September 10, 2008 

For some time now I've heard talk about the famous Vibrato sound of the Magnatone amps. So I started watching eBay and spotted this one. It was advertised as "recently had been playing through the amp and slowly the sound went out and the light under the logo went dim. From then on the amp will not turn on and is being sold as is." I decided to gamble on it and won the auction for $204.49. I finally received the amp about 3 weeks later. I was fortunate that the amp was not completely destroyed during shipping. The guy that boxed it up didn't bolt down either of the chassis! And he left a lot of empty space inside the shipping box. The power supply banged around inside the speaker compartment and broke the cap can and fuse holder. The main chassis broke a cast aluminum mounting bracket, cracked the plastic case, and smashed a couple tubes. The main cabinet survived and luckily there was no major damage to the circuitry.

So, I ordered some cap cans from Doug and began using J.B. Weld to repair the broken parts. I got the caps before the J.B. Weld cured! The power supply chassis originally had a 4 section can with 4 x 40uF caps. I couldn't find a replacement, so I opted for two cans with 2 x 50uF caps.  With a little planning I was able to squeeze both cans into the little chassis. I had to punch new holes to mount the cans. It really turned out looking neater than the original.

Next I reassembled the main chassis, top case, and mounting brackets, and finally put everything back together. Everything fit together and the amp is once again sound and sturdy. I was skeptical initially, but the mechanical repairs turned out better than I had hoped for. Finally, I could focus on the electronics!

I was lucky to find a schematic for this amp. There were several component values on the original schematic that were unreadable so I proceeded to redraw with Visio. It was quite a challenge to redraw and verify that the new drawing was correct and component values matched what was actually in the circuit. The process took a long time but the results were worth it. And as a bonus, I became intimate with this circuit. The circuitry is more complex than any other tube amp I've tackled and the actual point to point construction would make troubleshooting this amp very hard without a good schematic. You can download the original schematic and my drawings in a single PDF at the bottom of this page.

 During the visual inspection I found a few shoddy repair attempts using totally incorrect components. Someone had replaced a few 47Ks and 68Ks with 330Ks using snip and clip and bomb soldering techniques. It took some patience to undo this shoddy workmanship and make it right. Luckily the majority of the circuit was untouched. I was also able to 'date' the amp using the pot codes. The Stackpole pots were all made during the 32nd week of 1965. (Hey, that's the year I got involved with this stuff!)

So finally, I replaced the broken tubes and fired it up. Surprisingly, the amp sprung to life, although it wasn't a totally joyful noise.  One preamp was dead, reverb just made an awful sound. But the vibrato was working fine! That was my biggest fear because the varistors are rare. Next I loaded a fresh set of tubes and all the above problems cleared up. I still had a couple dirty tube sockets and several scratchy pots, but a good cleaning took care of those problems as well.

I spent several hours playing my Strat through the amp and exploring the tone controls and vibrato. My first impressions were WOW! This is a really clean, refined sounding amp. And the phase shifting vibrato sound is excellent. The reverb is OK, definitely not Fenderish. It's more along the sound of an old Sunn amp. After a week of playing through this amp I found myself missing the growl of my Marshall inspired November Project. I'm sure someone like Chet would have a better appreciation for the clean sound of this amp. However, the vibrato sound makes this a keeper. And I really have a huge appreciation for the circuit design of this amp. It's refreshing to get away from the 100K/1.5K circuits for a while.

Here's the eye candy. The Oxblood grill cloth is obviously not original, but it has a certain tasteful appeal. I'm also considering a new faceplate. I really had a good time with this project.

 Update November 16, 2008      Well, eBay is amazing. You really can find anything for sale if you're patient. I just picked up a top case for my M10-A. This one is in better condition than the original. Now I can actually read all the writing on the face plate. I've updated the pictures to show the new top case.



Here's a close-up of the innards. M10A_guts.jpg

Download the original schematic and my Visio created schematic in a single PDF. Magnatone_M10A.pdf (668KB)

Want to know how the vibrato works? Download the theory of operation. NON_LINEAR_IMPEDANCE_ELEMENTS.pdf (442KB)

I found a lot of information about Magnatone at this site. Magnatone, The Unofficial Amp Site.

Another informative site with a lot of history. MagnatoneAmps.com

And here's a public user group with lots of discussion, pictures, and files. Magnatone-Valco

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