Capacitor Job -- Fad or Necessity?

Alan Kirk · From: Scotia, CA, USA

I took a '61 Princeton to a Tube Amp Repair class in Santa Barbara, CA. I told the guy I wanted to do a cap job. He asked why? I gave him the standard 10-20 years and they wear out spiel. He shook his head and said "If it ain't broke, don't fix it." He got out his digital VOM and started measuring voltages across the caps. One was suspect, but after looking at the schematic and doing some math, he said it was all right. He said my caps were working fine and I shouldn't mess with them.

The amp works okay, but needs a new set of power tubes (one of them has a cracked base but is still working -- and after taking voltage measurements he said the tubes weren't very well matched).

Good advice or bad advice? As I said, the amp works okay. No dramatic problems.

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[This message was edited by Alan Kirk on 26 October 2004 at 12:40 PM.]

Bill Hatcher · From: Atlanta Ga. USA

Fender amps will "work" under the worst case of worn out caps, tubes, resistors etc.

They will sound incredible and work to the specs they were designed to if you do the routine maintenance every so often. I don't agree with your techs creed of "if it ain't broke". Do you drive your car with bald tires--they ain't really broke, and the car still rolls---right?

I think 20 years on a set of filter caps in an amp that has seen a bit of use is long enough. You will get folks with all the usual tales about running an amp for umteen years on the same tubes and caps,yes they will do that, but the old Fender amps have so much more potential with fresh caps and tubes and such. Also one thing to consider is that new parts built today are better in regards to specs and construction.

Ray Minich · From: Bradford, Pa. Frozen Tundra

Did he take the cap connections apart and measure leakage currents?

Blake Hawkins · From: Florida

Since I did a filter cap job on my 1953 Fender Deluxe last week, I'll jump in here.

I had noticed a little intermittent noise and faint hum.
The last time the caps were replaced was in 1968.
Put in a new set and the change was dramatic.
The amp is now completely quiet.
Checking my old caps on my Sencore Z meter showed that one was pretty normal,one was completely open, and the other was intermittent.
I agree with Bill, Fender amps will continue to work with many old components that have shifted in value.
In a guitar amplifier simply measuring the voltage across the cap does not tell the whole story. The leakage is important as well as the ESR.
Bill's analogy to tires is a good one....caps do decline in performance over their life.
Don't worry about a cracked plastic base on a 6V6GT..they run so hot that it is not uncommon.
Blake

[This message was edited by Blake Hawkins on 23 October 2004 at 09:37 PM.]

Bob Lawrence · From: Beaver Bank, Nova Scotia, Canada

It's called prevenative maintenance. If your tech doesn't understand the term, find a new tech. Why have you amp blow up (yes capacitors explode) on stage when it could have been prevented? Even a capacitor that sits in storage has a shelf life. When it comes to identifying faulty electros.
forget about capcitance meters and volt meters; an ESR meter is the way to go. The ESR meter (mentioned above) allowes service technicians to quickly and easily identify defective electrolytic capacitors while they were still in circuit. It measures a characteristic of electrolytic capacitors called "equivalent series resistance" or ESR. The ESR values are different for the different capacitors values. Therefore, a lookup chart is required. ESR meters will find faults that other meters will miss totally.

Trivia:
The first ESR meter design was published in the January 1996 edition of "Electronics Australia".by Bob Parker.

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Andy Zynda · From: Wisconsin

Electrolytic capacitors (In the power supply filtering, and preamp cathodes) have an emulsion between their plates, that is the insulator that allows the plates to hold their charge. This emulsion will dry out eventually, there's no getting around that fact. The older the capacitor, the faster they dry out. The paper covered electrolytics I dont even bother checking in the tester. They get replaced pronto. Plastic molded and metal cans, I'll check, and if they have drifted more than 30%, I replace them.
2 cents
-andy-

Donny Hinson · From: Glen Burnie, Md. U.S.A.

(I can hear the moans already..."Here he goes again!")

I suspect the teacher may have been taking value into consideration, something our players and techs here on the Forum may not have thought of. If that amp is in good-to-excellent cosmetic condition, and it's working, I'd leave it alone! A nice 1961 Princeton, working, all-original amp, is now bringing between between (get this) between $600 and $900 in the "collectors" market. The more stuff you replace (regardless of the reason), the less it will be worth on the collector's market. Old amps are like classic cars, and you have to (or rather, you should) take collectors' value into consideration when you repair them. "Original", is the key. "Original style", or "works like new" just don't cut it in the eyes of today's shrewd collectors.

I have a friend that lost thousands in the value of his classic Corvette when he replaced the "old and tired" original engine with a brand-new crate motor. Oh yeah, it ran great, but since the serial numbers of the car and the engine no longer matched, the "finicky" (but rich) collectors didn't want it when he decided to part with it. (No, he wasn't smart enough to keep the old motor. )

Think about this the next time you're considering "preventive maintenance" on an old classic piece of gear.

