Magnatone Custom M10 Mess

[plexi50]

This is pretty beat up but once i can get to the power supply i will know more / It powers up but no sound

I can hear some thing rattling inside the cab itself. Crayon? PS cap? Petrified Twinkie? I hear these are a pain to work on

Personally i think this amp will need a lot of work to get it working and looking decent again

[jjasilli]

no worse inside than an old silvertone    should be a great project!

[eleventeen]

Those old Magnas had actual pitch-changing vibrato, did they not?

Yup, insides looks like someone swept up the floor in an electronics factory & dumped the sweepings into the chassis. I've actually never seen the insides of one, doesn't look like I missed much!

[bluesbear]

Looks simple and easy compared to old Gibsons. I've got a '59 Les Paul GA-40 with the componants mounted on BOTH sides of the circuit board. Fun!
Dave

[plexi50]

The amp has no hum at all.  I really thought the PS cap can would be DOA for sure. All voltages are present throughout the preamp and power section

I need to put an anolog meter on the can terminals and rule out any saturation but the amp is quiet. Maybe this means nothing as far as a cap doing it's job though and may be a problem

I have the bias grid at -16.50 VDC.  The volume is very faint and really distorted and the volume fades out pretty fast

Been going over the cathode voltages and one tube is flashing a - VDC up and down. Im not sure at this moment if it is V1 or not

The bias was -14.65 VDC before i changed the drifted 1K grid resistors (1.5K) to new 1K's. It is now -16.50 VDC

So the bias is not my problem as i thought it was.  Boy i get lost some times tracing all the very tight and over lapping resistors,Caps & wires

[plexi50]

Looks simple and easy compared to old Gibsons. I've got a '59 Les Paul GA-40 with the componants mounted on BOTH sides of the circuit board. Fun!
Dave

It's fun. The Vibrato from what i can hear sounds very sweet and unique. The amp wants to bust loose with full volume but is being dragged down by one of the preamp tube circuits i believe

[plexi50]

Please look at this PDF of the Magnatone M10. The two bias point connections are label (F)

http://www.triodeamplification.com/files/magnatone_m10a.pdf

On the PDF both show -17 VDC

I get -17 at point (F) at the 270K bias grid

I get -47.0 at point (F) 22K & 25UF 50volt cap to ground

Whats up? I have checked and rechecked the resistor values and the electrolytics

[PRR]

I think you are just lost. The PT bias winding probably gives ~~48V from the power chassis. Then a 22K+12K divider in the amp chassis whacks that down to 17V.

[plexi50]

Thats what i was thinking. I wanted some confirmation on that. What about V1's cathode negative voltage running up and down?

All the rest of the preamp tubes cathodes read between 1.000 to 2.245 VDC on them / Just V1 is all over the place

A ton of ceramic caps in this amp

[RicharD]

If V1's cathode is going up and down, you've probably got a bad socket connection of a dieing tube.  Channel 1 should act just like channel 2 up through the 1st 3 stages.  Do either channel 1 or channel 2 work?  Channel 2 will be simpler to bug out because it lacks the reverb circuitry and that very interesting vibrato circuit.  What the heck are those parallelogram symbols?  

I always bug out amps from the output back after I've verified the PSU.  Sound's like the output tubes are sitting pretty.  Look at the PI and it's driver.  Inject a tone into pin 7 of the PI driver and make sure you're good to there, then work on the easier channel.

Personally, I'd change the can cap regardless if it's acting fine.  At some point it's gonna fail so might as well get it while the getting is good.  I'd also replace the diodes with good ol 1N4007's.  That's a dollar in parts that could save you grief later.  Them old germanium diodes sure are cool lookin.  After getting the pi55 knocked out of me on a Silvertone split chassis, I'm a big believer in redundant grounding between the 2 chassis.  Of course put a grounded cord on it.  I'm also a believer in leaving the "life" cap on the line power.  It's real nice to have if you plug into a reverse polarity receptacle.

