Blues Jr. Trouble

[JlMMY]

I am working on a friend's Blues Jr. When he gave it to me I noticed one of the EL84 power tubes was almost out of the socket. To my eye it looked as though the tube was not pushed in far enough to make contact. The socket was not loose and the tube probably got knocked out of the socket by the power cord after a jam. I tested the tube and found it has a short. I also found a resistor that had scorched the PCB. The schematic identifies it as R47 (470k) and the resistor is also testing bad. I replaced the tube and the resistor and still don't get any sound. Any ideas what to check next?

[Miles19]

The power supply. Then start at the first stage and check for output (using a scope), and move on to subsequent stages. (Obligatory warning: high voltage can KILL YOU, don't do any of this unless you know what you're doing!!)

R47 is on the reverb return, so it would knock out the reverb only. Beats me how that could get scorched though - the voltage is very low in that section.

Miles

[SatelliteAmps]

If you are unsure of working on amps that are live, dangerous, and potentially lethal, then take this to a proper tech or get someone's help who knows what they are doing.

Before even bothering hooking up a scope, just do a basic check for voltages and make sure they are all there. Then, if you want to hook up a scope, you start at the end, not the beginning, and work backwards. Make sure the power section is working before trying to troubleshoot the preamp.

[JlMMY]

I was trying to avoid pulling the PCB. It looks like a lot of work (and will be my first time doing this) but I cut the burned resistor with enough lead left on the board to attach a resistor from the top side and it didn't fix the problem. I found a schematic and a thread on another forum that says R47 is a fuse. I am going to pull the board and go from there. I am personally not skilled enough to debate whether R47 is part of the reverb circuit but here is a quote from the other forum.

"The next most common failure is a screen short in one of the EL84s. I think Fender's intent was for the little screen reisistors, R35 and R36, to act as fuses and burn out if there's too much screen current. But they always survive and R47 burns. Is heat a factor in screen failure? Seems likely to me."

[phsyconoodler]

Did you check the usual suspects like speaker,jack,preamp tubes,input jack and measure all the voltages?
Voltages will tell you a lot.

[JlMMY]

Originally Posted by phsyconoodler

Did you check the usual suspects like speaker,jack,preamp tubes,input jack and measure all the voltages?
Voltages will tell you a lot.

Everything except measure all the voltages. When I started the thread I was hoping it was a common problem that would have a very specific fix. I was also a little afraid of pulling the PCB, since it is such a pain to approach. I guess I am at the point now where I check the voltages. Speaker is good. One EL84 is bad. Preamp tubes are good.

[hasserl]

Originally Posted by JlMMY

I was trying to avoid pulling the PCB. It looks like a lot of work (and will be my first time doing this) but I cut the burned resistor with enough lead left on the board to attach a resistor from the top side and it didn't fix the problem. I found a schematic and a thread on another forum that says R47 is a fuse. I am going to pull the board and go from there. I am personally not skilled enough to debate whether R47 is part of the reverb circuit but here is a quote from the other forum.

"The next most common failure is a screen short in one of the EL84s. I think Fender's intent was for the little screen reisistors, R35 and R36, to act as fuses and burn out if there's too much screen current. But they always survive and R47 burns. Is heat a factor in screen failure? Seems likely to me."

The schematic I have shows R47 as a 470k resistor in a voltage divider in the reverb return circuit. It also shows the screen grdi resistors as R24 & R25 (100 ohm, 1/2 watt). This schematic is marked Rev B and drawing #049301. Do you have a different schematic?

[JlMMY]

Originally Posted by hasserl

The schematic I have shows R47 as a 470k resistor in a voltage divider in the reverb return circuit. It also shows the screen grdi resistors as R24 & R25 (100 ohm, 1/2 watt). This schematic is marked Rev B and drawing #049301. Do you have a different schematic?

My schematic is the same, Rev. B. I can see now that P3 comes from the reverb tank and should read 191mv just before R47. I am a bit confused by schematics.

[cmadragon]

If schematics confuse you, you probably should not be messing around inside of a tube amp.

[Bob V]

BillM's web site has great instructions on how to get at the flip side of the PCB so you don't have to do anything messy like trying to solder to a stub of a cut off lead for a burned resistor.

Make sure you've got the right schematic, there's a green board and a cream colored board, and several revisions of each one.

[tim boehlert]

Usually a burnt resistor in conjunction with a bad power tube is an indication of a short - and it usually takes the complementary paired tube out (in this case the other screen grid resistor AND power tube...). From the advice above, I'd guess that this doesn't hold true for the EL84's? At any rate, I'd check for other toasty parts - look for cracked components, not necessarily well-done as was the case with the first resistor. My gut feeling is that if the first Power tube went, you'd want to re-tube it with a new set, rather than just one tube.

[JlMMY]

I re-tubed and replaced the visibly bad R47 (470k) resistor. I pulled the PCB, which was a lot of work and still nothing. I guess it is time to buy an oscilloscope? I work on it for awhile than put it aside. Pulling the PCB wasn't as hard as I thought it would be. Thanks for the tips guys.

[bgoez]

all you need is a volt meter to start this project. probably the large resistors on the tube board. or perhaps the traces to the tube jack have been broken when the tube was jarred. look them over real well. you'll need to take out the 4 or 6 screws holding the board in. Remember as everyone else has stated. high voltage here, make sure the amp is unplugged and all the caps are drained. good luck!

[JlMMY]

Originally Posted by bgoez

all you need is a volt meter to start this project. probably the large resistors on the tube board. or perhaps the traces to the tube jack have been broken when the tube was jarred. look them over real well. you'll need to take out the 4 or 6 screws holding the board in. Remember as everyone else has stated. high voltage here, make sure the amp is unplugged and all the caps are drained. good luck!

I am glad this thread is still alive. I have fixed 10-20 amps so far but this one is giving me trouble. I check the power tubes from the traces to the sockets with a continuity checker and all is working. I haven't done the same for the 12AX7s yet. I did change the power tubes when I replaced the resistor that was burnt at R47. I pick this amp up about once a week and try fixing it. I am frustrated but don't want to give up.

[phsyconoodler]

Voltages,voltages,voltages.
The plate voltages at each tube,and the filament voltages and the cathode voltages at the preamp tubes.That will tell you if the preamp tubes are conducting current,are getting voltage to the plates and whether the power tubes are getting bias voltage.
At the power tubes,you need plate voltages,bias voltages(this will be about -11v)filament voltages.
Then you need to measure the output jack for AC voltage.When you plug in your guitar and strum it,you should have a fluctuating AC voltage at the output jack.