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Mercury must have been in retrograde and taking its toll on out of date gear.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

1978 Marshall JMP 50 watt combo

Yes. That's all I need to say.

These are fantastic amps. This one was blowing the H.T. or high voltage B+ fuse. We had hoped it was one of the 6550 tubes but unfortunately the output transformer was cooked.

When the H.T. fuse pops 9 times out of ten it's just a power tube. Once in a while it's a shorted filter can but every so often it's the output transformer. That makes for an expensive repair. And the sad part is it does tend to hurt the value whether it sounds better or not.

Here's the ultra simple control panel. Gain, Master, EQ and presence. You really don't need any more with an amp that has this much sound. These are known as rock n roll amps but in the right hands can be good with any style. Someone clipped out one of the treble caps, the one on the volume knob. I owned one and clipped out both of them. I don't need that much treble and clipped out you can simply turn up your treble and presence if you need it.

This one has the original speaker intact. A pair of lovely 30 watt "Black Back" Celestion speakers:

So what output transformers do I like these days? I'm a Mercury dealer and I also deal Mojotone as well. For the price you can't beat the Mojo. I believe they are made by Heyboer and they are first class transformers. Friendly right out of the box. The Mercury product is expensive. Very expensive. I like there iron very much but there are only a few things to me that are worth that much more. When I was building amps for people I used their ultra expensive "Radio Spares" iron in a JTM 45 I built and always used their "Radio Spares" output transformers in my 18 Watters. They simply can't be beat in that capacity. I had the pleasure of playing my JTM amp next to a real "coffin logo" '62 model and mine was so close. I sold that amp for about $1300. The '62, granted is ultra rare, but cost around $20,000. So parts makers like Mercury and Heyboer helped me lose my lust for old amps! Why spend all that money if my intent is to enjoy it?

But for general repairs and price, the Mojotone iron can't be beat. I've used many of their transformers with dynamite results. In this Marshall I can say DYN-O-MITE! It's got it where it counts and was a relatively easy install!

JB 

Stereo Memory Man

  It's been a minute since I've written anything so I'm up late catching up.

  Just re-capped this tonight. It was making this wum-wum-wum-wum slow pulsating sound and has been slowly dying over the years. Dead filter caps. Photo after:

Nice thing about modern electrolytics is they are so much smaller than the originals. Just makes everything fit better.

These are lovely sounding units. I prefer them over the "Deluxe" version. They are built like total crap, which is part of the Electro Harmonix vibe. I've heard some that are really noisy, this one after the recap is much better now. If you open one up beware. They use this thin solid core wire that just loves to break. I only lost one and had to look at a schematic to figure out where it went. I also had to open it back up cause of a previous hack job on the delay pot. If you look at the picture you can see it,  the one in back with the blue shaft. It's really difficult to find proper pots for these with the ultra long legs, and 2 of these have been replaced. So people usually cut the old ones out and solder the new pots to the old legs. Not a problem, unless it was poorly soldered like this one! A leg joint simply broke upon re-assembly and the delay speed knob was no longer working. No biggie!

Anyway, here's a schematic.

Fun! It's been on for 3 hours now. No hum! I win!

JB