Retube Or Not To Retube

[ELVIS]

my dad is an old military electronics technician and he helped me work on amps when I was a kid...

BS is correct...

Do not discharge a filter cap with a screwdriver...

You might end up with the same results as laying a wrench across a car battery...

[BrownSound1]

Originally posted by Hardrock69
My Super Lead head uses EL-34s.

I have had the current tubes since I bought the thing about 9 years ago.

When I got it originally it was kinda fucked up, but I took it in to the local Marshall Magician, and he fixed me right up.

Been kicking ass ever since!

Well they quit putting the 6550s in the US Marshalls back around 1985 or '86. Started the practice sometime in the mid-70s.


One other saftey tip for those working inside their amps. Always use the one hand rule, so you don't get across high voltage and it go through your heart. That's a bad day if that happens.

Take off rings and watches too.

[Hardrock69]

Personally if I need an amp worked on, I take it to the local guru.....though my last guru here in Gnashville retired a couple of years ago....

[Nitro Express]

Have have two books on amp repair. One says to bleed the capacitor using a resistor and the other book (writen by a former Marshall R&D technician) says to use an electricians screwdriver.

I always used the screwdriver method because it's obviouse that the caps are discarged because you can hear it. With a resistor you just don't know for sure and plus you have those leads in the way of things and possibly more wires to get shocked by.

I've never heard of damaging capacitors using the screwdriver trick. I knew an electrical engineer who liked to screw around with old tube amps who did the screwdriver trick.

I was told in the 90's that Marshall had gone to using 5881 tubes in their new JCM 900 line because a source of reliable EL-34s was not available. There was a period where the only good El-34s were new old stock.

But yeah, a bleeder resistor will do the trick.

I think it's best to treat a tube amp just like how you treat a firearm. Even if it's unloaded you treat it like it is because plenty of people have been shot by supossedly unloaded guns. It's the same with those big filter capacitors. Treat them like they are always hot and you won't have a problem.

[Matt White]

Good advice....


TANX

[BrownSound1]

Originally posted by Nitro Express
Have have two books on amp repair. One says to bleed the capacitor using a resistor and the other book (writen by a former Marshall R&D technician) says to use an electricians screwdriver.

I always used the screwdriver method because it's obviouse that the caps are discarged because you can hear it. With a resistor you just don't know for sure and plus you have those leads in the way of things and possibly more wires to get shocked by.

I've never heard of damaging capacitors using the screwdriver trick. I knew an electrical engineer who liked to screw around with old tube amps who did the screwdriver trick.

I was told in the 90's that Marshall had gone to using 5881 tubes in their new JCM 900 line because a source of reliable EL-34s was not available. There was a period where the only good El-34s were new old stock.

But yeah, a bleeder resistor will do the trick.

I think it's best to treat a tube amp just like how you treat a firearm. Even if it's unloaded you treat it like it is because plenty of people have been shot by supossedly unloaded guns. It's the same with those big filter capacitors. Treat them like they are always hot and you won't have a problem.

A quick discharge to ground most definitely can damage filter caps, I don't care what that book said. I've seen it with my own eyes. You might get away with it 4 or 5 times, but it'll eventually get you. Plus, who wants to risk welding the screwdriver to the chassis. Some amps automatically bleed off the caps when you shut it off...and they use a bleed off resistor in the circuit. It doesn't take as long as you think, and if there is any question, then check it with your multimeter. (You shouldn't be probing around in the amp without one)

Now, the EL34 shortage for the JCM900 is probably true...but c'mon a JCM900 ain't a real Marshall. I've seen them with both EL34s and 5881s (6L6..same thing), and it still sounded like ass with either one. The Plexi reissues from that period still had EL34s, so who knows. The first JTM45s had 5881s as did the very first 100 watt amp that Marshall built...complete with dual output transformers.

Nowdays you've got plenty of sources for EL34s, thankfully. No, they aren't as good as those old Mullards, but I think they're much better than the late 80's/early 90's offerings.

[GAR]

Originally posted by ELVIS
More specifically, filter caps...

Good gawd, are you and BS1 the only ones with some kinda common sense in here?

He needs to replace the filter caps. I'd do that before replacing any tubes whatsoever.

Seldom have I ever replaced power tubes. Eddie dropped the voltage input with a Variac because he wasn't deep in the money like Roth and needed to stretch the tube life.

