Diagnose my 5150

  • Thread starter Thread starter faisalbaig
  • Start date Start date
F

faisalbaig

New member
Just when i was ready to sell my 5150 from scrap, i decided to move the 2 power tubes from the outer sockets to the inner sockets.
Suddenly there was no more volume drop, no choking of the gain etc!!!! :)

I still get some crackles and small popping sounds. Also the head still does not have the clarity it is supposed to have, notes have a certain "scratchy aura" surrounding them.

The RETURN in is still not working. A few weeks ago with my JMP1 connected to the RETURN, i got a big popping sound and since then there is enormous Hum when anything is connected to the RETURN.........

Also installed a Bias mod and running at 30ma, the Tubes are glowing a lot brighter than usual it seems.........

Advise please......
 
popping and crackling are symptoms of bad preamp tubes or poweramp tubes for that matter. if they are stupid loud popping/cracking then it sounds like the OT could be 180* out, was it recently serviced before all of this happened? seems to me someone might have it wrong if they replaced the connector.

as far as the JMP 1 in the return, did you adjust the send level at all in the unit?

you will get a shit ton of hum if the cable is connected directly into the return jack with no proper grounding - you're sticking an antenna directly into the front end of a 120W amplifier wide open balls to the wall - i would be skeptical if it didnt do this. what you need to do is loopback the effects send/return. if there is still humming then something is wrong, if there is a volume drop when plugging/unplugging then get some good contact cleaner and clean the jacks when the amplifier is off. check the effects loop tubes - sounds like the popping/cracking could be tube plate oriented there as well.

check your impedance settings - scratchy, volume drops, choking, etc are all attitudes of improperly mis-matched loads to the secondary of the OT.

mainly though, you need to be 100% positive every tube is in working order. especially if you move it around frequently or do not let it warm up before playing.

post back and we can go from there :)
 
thanks for ur reply. i'll post back more details when i get home.
:thumbsup:
 
When you plug just a jumper cord int he loop jacks what happens?
Pops and crackles can be tubes, usually power tubes. It can also be a poor connection which happen sometimes with the ribbon cables on the 5150's. Could also be a broken solder joint somewhere in the amp too. A lot of times a poor connection in the high voltage circuits of the amp will cause a crackling arc noise.
If everything was fine before you modded the bias I would go back and retrace your steps to make sure you did it right and there are no issues with the work you did.
Jerry
 
Pardon my lack of technical knowledge, whats a Jumper cord ???

Things weren't fine before i bias modded the amp. It was very moody. It would drop volume suddenly, get noisy and scratchy, the distortion would sound more like busted speakers sometimes. At other times it would suddenly sound fine.
After the bias mod and cameron mods, the amp was running just fine for a few hours, then i connected the jmp1 into the Return (Guitar-->Jmp1--JMP OUT---5150 Return), no input cable to 5150, it was fine till i raised the volume a bit, got a loud pop and then HUMMMMMMMMMMMM!!!!! It also became very noisy when just plugging into the 5150.

Then i switched my pair of tubes from the inner to the outer sockets and the amp came back alive!! But still hives Hum on the return.....

I cant gig with this amp coz i feel its not reliable right now. No pro amp techs here, just some old timers who know about tubes and tube audio, not specifically guitar amp techs. So i rely on knowledge i find myself and try to help them identify the fix.

Am thinking of replacing ALL tubes, Pre and Power. Should i replace the power tube sockets as well ?? Any other parts i should replace ?

Oh, since reading another thread here, i have minimized the Presence knob. Today i realized it does add a lot of scratchy noise, not just hiss or fizz.

Am hoping that replacing tubes should fix all this. How can i check if my trannys are fit ?

Any ideas, please help.

Thanks
Faisal
 
by jumper chord he means normal short 1/4" instrument cable going out the send back into the return. follow the directions i mentioned above on what to look for in specific and as to why you would want to do this.

leave the transformer idea to a technician - they involve direct rectified voltages right off the PT and can be in excess of 450V or more. one slip of the probe (or your hand) and you could cause more damage, or at worst kill yourself.

It sounds to me to be tube related, the pop and hum is exactly what it sounds like when a tube shorts grid to cathode, as the voltage you are hearing is the indirect heater hum.

replace the power and preamp tubes both with known working tubes - used or new as long as you know they work. the sockets should be fine if you have not touched them - i would take one of your old tubes and with the amplifier off, take contact cleaner, spray the tube pins, shake off the excess cleaner off the tube, and put the tube in and out of each power socket repeated times. wipe off the pins of the power tube, respray it/shake off the excess between each socket. do not wiggle the tubes in and out, pull them and push them!!! it should be a tight fit for each socket. DO NOT put the tube in the socket with it dripping wet

double check that your bias modification is done correctly - regardless whether the symptoms were there before or not. just because they are there before does not mean you could have created more problems for yourself down the road.

i would also check the cap you installed on the board and make sure it is not shorting out close to anything around it and assure all of your soldering work is not cold-soldered.
 
:thumbsup: I will make my tech follow all these steps. thank you so much.
 
Just did the jumper cord test: connected a short cable from the send and barely touched the return socket that i had a very very very loud Hum/Noise. I disconnected immediately. Its 5am here!!!! :doh:
help.....
 
faisalbaig":24ndv849 said:
Just did the jumper cord test: connected a short cable from the send and barely touched the return socket that i had a very very very loud Hum/Noise. I disconnected immediately. Its 5am here!!!! :doh:
help.....
Click to expand...

when daytime comes pull the effects loop tube out of the socket and fire it up - plug the chord in and see what it does. still humming then you've got a coupling cap shorted/defective.

been there done that before, i remember biasing an amplifier on my bench once and all of a sudden the rectifier diode let go and then the fuse went behind it - talk about making you jump out of your pants :lol: :LOL:
 
Don't start throwing parts at it, you will make things harder to get fixed. Start at the begining and go thru it logically.

The problems you describe having before you touched it sound like dirty loop jacks. Clean them.

The problems you had after the JMP1 in the loop could be many things if the JMP1 is in fact what caused the problem. I would start by changing the 12AX7 for the loop. It's the second tube in from the power tubes. It's V3 in the pictures here http://www.fjamods.com/5150Models.html

I would check over everything that was done during the bias mod and the cameron mods. My first guess is you screwed something up or have a bad solder joint, solder dripped across something it shouldn't have and is causing a short, take a good look here.

Try the cord in the loop, see what that does. Does it still hum when plugged into the return jack that way?

Try running out of the send jack into another amps loop return jack.
Try plugging the guitar right into the return jack.
Jerry
 
You must log in or register to reply here.
Back
Top