Magnificent Magnavox

stankmuffin

New Member
I bought a Magnificent Magnavox console tuner/turn table at an auction for a couple of dollars. The tube amp is a 132AA. All the tubes appear to be working but there is no sound. I get static when I wiggle one of the tubes. Its pretty dusty and I need to clean the contacts. Does anyone know anything about the tube amp? I'm real new at this whole thing, someone was telling me tube is the way to go so I bought it because it was so cheap and it had tubes in it. For two bucks what the heck.
 
Stankmuffin,

You know the drill, if you want help ya gotta post pictures of the gear so we know what you're talkin' about. :)

That noise when wiggling tubes is common with a tube set that has been sitting a long time and is just minor corrosion/dirt on the socket contacts/tube pins. Removal and re-insertion a dozen times, aided by some control cleaner sprayed on the tube pins and or socket if really bad will usually cure the problem. Use a non-residue cleaner. 99% isopropyl alcohol is what I use for this.

Best to bring up a vintage piece of tube gear on a variac slowly to save/restore the electrolytics. If you have been running this it is probably too late. If the electrolytic can capacitor gets warm/hot you've pooched it and it will need replacement.

Congrats on your fine purchase! Magnavox made nice stuff.
 
Originally posted by stankmuffin
There are 4 - 6V6 tubes and 2-5Y3GT. All magnavox.

Ah-HA! THERE'S your problem! Betcha a donut one of those 5Y3's is supposed to be a 6SN7. And it wouldn't be the one close to the power transformer.

Some knucklehead put the wrong tube in. You need a dual triode for 1) voltage gain and 2) phase inversion. A double-diode like the 5Y3 just isn't up to the task. ;) But I bet you didn't hurt anything.

Tubino
 
Ooops. I ... uh... looked at your picture this time. You didn't mention the 9-pin mini tube in the corner. 12A?7?

And it looks like the tube sockets are marked, I presume two are marked 5Y3, so there are supposed to be two rectifiers in there.

Okay, are you sure you have the power and inputs all sorted out? Is there a switch that must be completing a circuit for input or power?

When there are tube rectifiers I like to start by pulling them and making sure you have all filaments lighting up. NO point in hitting a cold tube with B+.

Tubino
 
I forgot to mention the tube in the corner because it isn't marked on the amp. The tube has 12 AX on it though. Everything else is marked on the amp. I didn't know I could damage it and powered it up when I first got it to see if it worked. All the tubes lit up. Since then the tubes have been over at Dingus and I just put them back in today for the picture. I need things explained like I'm in kindergarten. Like first plug it in. Then turn power on etc.... I was a machinist before I got laid off and knew very little about electricity and less about audio. I've never even listened to a tube stereo and I am looking forward to it so any help and patience would be great! Thanks.
 
There are really two 5Y3's in this amp! They are either in parallel or each tube serves as one side of the full wave rectifier. The two 5Y3's are teamed up to provide more current capability.
Likewise, there are 2 6V6's (4 total) on each side of the push pull transformer for more power output. This is a mono amp. The 12AX7 is the 1st stage audio and phase invertor. Some of these Magnavoxes using a magnetic cartridge have an additional preamp (maybe 6AV6) mounted in the control panel chassis.
 
Read the opening statements as there is an article on how to bring old eqipment up to voltage without damaging it.
For a mono bloc I would say you have a nice one, as a matter of fact those Magnavox electrolitic capacitors (aluminium cans) were usually what came with Heath Kits which were TOTL kits when this was made.
Most of what I would tell you is in the interductory post, come back with specifics and we will help you all we can.
As far as the use of this unit with todays systems you can keep it as is for the most part and leave it a mono system, or you could make a great center channel for DVDs out of it.
Another option is to find exactly the same power amp and use it in a stereo set up. That is often the most difficult.
 
Magnavox Mono Amplifier

Congratulations on the great buy.

I used to have such an amplifier but ended up selling it because I only had one. That was some time ago. Now with ebay the chance of finding a mate is good. I have seen a number of amplifiers like this on ebay in the past. Be patient, one is sure to turn up soon.

Magnavox made a number of amplifiers that were similar to the one you have. I found a SAMS Photofact in my files of an AMP-142AB. It shares the same layout and tube complement. The only noticeable difference is that there is a standard input jack on the chassis between the 12AX7 and the electrolytic filter capacitor. The fact that your amplifier does not have the jack would suggest to me that the input is bundled in the wires attached to the octal socket on the other end of the chassis.

At this point you do not know if the fault is in the control section or the amplifier section. Have you tried all the inputs? Is there an AUX input on the back of the control section? If there is, try plugging the output of a CD player into this jack, switch the input to AUX and see if you have sound.

