Replacing old multi-capacitors in guitar amps (2024)

I have three old guitar amps (2 are tube, one is solid-state) that
have that characteristic 60-cycle hum. I presume this means that the
electrolytic filter caps are in need of replacement. Trouble is, all
three amps have those old multi-caps in them (each about the size
of a D-cell, with 4 wires coming out of 'em). I'm not so dedicated
to originality that I'd want to spend hours searching for the exact
replacements--if they're even available--nor spending $20 to $30 for
each one, as I've seen some multi-caps for amps cost. Of course, IF
they are readily available for reasonable prices, I'll happily use
multi-caps.

So my question is--how should I go about replacing these things?
For example, one of the multi-caps is 20, 40 and 60 mf. With 4 wires
going into it, that means (I presume) that one wire acts as a common
wire for all 3, and the other 3 wires are specific to each cap.
Should I just solder the 3 leads of the correct sides of each cap
together and to the common wire? Can anyone tell me the correct way
to discharge a used cap before unsoldering it? Are there other
things to be concerned about? Should I go ahead and replace all the
electrolytics in the amps at the same time? How about the other
caps? And once they're replaced, should I slowly bring the amps up
to full current on the variac?

Thanks in advance for any advice.

Let's start at the end,
Variac power-up: if you have one you can do this, slowly and check
for hot caps.

replacing all:
dunno if it is really necessary. old caps are defnitely dryer but so
far all i checked were ok on the ESR meter.
I wouldn't exchange them, but then i don't have good audio amps.

yes, multi-caps usually have one common, it is often labeled on the cap.
You can simply use individual caps and connect the common, make sure
the capacity is the same and the voltage rating is the same or better.
mind the polarity, and then double check it again... don't ask ;-).

Discharging caps: often not needed if you unplug the unit for a while.
Always discharge caps which could carry over 50V, use around 100Ohms and
check with your voltmeter.
A capacitor recovers some of the voltage after discharge to zero, so discharge
the nasty ones (e.g. in a switchmode supply) at leat 10s after they reach zero.

You could build a ESR-meter very cheap and find the bad caps if you don't want
to change them all. if you search the archives you will find it....

ST

Roy J. Tellason

On Wednesday 07 July 2004 06:08 pm, Robert wrote:

I have three old guitar amps (2 are tube, one is solid-state) that
have that characteristic 60-cycle hum. I presume this means that the
electrolytic filter caps are in need of replacement. Trouble is, all
three amps have those old multi-caps in them (each about the size
of a D-cell, with 4 wires coming out of 'em). I'm not so dedicated
to originality that I'd want to spend hours searching for the exact
replacements--if they're even available--

They weren't, the last time I looked.

nor spending $20 to $30 for each one, as I've seen some multi-caps for amps
cost. Of course, IF they are readily available for reasonable prices, I'll
happily use multi-caps.

These things seem to have gone completely out of favor in recent years.

So my question is--how should I go about replacing these things?
For example, one of the multi-caps is 20, 40 and 60 mf. With 4 wires
going into it, that means (I presume) that one wire acts as a common
wire for all 3, and the other 3 wires are specific to each cap.

Yep. One of those connections is to the physical outside of the can. Clarify
something for me: Are we talking about units in metal cans? Units that are
mounted vertically, like the tubes are? Or are we talking about units that
are underneath the chassis, and have wires coming out of them? I've seen
the latter used in things like radios, but not recently.

Should I just solder the 3 leads of the correct sides of each cap together
and to the common wire?

There are a few ways you could approach this, depending on the construction
of the unit, I'll elaborate when you've answered the above.

Odds are that you've got one common ground connection, so the replacement
units don't necessarily need to be grounded to the same point -- just use one
that's convenient to where you want to mount them.

Can anyone tell me the correct way to discharge a used cap before
unsoldering it?

I've seen people use a screwdriver. <g> I wouldn't recommend that, though,
it tends to chew up the screwdriver if there's any spark. A couple of clip
leads and a resistor ought to be safe enough. Resistor should be something
like 10K, 2W.

