Filter cap question.

guitarcapo

Senior Member
Messages
2,326
I just purchased an almost 50 year old Magnatone 410 amp. It's basically like an old Fender Vibro Champ but with better vibrato and speakers.

Anyway it was never worked on and in very clean condition.

It looks like it sat in someone's closet mostly.

Before the amp arrived I was pretty sure I was going to install a 3-prong cord, which I did. The vibrato circuit wasn't working....and I was pretty certain it was due to dried up larger caps. I found that after replacing a few of the larger electrolytic caps inside the amp it started sounding better and better.

The vibrato returned and the amp sounded less distorted. I was getting a better sound as I went so I replaced about 7 or 8. I used orange drops because I like their definition with this kind of vibrato I know some people discourage replacing caps only as a "last resort" but what can I say? It sounded better.

Anyway getting to my question: I assumed this amp would probably need a new set of the larger filter caps because of its age. It had this "can-cap" and I was dreading pulling the thing and "re-stuffing" it with new caps. I decided to just buy some and install these inside the amp. I even bought the caps before the amp arrived. This amp is 50 years old and I was certain it would probably need a cap job.

But right now with the amp cranked to maximum volume, I hear absolutely no 60 Hz hum other than a slight magnetostriction hum from the transformer which is there whether the amp volume is at 0 or maxed out.

It's dead quiet.

Now some of the smaller caps inside the amp were obviously gone with residue on them. How is it that the larger caps inside the can are working fine? I know for a fact the amp has never been touched in 50 years.
Can filter caps last that long? The amp also plays fine now. Is there some sound that would happen while playing that would indicate that the caps need to be replaced that you WOULDN'T hear when you aren't playing?

I'm probably going to leave the amp alone right now. I guess my question is: Is it possible that an amp with filter caps failing can show symptoms of certain sounds while playing that I should listen for...but still be dead quiet when not playing?
I'm using the fact that it's dead quiet with the amp cranked but no playing as a guide that the caps are fine.

BTW it's possible that the amp just sat unplayed for a long time and someone brought it up slowly with a variac, reforming the caps...but I don't know.
 

Blue Strat

Member
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30,687
Lots of electrolytics last that long. The point is that they don't last forever and when they go, they can take out expensive or irreplaceable items like OT's, PT, etc.

If it were my amp I would replace them.
 

Structo

Member
Messages
9,553
Electrolytic caps actually last longer if the amp is played regularly.
The smaller caps inside are probably the bypass caps on the preamp tubes. These should also be replaced.
As for the filter caps, I like to drag this picture out when someone is hesitant about replacing filter caps.

DSC01888.jpg


DSC01889.jpg


I believe this was a JTM45 amp that the filter cap exploded in.

Not only is this a mess to clean up, the electrolytic paste is caustic and can be injurious
to the technician if he happens to be looking down into the amp when it explodes.

As mentioned, a shorted cap can also take out tubes, transformers or other expensive components.

Better safe than sorry.
 

Jerry Glass

Member
Messages
869
Those metal can caps have a tendency to last a really long time. I can probably count on one hand how many times I have seen a bad one in all the amps I've had in the shop over the last 15 years. I don't replace them unless they are bad (based on value and ESR...and performance to some degree). When I do replace them, I tend to prefer to use discrete components, keeping the original cap in position but disconnected; this preserves the external aesthetics of the amp.

I have personal equipment from the 50's and 60's (old Fisher and McIntosh amps) that still have the original cap cans in them with no issues what so ever.
 

guitarcapo

Senior Member
Messages
2,326
I guess what I'm asking is would there be some sort of sounds you would hear to give you some warning as they are on the way out...or would it just be "BLAM" one day?

BTW this is a 6 watt amp at best. The 3 caps in the can are something like a pair of 20uF and a 10uF at 450 volts.
 

Jerry Glass

Member
Messages
869
I guess what I'm asking is would there be some sort of sounds you would hear to give you some warning as they are on the way out...or would it just be "BLAM" one day?

BTW this is a 6 watt amp at best. The 3 caps in the can are something like a pair of 20uF and a 10uF at 450 volts.

First of all...snaps and pops that are not tube related. This is the biggest sign of an impending failure.