I'd redo the caps and see what happens. I never replaced a preamp tube once I got an amp setup properly and a good set under casual use should last at least 10 years.

Unfortunately capacitors last to spec 7 years at most and it's difficult to explain this to people who think they can just throw a new tube in there when the juice to run it just ain't there off the juice end of the caps.

[Nitro Express]

I got one of those old Philco radios from the late 1930's that is in the big wood box that was suppossed to look like a piece of furniture. It was my grandparents and the only thing it took to get it working was replacing the power cord and new electrolytic capacitors. It has all the original Tung-Sol and RCA tubes that were in it. Of course all the cobwebs and shit were cleaned out of it but it still works and that big ass speaker sounds great.

I don't think anyone will be using my iPod 60 years from now. Got to love tubes!

I was told electrolytic capacitors will last a long time if they are used but if they just sit there unused, they dry out and fail.

I've never done a cap job but I heard you need to bring the new capacitors up slowly with a variac after you install them.

[Nitro Express]

I think the only JCM 900 I've ever heard sound good is the one Henry Garza uses in the Los Lonely Boys. He runs it through a Bogner 2x12. I mean he's using a high gain amp to get a fairly clean Tex Mex sound but it works so what the hell.

[BrownSound1]

Nitro, you're right about electrolytic caps drying out...they definitely will do that. On the variac thing...I know some people say you have to do that, but it is bullshit. The only way I could see you needing to use a variac is if the caps had been sitting up for a long time. You might have to use a variac to reform them. New caps though...forget about it, definitely not necessary. Hell, when most electronic equipment gets manufactured the caps go straight from the box, so to speak, into the unit and fired up to full power. They don't have time to do a slow ramp up.

[Nitro Express]

LOL! The guy that restored that old radio uses a Klein electricians screwdriver to discharge capacitors with a big POP! But he's tottaly anal about using a variac to bring new capacitors up to working voltage.

My head is spinning. Who to believe? LOL!

[Nitro Express]

I've heard stories of capacitors exploding and shit like that. Then you hear the stories of capacitors oozing nasty electrolyte when they are shot. Frankly I've seen none of this.

On really old amps that haven't ran in years, they just don't work or the main fuse blows. I've never seen the oozing and the only capacitors I've heard make loud noises are the ones I've shorted out to ground with a screwdriver. Actually I never liked doing it. It's scary so I guess I sould solder up some bleeder resistors.

[GAR]

Any screwdriver bladed across the capacitors terminals will cause an arc during discharge. They do this to make the piece safe to desolder and remove.

Every tech brings up the voltage on the bench with a new recapping by stepping it up on a Variac.

Just do it. Check mojotone.com for prices maybe you could do it yourself.

[Sleeper Cell]

here are a couple of tube tips I can think of to pass on.

1. Look for discoloration in the plate when the tubes are working--could be a sign of a bad tube

2. Look for burn marks--have you ever heard of anything that looked burnt being good?.

3. Try if you got the means to get a 'matched' set of tubes. This just means that the tubes have been tested and determined to have close to the same break up point and draw.

4. Your amp may need to be bised once you put new tubes in. If you are a novice electrician/tinkerer, do not do this by yourself...

5. Some amps are harder on tubes, and the life is determined by usage, so if you play every day or several times a week, you may get anywhere from 6-12 months out of 'em. My Matchless EATS tubes. I get noticeable loss after about 8 months if i am gigging or playing my usual 1-2 hrs a day.

6. Check the net-there's lots of good tube sites out there full o knowledge.

I am just guessing, but an amp as old as yours probably needs new tubes, especially pre-amp section, but probably all new tunes..

my .02$

[Nitro Express]

What really beats tubes up is the heating and cooling. That's hard on them. That's why they started putting Standby switches in amps to keep the tubes warm during breaks when the amp was going to be used again.

Also bumping tubes when they are hot is hard on them. That's why you should let your amp cool before moving it.

Combo amps are especially hard on tubes because the tubes are usually by a speaker. If you play hard rock and roll with a cranked combo amp, the tubes are being rattled by the speaker when hot and they tend to not last as long. That's why it's better to have the amp sepparate from the speaker.

Maybe Pete Townsend had his Hiwatts off to the side and away from his speakers for a reason.