Monday, November 9, 2015

1959-60 Ampeg M-15! This is a great amp!



Mercury must have been in retrograde and taking its toll on out of date gear. This old Ampeg M-15 showed up a few weeks back. It was blowing fuses. Once againI hoped it was just a dead tube but alas, it was a burnt power transformer. You could smell it!

These early Ampegs are really likable in the sense that they are really well made and use fairly common tubes unlike the later models. This one has 3 6SL7 tubes, a pair of 6L6 tubes in cathode bias and a 5U4 rectifier. All are easily sourced and not terribly expensive.


Here's a shot of the back:


Amp has the original 15" speaker though it was re-coned in 1999. The filter can was replaced in 1994, it's dated. I usually like to replace filters after 20 years but the amp after it was fixed has no hum and a lovely voice.

So first, the transformer replacement. Again I could have purchased an expensive Mercury Magnetics part or Hammond makes a suitable one as well. But we're on a bit of a budget so I used a nice US built part supplied by a company on Ebay. It's slightly smaller but has the 140ma I need to feel good about this job.

So first the burnt out culprit:



Remove it. Unsolder everything first:





The second shot is the rectifier tube socket. The unused pins have the 2 black wires from the power transformer to the wall, 120 VAC. I don't like this arrangement. I doubt it's factory. So I found a solution. Simply put one of the 120 V in wires on the fuse, the other on the power switch. Makes for a tidier amp!

Next measure:




Simple. Use the new part as your guide. The new part has wires coming out both sides whereas the old part only one side. I'll need to drill a hole for that side, hence the blue dot on the left towards the middle.

  Next drill away! Start with 1/8" pilot holes. Use oil on your drill bit if needed.





The big hole I made with a step drill bit. I think I paid about $60 for it and thought I was crazy. But the first job using it the thing paid for itself. No more filing and wasting time!

Once it's in, start wiring it up:




Now before you get your collector panties up in a bunch about drilling a chassis, here's the deal. This is a beat up old Ampeg. They sell for about $450-$750. They're great player amps and you aren't going to hurt the value by making them playable!

So next, put it on the Variac with no tubes in it and see if that pilot light comes on:




Next, drop in that rectifier tube and bring it up slowly again and test the B+, or pin 8 on that 5U4:




Yup. 471 volts. Not for the weak. Don't mess with a tube amp if you like to drink while you work. Can be fatal.

Once I got this amp working another couple issues arose. The tremolo was weak. 3 out of 4 of the tremolo caps were replaced but they were replaced with some real crap. So I replaced all of them along with some other dead caps and cheap craps. I also replaced the tremolo feed resistor (470K on the schematic feeding the intensity knob) with a 220k. It's good to experiment. Too high a value and you have some weak ass tremolo. Too low and you have a 'WOOF WOOF WOOF THUD THUD THUD' sound. And that can actually damage your speaker. I've seen it happen. Rare, but possible!

Here is a beautiful schematic drawn by Joe Piazza. The tremolo circuit is in the lower left corner.



All in all, this is a really great amp. Ampegs have a totally different thing going on that Fender or Gibson. They lack the 'bite' but the mids make up for it. Lovely, warm sound, euphonic. This model had 5 inputs going into 2 channels. Accordion or Instrument. I prefer the bigger and deeper sound of the Accordion channel myself. Makes my Esquire sound bigger than one would expect.

So if you find one of these at a good price, these are a good deal. Like a tweed Pro with tremolo, more interesting of a choice. They are hand wired with a board not unlike a Fender amp. I imagine a pedal steel would be a great fit for this amp. My friend plays her violin and guitar through this amp. Lovely choice for a fiddle! Big and warm, that's what you want!

JB

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