Amp repair advice

[Bret608]

Hi everyone,

So, I've been playing a Fender Blues Jr. for about 17 years. I changed the power tubes once about six years ago, and the preamp tubes this summer. I've never had any mod or repair work done on it. I think I may be having my first issue with it ever.

The second to last time I played it, I was hearing some slight popping sounds here and there while plugged in. The last time I played, after I turned it off, I went and did something else for about ten minutes and then came back to unplug my guitar, etc. I noticed the control panel was quite warm when I touched it, and the tip of my cable was also warm when I pulled it out. I've never noticed this before, even when unplugging the cable right after turning off the amp. It wasn't burning hot or anything, but I was concerned nonetheless.

Any pointers from those of you who know amps would be much appreciated!

Cheers,

Bret

[pryde]

Well there is a number of things/possibilities that could be an issue. Here is what I would do:

1. New power tubes installed and BIASED correctly: Your tubes are 6 years old which is a long life if played regular. Also, over time the bias will drift and further compromise the amp.

2. After power tubes, replace 1 pre-amp tube at a time if the noise/issue still persists as you may have a bad preamp tube in there as well.

I recommend getting new power tubes first and foremost due to the age of your original ones. If they are failing and actually short-out, there can be more significant damage/repair cost for you. If you still have issues after re-tubing, then you have eliminated the most likely cause first.

[gtr2]

For the popping on startup, you likely need a cap job.  The illinois caps they put in those don't last long from what I've found.  I just recapped my friends with some F&T's.  3 out of 4 were leaking.  The hardest part is getting the PCB out

Also, those amps are biased unnecessarily hot.  You can add an adjustable trimpot so that it the dissipation is proper for the EL84's.

Plenty of info is available on BillM's site.

Josh

[Bret608]

Thanks you two! I really appreciate it.

I will go for some new power tubes first then. So could the tubes beginning to fail/short be the cause of that heat I was noticing?

Duly noted on the popping--this amp has never been recapped, so I bet you are right. I've been wanting to do the BillM mods for awhile now, but there always seems to be some pedal to build that I give a higher priority to!

[pryde]

Possibly but excess heat can be caused by many factors. A surefire tube toaster is bias current way out of spec.

As said the Chinese made Illinois filter caps in those may be at their end even though on paper they spec a long life. If you are going to rip-er open, definitely change them to F&T caps.

[gordo]

I'll dig thru my notes when I get a chance but there are a pair of resistors that start to break down and they get worse as they get hotter.  You may notice one of the power tubes is screaming hot and the other is almost cool.  It's an easy fix, but you have to take boards out of the amp.  The Bill M mods are subtle but nice and his prices are reasonable.  I've installed a bunch of his kits and while it would be easier and cheaper to just get the stuff yourself he's been great to deal with and I suspect he's not getting rich anytime soon.

[rullywowr]

I have a blues deluxe (usa, early 90's) and while it's a great sounding amp there are some serious design issues with it. As the blues deluxe and blues junior share a lot of the same design I hope this will help.

1.  Input jacks. These are notorious for flaking out. The jack gets tugged by the cable and causes the metal nut to loosen. This then allows the PCB mounted jack to flex and break the solder joints on the PCB. You may get lucky and just have to reflow the joints but its more likely that the jack needs replacing.
2.  Switchable channel problem. The relays sometimes go in these amps and it causes the amp to not switch properly and get stuck between drive and clean. Need to replace.
3.  Screen grid resistors. These look like square ceramic blocks and are sometimes underpowered in these amps. I had to replace mine and they got so hot they actually burned a spot on the PCB. There is a bunch of info on the web about replacing and upgrading these to a higher wattage rating.   I ended up replacing them and floating them off the PCB with some silicone to help dissipate the heat. This is a crappy design on Fender's part.

