One of the EL84 tubes on my Blues Jr has worked itself loose, and one of the springs on the metal plate holding the tube in has come off also. I've tried fitting the tube back in, but no luck so far. What would happen if I took the back off the amp? Would it be easier to get the tube and spring back in?? Or would it be highly dangerous? Any advice/tips appreciated guys. This should be a straightforward job, but I'm a little nervous. :oops:
As long as you work carefully (so as not to create damage) and you have the amp unplugged while you have your hands among the innards, you should be safe.
There are some components (I.E. Capacitors) which can store electricity for a period of time after the amp is powered off, so you might wait a short period of time if you will be operating near exposed electrical connections.
You should wait a bit before touching the tubes anyway. They get very hot when in use. Burning flesh does not smell pleasant.
I wrapped a newspaper ’round my head
So I looked like I was deep
If you really want to stick your fingers into the back of the amp then you really need to drain the capacitors. This site will show you how and explain a whole bunch of other mods to the Blues Jr. too. http://billmaudio.com/wp/?page_id=17
If you just want to take the back panel off to get the tube back in and will not touch the electronic components inside then you don't need to drain the capacitors. Taking the back panel off will expose the electronic components including the capacitors. However, keep one hand away from the amp so that you don't accidentally touch something live with one hand and something grounded with the other - could very well be fatal. The standard practice is to stick one hand in your back pocket.
As for fitting the EL84, are you sure that you've got the pins lined up right? There's actually space for 10 pins on an EL84 but one is missing - also on the socket too. You need to line up that missing pin space to get the tube to go in.
Pop music is about stealing pocket money from children. - Ian Anderson
Yes, by all means be safe! Those caps can hold a charge for a long time. Tubes are usually on the outside of the danger zone though so you don't need to expose the inside to work on them. I'm just not familiar with the Blues Junior is all to know what it looks like back there.
"Work hard, rock hard, eat hard, sleep hard,
grow big, wear glasses if you need 'em."
-- The Webb Wilder Credo --
+1 on all Hyper says.
(inaccurate info removed!)
in addition to "hand in the back pocket," wear rubber soled trainers and don't lean on anything concrete or metal.
-=tension & release=-
Hmmm, now that I look at the schematic, it looks like the plate voltage doesn't have a drain resistor to bleed off the PSU caps.
If one was going to be working on the "wiring side" of the chassis, then draining the caps would be absolutely the first thing to do.
As long as you are staying on the component side (no exposed un-insulated connections) then one should be fairly safe from any stray voltage.
One hand i the pocket is always good advice, though.
I wrapped a newspaper ’round my head
So I looked like I was deep
just popped the back off of mine (there is only one piece -- I've just nearly broken it into two :roll: ). anyway -- taking the back off pretty much exposes all the dangerous bits -- so be very careful.
-=tension & release=-
I play through my Marshal for 24 hours, and come back to find 6 replies! Thanks guys!
Okay, I'll take another crack at fixing this without taking the back off. The problem is getting the space to jiggle the tube back into place and reattach the little spring - there just doesn't seem to be room to do it. The tube is definitely loose though, as it isn't lighting up and the amp is fuzzing up and going silent intermittently, hence my using the Marshall for a bit instead. I'll report back later and let you know how I get on.
P.S. This forum really is bloody useful...
Sweet! Job done. Back off, tube back in and amp ready to go. And I'm still alive to tell the tale. Nice way to start the morning. :D
I had to take the back off to get working room toe fit the springs and metal cover cap for the ELs back on - I know what you mean about it being pretty dangerous opened up! Fortunately, I didn't have to go anywhere near the danger area, I just needed the back off as the grille which hangs down over the tubes was preventing me from fixing the problem. With that out of the way, fitting everything back together was easy, and I could screw the back on to the amp again safe in the knowledge that I hadn't done anything riskier than replacing the tubes with the back cover still on would have been.
Thanks for the responses. I am (quite rightly) still a bit of a wuss when it comes to amplifier work, so I appreciate the comments and pointers towards schematics and modding info. But I think I'll be leaving the build my own tube amp project until I've gone over physics 101. By which I mean "completed a degree in electrical engineering."
Glad you got it squared away. I would hate to think of you electrocuting yourself. My Peavey Classic 30 has a nice setup. There is a metal cage that covers the tubes to protect them as well as protects you from accidentally touching them when hot. You take that off and there is ample access to them without being inside the amp. I don't think they origionally had them, but I have seen aftermarket ones. The newer ones come with it. I kind of remember checking out a Blues Junior once and thought the tubes looked a little vulnerable and unprotected. I was looking at Blues Juniors online recently. They have gotten expensive! Bet it sounds good though. 8)
Oh ... here is a pic of the thing
"Work hard, rock hard, eat hard, sleep hard,
grow big, wear glasses if you need 'em."
-- The Webb Wilder Credo --
Installing a high value resistor to automatically drain the caps after the amp's been shut down for a few minutes is an easy mod and a nice safety precaution. Some amps come with one.
"A cheerful heart is good medicine."
Installing a high value resistor to automatically drain the caps after the amp's been shut down for a few minutes is an easy mod and a nice safety precaution. Some amps come with one.
It's something that all HV power supplies should be designed with.
But, no. They're saving $0.14 per unit. :evil:
I wrapped a newspaper ’round my head
So I looked like I was deep
Well, 14 cents vs a life ..... the economy is bad you know. :lol:
"Work hard, rock hard, eat hard, sleep hard,
grow big, wear glasses if you need 'em."
-- The Webb Wilder Credo --
designers/manufacturers are pretty tuned in to liability. I suspect there's another reason for the omission. I can't read the schematic imaged here. you sure there is not another sneak path for discharge?
and resistors cost no where near $0.14 -- more like $0.002 to $0.02 depending upon specifics. and this one ain't special.
-=tension & release=-
Yeah, anyone tried to use one of those new gas cans? Manufacturers are very liability concious indeed.
"Work hard, rock hard, eat hard, sleep hard,
grow big, wear glasses if you need 'em."
-- The Webb Wilder Credo --