Are there other things to be concerned about?

If you're looking at the kinds of units that are in metal cans and that are
mounted to the chassis, then it's likely that the terminals were used to tie
a lot of wires to. DO NOT be tempted to use the terminals of the original
part for tie points, as there's always the chance that it'll short out or
otherwise affect the performance of things. If this is what you've got
you'll probably have to add some kind of a terminal strip in there or use
other tie points.

Should I go ahead and replace all the electrolytics in the amps at the same
time?

How old is the unit? I would, probably, or you may end up having a problem
in the future, or less than optimum performance. For stuff that's oh, 20
years old or older then I'd usually make it a point to go in there and change
them all out.

How about the other caps?

Depends on what kind they are. If they're the really old-style "paper" caps
(a cardboard tube sealed at both ends and coated all over with wax!) then
yeah, I'd replace those, too. That's what all was in use in most stuff when
I started working on this stuff...:-)

If they're "molded" types, I wouldn't necessarily bother.

And once they're replaced, should I slowly bring the amps up to full
current on the variac?

For tube gear? That's a tough one.You won't get any kind of a load on things
until the tubes warm up, and they won't warm up until the heaters are up to
temperature. If you have a variac handy it probably won't hurt to run it at
75 or 80% of full, and bring it up gradually over a period of time, just in
case the caps you put in there are "old stock". This will give the
electrolytics a chance to "form" properly. This is probably more important
if you've got solid-state rectifiers than if you have a tube rectifier.
Personally I've never bothered with this, but I've also not dealt with any
tube gear in a bunch of years, and what's out there now in terms of parts
stock for those higher-rated caps may have been sitting on the shelf for a
*while*.

Roy J. Tellason

On Wednesday 07 July 2004 06:38 pm, Stefan Trethan wrote:

Let's start at the end,
Variac power-up: if you have one you can do this, slowly and check
for hot caps.

Depends on the "tube gear"'s design. If it uses a tube rectifier it won't be
applying much in the way of voltage.

replacing all: dunno if it is really necessary. old caps are defnitely dryer
but so far all i checked were ok on the ESR meter. I wouldn't exchange them,
but then i don't have good audio amps.

For electrolytics, it depends on the age of the equipment.

yes, multi-caps usually have one common, it is often labeled on the cap.
You can simply use individual caps and connect the common, make sure
the capacity is the same and the voltage rating is the same or better.
mind the polarity, and then double check it again... don't ask ;-).

The capacity has to be "somewhere close" but isn't at all that critical. As
much as 100% over is probably okay.

<...>

You could build a ESR-meter very cheap and find the bad caps if you don't
want to change them all. if you search the archives you will find it....

For stuff that's pretty old I wouldn't bother with this. I'd just go ahead
and change them out...

Odds are that if it hasn't failed, it's going to.

JanRwl@AOL.COM

I am not a "serviceman", but I DO know a littleabout such stuff. You didn't mention the "Working Voltage" of these caps. Is it still readable on the package? If not, can you still "fire it up" and measure the DC from ground to the capacitor-pins? (Be careful if you are not electronically-oriented, as a "tube job" has some serious voltages!)

Get the same voltage (or next-higher) caps of same µF value, tape- or Krazy-glue 'em neatly together so that the ( - ) ends are all same end, and solder all those end-wires together, and to a nice piece of black stranded wire. Then perhaps slip some insulation stolen off the OLD cap, if possible, over the (+) wires. Simply solder that in place of the old one you removed.

Obviously, it is impossible to "tell you" how to work, particularly with a "salvage job" like this, but just use common sense. Work neatly, solder properly, and "Mount" the assembly some way "under there" so that it won't slop around and short to stuff when you move the amp!