{EDIT- munged font tag. --PRR}

[plexi50]

Thanks Butterylicious. Channel 1&2 both work but the volume is very low. It sounds just like a bias that is too cold. That far away sound. The chord when made is very distorted and then quickly fades out. I had this same problem a while back with a Gibson Lancer. It turned out to be speaker output jack and exhibited the exact same sound im having with this amp. Not to say it is related. I'll play around with it more today

[eleventeen]

"What the heck are those parallelogram symbols?"

They are varistors! (See lower half of schematic, center) The trem osc appears to be a phase-shift type of oscillator (giveaway: those three caps in a row between the plate & grid of the left section of the 12DW7 lower left) although why there are TWO networks of the 330K resistor and the .0047 to ground right after the footswitch jack is sort of odd.

These had actual pitch changing vibrato! Yes! When I was a kid I built an actual pitch-changing vibrato thingy on a chassis, the schematic was in Radio-Electronics. It actually worked! The effect was kind of subtle, though, and I believe you'd want to have a pretty darn clean sine wave as the modulating waveform or you'll get weird phase-shift and phase-cancellation issues. That may have been the reason for the phase-shift oscillator which IIRC produces a clean sine output. Of course, a truly distorted form of vibrato would have been lots more entertaining to me back then, and, probably now come to think of it.

Right under the speaker connector, I'm looking at the rotary knob on-standby-antihum1-antihum2 and I'm thinking "antihum"? Antihum? What's that, a new metal alloy used only in Magnatones? OH, ANTI HUM! Brain-f**t.

[sluckey]

Plexi, you have the wrong schematic. The one you're using is for the M10A. Here's the correct schematic for a M10...

http://home.comcast.net/~seluckey/Magnatone_Custom_M-10.jpg

I rebuilt a M10A a couple years ago. The power supply is identical to your M10. There may be some useful info on the following link, but keep in mind that the amps have a lot of similarities , but also a lot of differences. The tube layout may be different too, since your amp has a transistor in it. So, stick to the correct schematic.

My rebuild...
http://home.comcast.net/~seluckey/magnatone/magnatone.htm

EDIT... Oops! The power supplies are not identical. Sorry.

[plexi50]

I remember when you were building that amp Steve. You sure are very meticulious and i like to be that way as well

Beautiful work. Thanks for the schematic. I saw 2 days ago that there were some parts that did not match up to the schematuc i had

Great link site: I will be spending all day tommorrow trying to sort my problem out

The vibrato though even at crappy volume level sounds indescribibley beautiful unlike anything i have ever heard

eleventeen it is a Buck Rogers dematerialization switch! Whatever that is

[Rev D]

 I remember one of the members here built a few of them, had the hard to find varisters if I recall and his amps sounded very good. Dinkotom was his name, here's his video and an example of the trem:

 Very warbly and different for sure. Almost like more a chorus type sound. Here's another:

&feature=related

 That one I guess per description is a old fender amp converted to a Maggie circuit. Neat stuff.

Regards,

Rev. D.

Ps: They'll do a darn cool copy of Lucy in the Sky with Diamonds intro too...

[plexi50]

I messed with it some this morning. Being a real B to find the problem. But after that i have almost completed a complete tube amp build from SCRATCH. Guyatone Junior kit. Im wiped out. Im going to sleep like a rock tonight

[punkykatt]

On projects like this I use Dougs listening tool to find where the signal gets weak. You can start at the input jack and go through the preamp or start at the OPT secondarys and work backwards.  It could save you some time.
 Punky

[RicharD]

>On projects like this I use Doug's listening tool

I finally built one of these using a broken guitar cord, a broken clip lead, a used capacitor, and a scrap of shrink tube.  I used it to check each individual voice of my Hammond Organ....... just for fun.

[plexi50]

Great idea. I need to make the time to make one. Is this in Dougs library? This will help me alot. Thanks you all for the tip. Thats the only way this is going to get fixed as far as i can see.

