Every now and then you come across something you decide you just have to have, and that was the case with this Ampeg M-15 a/k/a Big M. According to Hopkins and Moore, the Big M was a 20w version of the M-12 Mercury with 6L6 power tubes and a fifteen inch speaker from the house of Jensen, introduced in 1959. This one's a little later because it has the checked blue covering rather than the earlier random flair that my Mercury is covered in. I had a chance to preview one and decided I had to have it.
On the general subject of Ampeg combo amps of this era, they seem to be a bargain right now and about as low as the price may get. I've acquired this one, a random flair M12, and a J12D Jet. Once you get around the odd tube choices Ampeg sometimes made with some of their amps like the Jet (6BK11s, 7591s and 7868s) they're solid, reliable amps. I'm awaiting the arrival of a modestly priced Jet II reissue, but more about that later. Now, if Ampeg hadn't smeared all that rubber cement on the phenolic circuit board in this amp to keep the components from vibrating-but I digress.
Not too long after that, this one showed up on fleabay at a relatively modest price because the power transformer was deceased, and it had a substantial dent in the control panel. As it happened, I had a new Stancor PC8410 power transformer that would fill the bill. I'd acquired the transformer at an auction and it was sitting in a tray of junk from the garage, obviously new and never hooked up. It cost me all of a dollar for the whole tray which included a World War One German army belt buckle as well. Pounding out the dents in the control panel was fairly simple as was replacing the power transformer. While I was in there I installed a Twin Reverb output transformer on a lark to see if more iron was good. The cement bodied cathode resistor also fell apart in my hands so that got replaced.
One thing is important to note when replacing the can cap. Most Ampegs of this era have the Mallory can cap insulated from the chassis with a bakelite washer and an internal ground wire going off somewhere. If that wire is broken the can is live, and if you touch the can and the chassis at the same time you get to be the ground. It is not an enjoyable experience as I found out a few dozen amps ago.
The results were flat and disappointing, and there wasn't nearly enough volume for me. I spent about two weeks trying to figure out how to convert the amp to fixed bias, got it done and it made not a lick of difference so out it went into the junk box.
I figured since the amp is not collector grade and some parts had already been changed I might as well just clean up all the messes and dump all the capacitors and resistors, even though the savants will tell you that shotgunning an amp is the worst possible thing a person could do. In some cases it is justified, I own the amp and nobody'll ever know unless I tell them. So there. Building things back to print is a good way of starting your troubleshooting journey if the amp you've been working on has been bodged like this one.
I left the vibrato alone because that was working well and at this point in life I leave things that work well enough alone. Everything else got replaced, including the can cap which got swapped for a JJ can cap-inexpensive and effective, too. A new set of tubes, mostly Russian stuff that I had lying around filled up all the empty holes. While I was excavating from one side of the board to the other I did find that some previous bodger had installed a 560k plate load resistor on one side of the phase inverter instead of the specified 120k.That was something of a revelation. The circuit, all things being equal is pretty simple once you've spent as much time as I did on this amp.
Could that one resistor have made all the difference? Perhaps. It's reasonably tolerable in the volume department now, and I've spent a happy hour or two putting it through its paces. When you're working with the Joe Piazza schematic as I was, it's worth noting that the tube numbering is incorrect. The first preamp tube is V2, the vibrato tube is V1 and the phase inverter is V3.
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