Secondly, hit a low E gently at low volume. Then hit it again more aggressively at a higher volume. Note if the note goes flat at the higher volume and the degree that it does. More than a half step flat would indicate that you will note a tonal improvement with a cap change. This is a quick and dirty field test and should only be used as an indication of an issue but not as a substitute for actual measurement.
 

guitarcapo

Senior Member
Messages
2,326
I think I'll give that a try.

I'll probably just add a rail inside the chassis and add the 3 caps eventually.

I hate taking it away from stock but for me it's all about the sound and keeping it working.
 

Ronsonic

Member
Messages
3,303
Hum is only one symptom of failing filter and bias caps. In a push pull amp, power supply noise is largely canceled to the point that the first audible symptoms are heard when playing or observing the output on a scope in testing.

Replacement can caps are available if you're interested in keeping it original looking.
 

phsyconoodler

Member
Messages
4,301
Quote:" It had this "can-cap" and I was dreading pulling the thing and "re-stuffing" it with new caps."

Good luck with that!

I do not trust old cap cans just because the amp is quiet.I have worked on many 'quiet' amps that have taken out PT's and burned resistors because of 'quiet' old cap cans.
Change them.
 

guitarcapo

Senior Member
Messages
2,326
Just further away from stock in that you have this whole set of new caps set up inside the amp on new wiring. It's one thing to swap out components...but now it's further removed from that. Like I said...not a deal breaker...just less original.


Yea I saw this website that showed how to do re-stuff an old cap can to keep things looking original.

I kind of thought it would be cool for the amp to look like it hadn't been touched...but what's the point? The only person who would care is someone looking at the inside of the amp....and then he might think..."Hmm....probably needs a cap job...I'll have to consider that into the price estimate that this amp is worth."

Does anybody know if ceramic disc caps ever "go bad'? This amp has a few of those and I'm wondering if the amp would get any better sound if I swapped out some of the larger ones (maybe .01 or .02 uF) of those.


BTW the amp had Telefunken 12AX7 preamp tubes in there. Made in Germany. Cool.

The 6V6 was an RCA blackplate.
 

Ronsonic

Member
Messages
3,303
I've done the can restuff, and there was at least one company doing that commercially. There are enough cans on the market that for most of our amps we can find a reasonable replacement off the shelf.

Ceramics rarely go bad and despite (or because of) their microphonic tendencies are distinctly mojo parts. IMO you can change the sound of an amp slightly with a cap change - though there are a lot of arguments about this, here and elsewhere - I don't believe that there is any cap that can guarantee better tone.
 

donnyjaguar

Member
Messages
4,194
The last one I blew hissed like a cobra snake before the fuse blew. It vented all its dielectric goo inside the amplifier. I've heard many older capacitors had PCBs in them but guessing these are the exception and not the rule.

PCB = polychlorinated biphenyls
 

SatelliteAmps

Member
Messages
6,168
Changing out the electrolytics is keeping the amp much more original than it is to pull all of the tone shaping caps and replace them with orange drops. At this point, the amp is considered non-original, so changing the caps that should be changed does nothing to affect the value or originality of the amp.

Changing electrolytics is like changing the oil in a car, and it affects the calue and originality the same way. It needs to be done if you want to drive the car. If you don't the oil will turn to sludge and end up not lubricating the engine, and it will blow. The coupling caps, on the other hand, are more like the personality of the car. If it originally came in Plum Crazy Purple, and you decided to change the color to Orangey Orange, then the value of the car will be affected. Each small cap you replace will affect the value and originality. Obviously, if a cap is actually bad, then it should be replaced, just as if a car is scratched it should be repaired (or a tear in the seat, or a broken radio, etc.)
 

guitarcapo

Senior Member
Messages
2,326
Yea I guess I could have left those blue caps with white crusty powder on them in there and had a more valuable amp with no vibrato but lots of personality...but I guess I figured why own a vintage Magnatone if you can't hear that vibrato? Maybe it looks cool in someone's trophy room sitting there I guess.
 

SatelliteAmps

Member
Messages
6,168
I didn't say NOT to change them, I am saying that you shouldn't be worried about not changing the electroytics in some attempt at keeping the amp original when all that other stuff has been done.

If the blue caps weren't working correctly then there is no real choice, except to replace them.
 




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