Re: Need Mcintosh MC225 parts source
- From: Peter Wieck <pfjw@xxxxxxx>
- Date: Mon, 6 Oct 2008 11:31:22 -0700 (PDT)
On Oct 6, 12:58 pm, timp...@xxxxxxxxxx wrote:
Hello
I recently picked up an MC225 and also a C11 pre-amp from the original
owner. He told me that they both worked the last time he used them
but that was about 15 years ago. They are both in nice original
condition and haven't ever been modified or upgraded. I would like to
replace the power supply capacitors and rectifiers in both units and
maybe other capicitors. Can someone if there is a company that sells
upgrade kits for these amps. This is my first experience with tube
amps. I always wanted to have one but had never gotten one.
Thanks
Scott
Scott:
A couple of things:
a) There is _nothing special_ about the filter caps (power-supply
caps). You have several options depending on how much effort you want
to expend. You can "gut" the existing cans and install new, modern
caps inside - that is the middle-of the road expensive option. You can
purchase replacement cans outright, this is the most expensive option,
but is the easiest by far. Or, with some finagling, you can mount new
caps under the chassis leaving the old caps in place disconnected.
This is the cheapest (and sloppiest) option that also leaves the unit
vulnerable to possible shorts and other problems from crossed wires.
But, electrically, there is no functional difference.
Note: New caps should be within +20/-0% of capacitance and *AT LEAST*
the same operating voltage.
b) There is even less special about the rectifier. I believe that some
of these units, depending on vintage, used selenium rectifiers with
the associated voltage drop - but if you do have a selenium rectifier
system, get RID of it. Should it fail it will do so in a spectacular
and very nasty manner that may well render a room in your house
unlivable (stink) for a few days, at least. So, replace with silicon,
then measure your B+ (and/or Bias voltage) downline and under full
load to make sure that it is not excessive. If it is, you will have to
add a dropping resistor after the rectifier to reduce the voltage to
stay within parameters. NOTE: Wallplate voltage today is often
significantly higher than it was when these units were made, so this
is a concern in any case.
c) As to "other capacitors", also, nothing special. Just use exact
replacements as to capacitance, at least the same operating voltage-or-
greater. "Exact" is a moving target as today an 0.50uf cap is
expensive and hard to find, but an 0.47uf cap is readily available and
cheap. Unless marked otherwise, film-type caps are +/-10, so 0.47uf is
still well within tolerance. 0.20 will be 0.22, 0.30 will be 0.33. And
so forth.
There is no reason not to use a 600V cap in place of an OE 250V cap,
for instance. Just not the reverse.
d) DO use a schematic. It will help you make sense of what you are
doing and also help you double-check values, operating voltages and so
forth.
e) Purchase your parts from a reputable seller with good support.
Mouser, Newark, Digikey, Santech and several others come to mind. My
"favorite" is Mouser as they have very good (and savvy) customer
support, and have no minimum charges. But I have had good experiences
with all of the above.
Enjoy!
Peter Wieck
Melrose Park, PA
.
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