[Geezer]

http://www.el34world.com/Hoffman/tools.htm

[terminalgs]

The volume is very faint and really distorted and the volume fades out pretty fast

my M10-A performed like this when I first got it.  replaced the 7189A's and everything was fine. 

like sluckey pointed out, a big difference between the M10 and M10-A is  the transistor.     notice your (-) bias voltage power supply is also the power supply for the transistor.  I'd keep an eye on that voltage to the transistor if you are attempting to change that bias voltage.  You don't want to go over spec on the transistor, voltage wise.

[Fresh_Start]

http://www.el34world.com/Hoffman/tools.htm

FWIW I built the box Doug describes as "just theory" and it works like a champ:




"B" goes to bad amp
"G" goes to a little solid state amp I never use and don't really care about anymore.

Just make sure you ALWAYS start with the listening box's volume at "0"

Fantastic debugging tool.

Cheers,

Chip

[plexi50]

Wow thats a cool device. I will make one tommorrow. Im sure this will pay for itself many times over just in stress relief. I wondered what that transistor was doing. This is really weird. Today i pulled 4 of 6 premp tubes and left 2 preamp tubes in circuit. So one is the PI and the other? I think 1 of them is the vibrato and 2 are reverb. I still got the same signal and volume level. Cant wait to build that signal tracer

[sluckey]

That box can be used with any amp. I'm sure you have one sitting around somewhere.

Before you can fix this amp, you will have to identify all tubes. You must know which tube does what and where it's located. Take your time and get it right. You can start by simply labeling the tubes on the schematic V1, V2,,,V8. Then verify which tube on the chassis is actually V1 by studying the schematic and chassis wiring. For example, you arbitrarily label the Channel 1 preamp tube (7025) as V1 on the schematic. Now look at the chassis. Locate the Channel 1 input jacks and trace the wiring thru those 47K resistors to the grid of some tube. Label that tube as V1. Use a permanent sharpie and mark the chassis top and bottom. Also mark the tube layout on the schematic.

Repeat this procedure to locate and label the Channel 2 preamp tube. These two tubes should be easy to identify. And the two output tubes should also be easy to identify. Then the PI should be easy to identify. That only leaves 3 more tubes. You'll just have to compare the schematic to the actual circuit to identify the rest of the tubes. You should be able to complete the labeling in about 30 minutes or less.

Once you have the schematic and chassis properly labeled, it gets a lot easier. I suggest divide and conquer. I would disconnect the Channel 1. Look at the schematic and locate that transistor. See that 0.22uF cap connected between the collector of that transistor and pin 7 of the PI tube? Locate that cap on the chassis and disconnect one end. Now Channel 1, vibrato, and reverb circuits are totally disconnected. You are left with Channel 2 preamp, PI, and output tubes. Much simpler. Make that work right before attempting to fix channel 1.

Here's a schematic showing just the Channel 2 preamp, PI, PA, and PS...
http://home.comcast.net/~seluckey/M10_channel_2_only.gif

If you have trouble labelling the tubes, post some high rez closeups of chassis (top and bottom) and I'll give you a hand.

[plexi50]

Thanks Steve. It arrived with all the tubes tape labled as well as the chassis next to each tube.  I have done some tracing from the inputs of each channel and have been checking cathode voltages. I have been all over the amp instead of focusing in on one area at a time.  I will slow it down and make time today to write down and confirm the tube positions. Just a fast note: I was reading 47 VDC and 72 VDC on (2) of the preamp tube pins 8 cathodes. At least the bias grid is functioning properly

[plexi50]

Just got back on this today. Got the original schematic from the owner. It's going to be a long day

http://s356.photobucket.com/albums/oo5/plexijtm45/?action=view&current=M10.mp4

[sluckey]

Lo Q videos suck! Just take some good pics. BTW, there's an original schematic glued to the cab.

[plexi50]

That old schematic is a mess. Looks like coffee stains all over it. Im going to take some better actual pics and try something with the small schematic i have

[plexi50]

I will get this scanned tommorrow. I dont have one here