The first things to go generally on these old amps is the power filter capacitors and these are often all rolled into one single ‘Cap Can’. Here’s mine in place in a GA-5 Medalist.
These are (IIRC) wax paper and foil capacitors and are sealed with wax. It is electrolytic but has a single ground lead. And by the time you read this, it is already dead.
Replacing this (or original discrete capacitors) is the most common fix to these old amps and often is all that is required to get the amp working again.
Mine is 45 years old it it came to me dead (despite the sellers assurances that it plays perfect). No surprise there. I’ve recently had some questions asked about this job and since it is so common, lets cover it in some detail.
First, should you do it? If the amp works fine then it’s already been done or you have magic capacitors. Don’t do anything. If the amp starts off humming (after tube warm up of course) then you probably should do it. If you don’t want to know anything about the guts or don’t have any electronics skills then take it to an amp guy. I can’t recommend any, because I don’t use them, but most towns have someone. Ask at the mom and pop guitar shops, they’ll usually know.
The actual job is unsoldering 4 leads, and then soldering in 3 capacitors, six leads total. The tech will probably want to vacuum/dust in there. He/She should quote you no more than an hour of labor plus parts. The caps are $10 and $11 bucks each for 500V rated Sprague Atoms here and you really don’t need the best in the world. So I don’t know.. maybe a hundred bucks to redo the power caps and they should do some contact cleaning and tightening while they are at it.
However, if you were in the habit of paying professionals to do the job right, you wouldn’t be here. So lets get started.
Safety Preamble – Read me or die.
I pointed to some replacement caps above and those are great but don’t get fooled by any ‘audiophile’ nonsense here. These are power filters and are not in the audio path. I would love to double blind someone with mediocre filter caps vs. audiophile but anyway, a decent cap should last you at least another 50 years. The general rule of thumb for voltage rating is double the expected voltage. These about 260V tops so 500V is good.
You can’t really test the caps in place without de-soldering anyway so you might as well do that. Power down, unplug and bleed every cap you can reach. Lots of people bleed the power caps but leave the death cap energized. That can result in a tasty little shock. Not that I know from personal experience .. lately.
Find Cap Can and Leads
The picture above should be all you need to find the thing.. the blue cans are the replacements I already put in. Here is the power filter section from the MSM.
The cap can has those three capacitors (20/10/10) inside. First, the ground lead is black (you should be able to read it off the cap or from the picture). This is often tied to pin 2 of the 6X4 tube socket. The 6X4 doesn’t use pin 2 so builders would use the extras to avoid having to add another (or longer) tag strip.
Try to get them out of there without damaging anything else connected.
Solder in the New Ones
You can just note the location of the ground and the other leads and re-solder the new caps in exactly the same spots. This will definitely work and in theory should be no worse than the original performance.
In a perfect world, each sub-circuit (power amp, pre amp, power etc) would have a single ground point (like star grounding) and all of those star ground points would be connected to a single chassis ground (galaxy). The earth ground from the plug would attach to a different point and this ground point can then be ‘lifted’ if you want to add a switch.
Since your amp is certainly not setup this way the best you can do is to ground the new cap with the section it belongs to.
Here is a notional (meaning no real thought) diagram of what grounds would be tied together in star/galaxy mode. If you ground your cap to one of the grounds used with the same color, you should be fine. However, you can’t be sure until you hear it.
Why ? That’s a good question. I’ll wave my hands at that later.