Tube amps are fun. Good luck with your project.

Sincerely yours,

tcdriver
 
I noticed when I was puttin the tubes back in 2 of the 6V6 were rattling like a light bulb with the filament burnt out. Is this a bad sign? Is there an audio book for dummies?:p:
 
rattling tubes

Dear Stankmuffin,

A tube can rattle and still work OK. How bad is the rattle? Generally, it is best not to subject tubes to physical shocks. The innards can be jarred loose.

Did you try the test I suggested?

Sincerely yours,

tcdriver
 
Tubes should not rattle and usually if something about one seems wrong, it is. Dirty pins, loose bottle, bad colors and lots of other things can cause problems or be because of problems.
Be careful because you are playing with very high voltage, capacitors store current and you don't want to get zapped. As far as the amp goes, none of the tubes are expensive or hard to find and your first move should be to decide on whether you want to keep the console or to remove the gear you would use. Then you need to get a variac, test your tubes and using a schematic decide on how you will get a signal into the amp and listen to it.
That amp should easy to work on as it has a nice large chassis. Putting an amplifying triode and an on/off switch and an RCA input along with a new set of binding posts should be easy to do, and really help you out for getting comfortable in using tubes.
I truly can not design at all, but I have built amps with spare used parts and ones bought surplus. Works too!
I would bet that one of those 5Y3s is used to power the off of main chassis preamp. Not an uncommon thing as the transformers and capacitors were the most expensive parts. The engineer was just making double use of the power supply transformer.
 
No I didn't. Today I removed all the tubes and connectors and cleaned it up using the contact cleaner as suggested. That is when I heard the rattle. It's not too bad. I didnt see an aux input, but there is an input on the preamp marked ext. with nothing in it. Is there a way to test the amp apart from the rest of the unit?
Thanks,
Stankmuffin
 
Originally posted by stankmuffin
Is there a way to test the amp apart from the rest of the unit?
Yes, but... you should ALWAYS have some kind of load (e.g. speaker or big 5-20 ohm resistor) hooked to the speaker terminals on a tube amp.

If you can swing by my place this weekend I can look over the amp, check the tubes, loan you a variac, give you some binding posts, and generally get you going.

Tubino (one of several TOLEDO-based karmaholics!)
 
Stank,
You are one lucky muffin. Variacs and tube testers are a must, as well as a little help from your friends!
 
Tubino,

Stank's the buyer of the Fisher as well. My audio junior if you will.

I'm in process of cleaning it up. What do I clean the faceplate with? It's like it's tarnished. I've begun cleaning the inards and it may look pretty before it gets plugged in.

I got the lot of tubes in, I'll take some pics and post it on that thread. The carrier is beat but works.

Is there any reason not to replace the caps (when possible) on these older tube amps? Modern caps are so much better from what little I know.

Toledo is boring! What else are we going to do.
 
Magnavox

Dear stankmuffin,

The 5Y3GT’s are connected in parallel. Magnavox needed two of them to handle the current required of the four output tubes (about 148mA) as well as the lesser current required for the preamp tubes. It was very common for Magnavox to use the double rectifiers in this way. Based on the configuration of tubes on the amplifier you have and the similarity to the AMP-142AB for which I have a SAMS Photofact, your amplifier was made in about 1955 or 1956. One can sometimes decode the date code on the electrolytic capacitors to learn when it was built. Just a little later Magnavox switched to using a single 5U4 that could handle the extra current without needing to double up on the rectifiers. I have a stereo AMP-175 that was made in about 1957 that uses four 6V6GT’s (two per channel) and a single 5U4GB.

The “EXT” may work for the test I suggested. I have a later unit with a “Tape” input but no EXT or Aux input. If you get really desperate you can use the “phonograph” input. Though it adds a little gain and equalization, the phonograph input would still allow you test for sound.

The beauty of the amp you have is that with a couple of simple modifications you can separate it from the brown box and hook it up to your music system. In general terms you need to:

Install an audio input jack near the 12AX7 and connect to the input grid of the first gain stage. This will probably be pin 2 of the 12AX7.

Connect the power cord so that you do not need to be connected to the control section to turn on the amp.

Install a suitable terminal strip or banana jacks to allow easy connection to your speaker.

In an earlier post you asked about books for a person just getting into working with tube equipment. May I suggest checking out the library? Sometimes one can find some really good books left over from the tube era. Especially good are the local college libraries as they tend to keep some of the books longer and have more depth of catalog. Another good source that I have used in the past to purchase books and parts is Antique Electronic Supply (www.tubesandmore.com).

Good luck.

Sincerely yours,

tcdriver
 
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