Bill Terry · From: Bastrop, TX

John Daugherty · From: Rolla, Missouri, USA

I think Bob cut right to the chase on this issue. If you're going to use the amp at home, fix it when it breaks. If you use it on the job, do the preventative maintenance.

Blake Hawkins · From: Florida

In restoring antique radios and amps, it is a delimma as to wheather you should preserve the original performance or the original appearance.
Collector opinions vary all over the place.
A happy medium in the case of capacitors is to take the original part, remove the guts and install the replacement part inside the old one. If you are careful, the original look is maintained, yet you have a new functioning part in the circuit.
This works really well with the can type of caps that are mounted on top of the chassis.
Also with the late 1930's electrolytics that are in waxed cardboard boxes.
In collector circles with which I am familiar, this is an accepted practice.
With tubes, it is good if you can find replacements with the original manufacturer's name on them. That is very hard to do, but replacement tubes do not detract much from the value of the amp.
Blake

Donny Hinson · From: Glen Burnie, Md. U.S.A.

Bill Terry · From: Bastrop, TX

A) or B)?

I dunno.... Personally, I've never cared much for owning an amp that I couldn't play through if I wanted to. My criteria has pretty much been if I like the way it sounds I keep it (and maintain it as necessary), if I don't, I sell it. If a 'keeper' happens to come along that has vintage value too, that's a plus, but I'd still service it and keep it playable.

So if that 'collector amp' really knocked me out, I might option for (B).. Nothing cooler IMO than a really 'mint' vintage amp that also works. What's the price of Mojo??? haha.

I've seen the whole 'Recap a collectible amp?' question over on the Fender Forum a few times. It's always a huge debate, usually ends up looking like a Robert Randolph thread over here..

Jennings Ward · From: Edgewater, Florida, R.I.P.

The rule of Thumb is as follows,,,,, Change the Caps once every 10 years as you would change you drawyers...

jennings

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EMMONS D10 10-10 profex 2 deltafex ne1000 pv1000, pv 31 bd eq, +

James Morehead · From: Prague, Oklahoma, USA - R.I.P.

OR, You could do the cap job, ect, save the original parts, and reinstall them if you want to sell to the collectors if you ever decided too. Of course, they'd probably gripe because the solder isn't vintage now.

Alan Kirk · From: Scotia, CA, USA

Okay, so how do you replace the capacitors with ones as close to the originals as possible? Is there a good source for these?

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Brad Sarno · From: St. Louis, MO USA

Replacing filter caps in an old amp doesn't reduce it's value. It's required maintenance like replacing dead tubes. Generally, you just can't use a '57 Fender Deluxe Amp without replacing the old electrolytic filter caps. I'm sure there are some collectors who like to put old, unaltered amps on a shelf and never use them, but if you want them to operate properly as an amplifier, the filter cap's just have to be replaced. Not to mention the horrible toxicity of the stuff that oozes out of those old caps.

Now replacing signal cap's is another story. Signal cap's don't necessarily go bad. Some do, many don't. Cap technology has improved greatly over the years. Some people prefer the mellower sound of an old Fender film cap. Other's like the sparkle and clarity and detail of a new Orange Drop cap. There are lot's of cool signal cap's out there these days.

Brad Sarno

Brad Sarno · From: St. Louis, MO USA

Tube amp parts:
www.mojotone.com www.hoffmanamps.com www.webervst.com

and many more.

They've got the parts that would be appropriate for a vintage tube amp re-cap job. Sprague Atoms make for nice filter caps.

Brad Sarno

Mike Simpson · From: Gilbert, Arizona, USA

I had a similar experience when I bought a 1952 Fender Pro-Amp for $250 and It had been painted black and made loud popping noises and was sort of crackly. I took it to a knowlegeable well respected amp tech reccomended by many people and he said "you should not change anything because the amp still works". So after trying to use the amp I decided that these didn't get popular because they sounded like this. I replaced the tubes and had to go through a few 6SC7's to find some that were not microphonic in the amp and still had prioblems. I found a guy that was a player that knew how to work on amps and he changed all of the caps and a couple of resistors (I still have the old ones) and the amp once again has the killer sound that made them popular.

I have to agree with the statement that changing tubes and caps does not devalue a vintage amp.

What good is it if it does not work?

Recovering, painting, non original grill cloth and speakers will devalue an amp. Honest playwear is more acceptable than refinishing.

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Big Mike
http://www.blueswizards.net

Alan Kirk · From: Scotia, CA, USA

Well, I took my 64 Super Reverb in to class and we looked at the filter caps. A couple of them had dimples. The tech said to replace those but not to worry about the others. I don't know. It seems to me that they all should be replaced.

I used to do a lot of gigging with this amp in the 1970s and have never in thirty years changed even a tube. The reverb channel sounds pretty wimpy these days but the normal channel has good body to it.

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