Hope this helps, and please if you don't know how to drain the caps in your amp -read up on it or let a pro handle it. I've been zapped with 500vdc and it really really sucks.  It's something that most DIY pedal guys can do, working on amps, you just have to really respect the voltage that lies within - even with the power off and unplugged.

[rullywowr]

For the popping on startup, you likely need a cap job.  The illinois caps they put in those don't last long from what I've found.  I just recapped my friends with some F&T's.  3 out of 4 were leaking.  The hardest part is getting the PCB out

Also, those amps are biased unnecessarily hot.  You can add an adjustable trimpot so that it the dissipation is proper for the EL84's.

Plenty of info is available on BillM's site.

Josh

Absolutely. I added a bias trimmer in mine and it made a good difference. They do come really hot cold from the factory with fixed bias.

[Bret608]

Again, I appreciate all the help and pointers. Gordo, I would love it if you are able to find your notes on which resistors those are. I had the amp opened up last night and am starting to get a handle on what parts are where. By the way, mine is a green board, revision "b" version from about 1996.

When I had it opened last night, I set my DMM to the 200vdc range and took readings on all the filter caps. I was getting 16-17 volts on each of them. Is that a situation where I need to drain them, or is the voltage low enough not to worry about?

[pryde]

First, please respect the voltages in the amp. They can kill you,,,really. Properly drain the filter caps and when/if you are testing/probing a live circuit as you described you did, one hand in your pocket, other one does the probing/measuring. Never assume caps are drained fully unless you know they are right?

Rullywowr pretty much pointed out all the good and popular fixes with these amps. The 2 470 ohm 5w resistors are associated with 16v zener diodes (CR22/23 I believe). This network supplies proper voltage to many opamps in the circuit. Often if the big resistors have heated and burned the pcb then I would do as rullywowr advised and raise them and silicone under/between them. Also check/test those 16v zeners as well for damage.

Good luck and be careful 

[Bret608]

Thanks pryde--believe me, those voltages do make me uneasy. The only reason I felt okay doing what I did was because BillM showed how to do it safely on a video clip he has on his site. My B Jr. has no standby, so the filter caps drain out almost completely just by turning the amp off. I had it off and unplugged at the time.

That said, I am leaning toward just ordering parts from him but having a local tech install it. Meanwhile, I have fresh JJ EL84s on the way from Tube Depot, just to keep the amp safe from any tube shorting until I can get it into the shop.

[Bret608]

Just wanted to pop back in and update you all on this situation...

I replaced the power tubes with some nice JJs from Tube Depot. I have not had the heat issue since.

However, I still have that popping, and it's getting worse. It's an audio pop if that makes sense--like it's coming from the speaker, not a physical pop. That sound like the filter caps? I was in our bedroom and the sound got my wife's attention all the way from our living room! My guitar was not even plugged into the amp yet.

Thanks for any pointers, and Happy New Year! 

[pryde]

Just wanted to pop back in and update you all on this situation...

I replaced the power tubes with some nice JJs from Tube Depot. I have not had the heat issue since.

However, I still have that popping, and it's getting worse. It's an audio pop if that makes sense--like it's coming from the speaker, not a physical pop. That sound like the filter caps? I was in our bedroom and the sound got my wife's attention all the way from our living room! My guitar was not even plugged into the amp yet.

Thanks for any pointers, and Happy New Year! 

Did you order an extra preamp tube (i.e. 12ax7)? You can replace 1 preamp tube at a time to see if it solves the problem. Not saying it is but you want to start there before assuming other issues.

Again other good possibilities are mentioned above but try a simple tube swap first to rule it out.

[Bret608]

Unfortunately, I did not order new preamp tubes as I just put these ones in over the summer. Even worse, I did not keep the old ones so I don't have one to swap with and compare. I may check with a local repair shop and see if they can let me test out a few different ones.

[pryde]

Preamp tubes can last a very long time but it is always best to start with that for troubleshooting and I good idea to have an extra for backup. Popping/crackle/static can very well be from a bad tube (amongst many other things as well).