I get a buzzing or fuzzy type of sound....then as the note decays, the sound becomes clear.and if you strike the notes in question soft you do not get this fuzzy sound...only when you hit them hard...I tried another separate speaker cabinet....same thing...I even bridged the filter caps with another one..same thing...The main filter is a 47uf at 450..I bridged this one with a 220 uf 450V...same thing....however, when I did this , I noticed that the small amount of hum I was getting disappeared but I still get these fuzzy notes....suggestions, comments......anyone???
Cheers,
Bernie

[52 Bill]

Have you tried different tubes? Have you checked the voltages compared to the schematic? Bias voltage?

Be certain to test the voltages on the driver tube plates.

[bsco]

Gotta ya there 52.....here are the findings....B+=329V, Z=306V, Y=267V, X=242V Bias voltage =minus 10.9V, V1 Pin1=161V, V1 Pin6=163V, V2 pin6=160V, V3 Pin1=195V, V3 Pin6=194V, V4 Pin7=326V and V5 Pin7=330V.......pretty much in the ball park.....this doesn't seem to be a mechanical issue...at least i can't find anything and the same problem with two other different cabinets...I also noticed that as you increase the volume you don't have to strike the notes as hard as you would at lower volumes.. the fuzzy sound occurs a split second after the note begins to sound and then as the note(s) decay the fuzzy sound sort of drifts in and out until you reach a point when the note is very clean...
Cheers,
Bernie
P.S. I searched this forum for some other posts....found a few things but nothing that sorted this out....

[52 Bill]

Low voltage power supplies okay as well?

You didn't say anything about the tubes, have you tried different ones?
Did you check the screen grid output tube voltages? Pin 9.

I usually assume that these sorts of noises are in the power amp, but they don't have to be. Can you feed a signal into the reverb return cable? Does the noise happen there? Can you listen to the reverb drive signal going to the tank? Does the noise happen there?

[bsco]

I tried different tubes....actually all of them....I'll double check the LV supplies....pretty sure they are ok but I'll double check...I never checked pin 9 of the output tubes...I'll do that right after supper and never thought about the reverb return line...I'll let you know what I find....I have a quarter inch to rca male cable so I can plug the guitar right into it....thanks for that tip...
Cheers

[bsco]

52--------I screwed up......no, no...I didn't blow anything up....I had an open resistor....this was causing the screen voltage for one of the output tubes to be missing...I thought that if that happened your tube would red-plate..but I guess not....So.......... learned something new....Check ALL the damm voltages...every single one of them...Thank you for the tips especially about the reverb return and send.......the amp sounds great now......just buttoned it up......Once again, thank you very much.......
Cheers,
Bernie

[g1]

I had an open resistor....this was causing the screen voltage for one of the output tubes to be missing...I thought that if that happened your tube would red-plate..but I guess not

An open screen resistor will basically shut off the tube, the amp will behave as if that tube was removed. Very often they go open because of a defective power tube. So when you have an open screen resistor, be suspicious of the tube. Of course it is possible the bad tube was replaced since the resistor blew, so you don't want to blindly discard the tube unless you know it is the one that blew the resistor.
So a red-plating tube might blow a screen resistor, but a blown screen resistor won't make a tube red-plate.

[bsco]

The tubes in this amp are push-pull??If one of them isn't working or even removed, wouldn't that cause excessive distortion??..But that wasn't happening, so I did not suspect that there would be a bad tube...Anyway, I learned a few things again.....it is always good when you keep learning something new..thanks for the info g-one.......
Cheers.....

[bsco]

I just tried to post a reply but went missing...here goes again...I thought that whenever a tube red-plated the screen resistors would be suspect...along with the bias voltage....I will make a point to check this...actually I am going to make it a rule...I have to say that I always learn something new here....you guys are great and are very helpful....and most important is that you guys don't make people like me feel stupid...you just give advice.....and it is very, very much appreciated....

[g1]

Unplug one of the tubes and see how it affects the sound. It will depend on the volume level, class of operation, and bias.
I've maybe exaggerated a bit and the tube may work a little, but not much. Did you scope the output when the screen resistor was open?
Good excuse to do some experimentation, with and without one of the tubes, on speaker and on scope with dummy load, at which point one side of the wave goes flat, at which point it becomes audibly fuzzy or distorted. Then same tests with one screen resistor removed but both tubes in.
As far as the red-plating, the screen resistors are always suspect, but not as a cause but a result of the red-plating. The bias (lack of) can be the cause of red-plating, but it can also be pulled down as the result of a bad tube.

[bsco]

I sent it back but these blues jr's are always having problems...When I get another one (which won't be very long), I will do what you suggested just to see what happens....Thanks very much for the tips...greatly appreciated......
Cheers,
Bernie

[52 Bill]

Glad to hear it's working. When you described the sound, it made me think that you were only getting one side of the signal. That's why I suggested testing plate voltage of the driver tube, as that is the more common failure in modern Fender amps.

That type of distortion is very distinctive sounding and eventually you should be able to hear it right away and know what to look for in the amp. I've seen it caused by bad tubes, bad output transformer, open resistors, etc. But in most if not all cases, the telltale sign is that there are missing or wrong voltages in the power amp. That's why the first test should always be to check all of the voltages on the power tubes and in the driver stage.

Are you mixing up the control grid resistors and the screen grid resistors? If the control grid resistor goes open, the bias to the tube will be missing and the tube will redplate. If the screen grid resistor goes open the tube will not turn on, but will not redplate.

[bsco]

I realize the control grid resistors are different from the screen resistors...now I know that when a tube red-plates first check the bias......But I also thought the screen resistors were involved in that as well....I was wrong....now I will know for the next time....I did have an amp recently that had one tube red-platng...it was a Sunn 200S....I swapped the two tubes around and the red-plating followed the same tube.....so there was something wrong with the tube itself....When I had installed a new set of tubes in it, the amp worked great..of course I checked the bias just to make sure.........from now on I'll make sure to check all the voltages on the driver and outputs first......Thank you very much for the reply....this is a great site....everybody is very helpful........if I only knew one tenth of one percent of what you guys have forgotten.......all you guys are great.....keep up the good work......
Cheers,
Bernie

[d. spree]

I sent it back but these blues jr's are always having problems...When I get another one (which won't be very long), I will do what you suggested just to see what happens....Thanks very much for the tips...greatly appreciated......
Cheers,
Bernie

It was problems and expensive repairs on a blues junior that got me started building tweed deluxe clones!! am so happy about that now,