In a message dated 7/7/2004 5:11:07 PM Central Standard Time, rch427@... writes:

So my question is--how should I go about replacing these things?
For example, one of the multi-caps is 20, 40 and 60 mf. With 4 wires
going into it, that means (I presume) that one wire acts as a common
wire for all 3, and the other 3 wires are specific to each cap.<<

Almost-certainly.

Should I just solder the 3 leads of the correct sides of each cap
together and to the common wire?<<

The NEGATIVE ends, yes.

Can anyone tell me the correct way
to discharge a used cap before unsoldering it?<<

This size cap is almost-certainly LONG discharged within a minute of turning the amp off! Anyway, these are not large enough to do any earth-moving damage. The worst you might get is a tiny shock while fiddling with it all. But you can still use a clip-lead (jumper wire), and short the common-ground to each of the three (+) ends, to make yourself feel safe.

Are there other things to be concerned about?<<

Yeah! UNPLUG the thing before you go fiddling with the wiring "under there"!!!!!!!!!

Should I go ahead and replace all the electrolytics in the amps at the same time?<<

I'd say if replacing the power-supply filter-caps (the 4-wire jobs) fixes the humming, then no. But if you are secure in your soldering ability and have the $ to spare, and know where to get GOOD replacement-parts of correct WVDC, and you have the TIME to spare, GO for it.

How about the other caps?<<

Only if "bad".

And once they're replaced, should I slowly bring the amps up to full current on the variac?<<

No. Who does that? If you can do the replacements properly to begin with, simply "plug it in and turn it on!"

Jan Rowland

Thanks for all of the good advice!

Roy wrote:

Clarify

something for me: Are we talking about units in metal cans? Units

that are

mounted vertically, like the tubes are? Or are we talking about

units that

are underneath the chassis, and have wires coming out of them?

I've seen

the latter used in things like radios, but not recently.

Both. The tube amps use the multi-caps that are held to the chassis
on their sides, by a strap, whereas the solid-state amp's multi-cap
looks almost like a tube inside a metal housing--and is mounted
perpendicularly on the chassis right next to the row of tubes. The
two tube amps are cheap ones from the early '60s (Danelectro and
Gregory), and I've heard that the tubes they use are more common to
radios than those in "good" tube amps like Fenders or Voxes. Perhaps
that explains their use of radio-type multi-caps?

Should I go ahead and replace all the electrolytics in the amps

at the same

time?

...

How about the other caps?

Depends on what kind they are. If they're the really old-

style "paper" caps

(a cardboard tube sealed at both ends and coated all over with

wax!) then

yeah, I'd replace those, too. That's what all was in use in most

stuff when

I started working on this stuff...:-)

If they're "molded" types, I wouldn't necessarily bother..

Another project I'm getting ready to tackle is for an old solid-state
organ. I purchased it on eBay from someone who knew nothing about
it, and hadn't tested it. When it arrived, I plugged it in, and
started to test it. As I was playing it, there were all of these
little "tinkling" sounds coming out of the innards, then a pall of
white smoke. After unplugging it and opening the case, I saw that
its dozens of dipped ceramic caps had all split open, presumably when
I turned it on. That project's gonna be a bitch. If I had slowly
brought it up on the variac instead, would those caps have survived?

Daryl Mills

Just as a thought, what about using two multi caps- using two values that are still obtainable.

I just did a 20-40-40uf cap replacement with a20-20-20-40 and paralleled two of the 20uf's.

Daryl.

Another project I'm getting ready to tackle is for an old solid-state
organ. I purchased it on eBay from someone who knew nothing about
it, and hadn't tested it. When it arrived, I plugged it in, and
started to test it. As I was playing it, there were all of these
little "tinkling" sounds coming out of the innards, then a pall of
white smoke. After unplugging it and opening the case, I saw that
its dozens of dipped ceramic caps had all split open, presumably when
I turned it on. That project's gonna be a bitch. If I had slowly
brought it up on the variac instead, would those caps have survived?

looks like they got too high a voltage, maybe the supply is faulty, or you applied too
high voltage if there is no internal supply.

At hearing the tinkling you should have powered it down.

ST