Other Amps

NAD - Mystery amp?

1

I picked up an old Magnatone (?) amp on Reverb for a couple of hundred bucks. I wasn’t expecting much. Looked like a barn find. When I opened the box, I was a little surprised at how grimy it looked in person, but I don’t mind roughed up looking tweed.

I’m moderately sure that the cabinet is that of a Magnatone M-192-5, but the amp chassis and arrangement of tubes, knobs and jack doesn’t look like the pictures I can find for these amps. I’m thinking it’s been replaced with a totally different amp.The chassis is smaller than other pix of the M-192-5 I've seen and it only has one input jack instead of the normal 3. It looks almost home-made.

But here’s the kicker. It sounds fantastic. Creamy goodness. This old Jensen 10" speaker sounds so full.

After playing it awhile I think it’s maybe a little too scooped. It seems to be louder when I play the two high and the two low strings, louder than the D and G strings. Same on three different guitars that act normal on other amps. Also, once in awhile it pops. It makes a snap sound and the volume goes down for a split second and right back to normal. Does it sound like a failing tube?

2

Definitely not a Magnatone amp. Looks homebrew. Snapping/popping is often the sound of arcing. Might wanna take a peek inside. Could also be arcing inside a tube. Play it in the dark and watch for fireworks. Looks like a little screamer.

3

Thanks, PW. Hard to believe it's so loud and full.

4

Well, two 6V6's is belting out 12 watts or so. Plenty of juice. Hard to beat an old Jensen alnico tonally. And the cab? How cool is that?

5

But here’s the kicker. It sounds fantastic. Creamy goodness. This old Jensen 10" speaker sounds so full.

Good find, man! Get the tubes replaced. That (hopefully) will take care of the popping and arcing. Three pronged plug, too.

6

The 'm' shaped grill front looks sharp.

7

Check the caps as well. Definitely a Maggie cab, but the amp sure isn't. Just the slapped on control plates and the too shiny switch as well as the basic project box chassis say home brew. Still, could be a great little amp.

8

Thanks for input, guys. I've been playing for a few hours this morning without a single pop. It already has a 3 prong plug.

My 1950 Bacon Belmont with flat wound strings sounds smooth and creamy through this amp. My 2016 Squier Mustang sounds bright and Beach Boys spanky. I don't know how it can do both, but this nasty-looking old thing is a marvel.

Warning: The above is the voice of a guy in the honeymoon phase of new gear. Check back in a month or so for a more unbiased view.

9

The popping could be bad news. Some ideas:

  1. Swap out all tubes. If the problem solves, thank your stars and call it a day.

  2. Still popping? Could be a bad socket with dirt in it, loose pin or something closely touching that's arcing.

  3. Burnt out plate resistor?

  4. Still popping? could be a bad filter cap can.

You might need a tech for 2 through 4.

Hopefully the heat from the tubes won't cook that speaker. They seem to almost be touching the magnet.

10

Neat! So what are those controls? Volume, Treble, Bass?? If you get a chance, take some photos of the insides...maybe it's an undiscovered circuit topology with special mojo resistors and capacitors!

11

It's bass, treble and volume, left to right looking at the back of the amp. Soon I'm going to take it apart to move the speaker up a tad, to get it away from that tube then I will take some pictures .

12

What power tubes are those?

13

It's bass, treble and volume, left to right looking at the back of the amp. Soon I'm going to take it apart to move the speaker up a tad, to get it away from that tube then I will take some pictures .

– Bob Howard

Thanks! Look forward to seeing the inside.

14

The reason I ask about power tubes. If the amp is running a push-pull pair of 6V6's, the Stancor A-3824 output transformer has a secondary impedence of 2 ohms as connected. The Jensen P10T's I've come across were all 8 ohms. An impedence mismatch that big could certainly cause arcing if that is indeed the culprit behind the popping noise.

Just something to consider.

15

Here is the rats-nest that is the inside of this weirdo. And here are some shots of the tubes. Thanks, Powdog. The popping noise has not reoccured in 6 hours of playing, so it may not be a problem. Can you tell what I've got here from these snaps? For a larger view, see my Photobucket pix.

16

given the RCA audio jacks, I'm guessing it's either a PA or hi-fi audio amp out of one of those old entertainment consoles.

17

The RCA on the back is the speaker connection.

19

...and a close-up of the inside.

20

https://flic.kr/p/RFDMYE

Here's the OT hookup diagram. Those look like 6V6GT's, can just barely make it out. They're hooked push-pull and lt looks like the 2 ohm connections are hooked up to the RCA on the back. Have you checked the impedence of the speaker?

21

Yes, they say 6V6GT and no I didn't check the impedance. I don't have an Ohms checker. I don't even have a voltage meter. I have to pop out and get one this afternoon. I don't know much about electronics, I'm just a dumb artist. Thanks for all the help, Powdog.

22

Artists are far from dumb. Don't need a multimeter. Just found the Jensen code. ST119 is a 6-8 ohm speaker.

23

Well, I went out and picked up a multimeter and it measures 5.5 ohms. Thanks Pow. Do you think it's bad to play this amp with this config?

24

If you're able to solder, I'd move the green wire on the output transformer from lug 2 to lug 4. Probably the best situation.

25

Just to be clear, you are saying to do as the picture says...


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