• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Vintage Magnavox tube amp

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Hello, I was wondering if someone could help me I picked up a Magnavox tube amp out of a console stereo It has 4 el84 output tubes and two 6eu7 pre tubes . My question is that there is on knob on the right of the amp that says hum! What is it for??
Next is the amp seems to have all the original tubes and sound great, but sometimes when powering on it will make noise like when TT is to close to speakers but from one speaker or the other or both! then I power it down restart and its fine , or when played load ,or even If I know the amp . Could that be the Tube are just shot? The rectafire tube is the only one that i replaced . If some one could help me that would be great!!
The amp doesn't seem to have bias adjustment either .

Thanks
 
Probably a hum balance pot on the filaments.

sounds like a Push Pull EL84/6BQ5 version. They are pretty common and should have some numbers on the chassis that will help identify it.

I'm not sure what noise you are describing.

If hum is very loud, the caps may need replacing.
 
Electrolytic caps are pretty well guarenteed to be dried out in consumer gear after 25 years. They are full of acqueous chemical and sealed with rubber, which is attacked by oxygen running or not. These caps are tall cans with squares, circles, triangles for the plus segments, or in cardboard covered cans with a plus on one end. Paper, plastic, and ceramic caps, don't deteriorate much. Before changing any of these caps, read the high voltage for newbies sticky thread at the top. Voltages are stored by these caps and unless repeatedly and safely discharged after power down and unplugging, it can still shock you.
Televisions burned up tubes much worse than sound equipment. People focus on the tubes because they are easily replaceable, but at 20 years the electrolytic caps are much more of a problem. After it is running, you can check the B+ voltage with a DVM at full volume to check if the rectifier tube is bad. That is the one most likely to wear out, at several thousand hours. You will need a DVM or VOM with a 600 VDC scale to do any safe work on it. Voltage must be under 25 VDC before you touch any metal except the chassis.
I reworked a 1968 tube organ last year, put 70 electrolytic caps in it. I've recapped my ST70 tube amp 3 times since I bought it 1971, when it needed new caps; was producing 14 watts instead of 70. It was a 1961 model.
New radial lead caps can be bought with a life rating of 3000-8000 hours. These could eliminate the every decade replacement cycle. I put these under the chassis on terminal strips from tubesandmore.com. The caps they sell, including the tall cans, they don't publish a life rating. I get the life rated caps from mouser.com or newark.com.
Have fun. 6BQ5 push pull should be good for about 13 watts.
 
Last edited:
"Gimpy" is RIGHT about replacing all the electrolytic caps., as they go bad with age. Replacing interstage coupling caps. with 716P series "Orange Drops" is probably a good idea. Check carbon composition resistors for drift in value. Replace out of spec. CC resistors with carbon film parts of the same nominal value and wattage rating.

The 6EU7 can be troublesome to source. Rewire those sockets for 12AX7s and install (specifically) Sovtek 12AX7LPS stock. No parts value changes are needed. ;)

The absence of a bias adjustment is a non-issue. The amp undoubtedly uses self (cathode) biased O/P tubes.

An inexpensive replacement (with decent sonics) for the EL84s is the Russian 6П14П-EB (6p14p-ev), AKA EL84M.
 
Ok guys thanks ! But Im a nubbie so Here i go. yes it sound like the same amp. The electrolytic cap is the tall silver one right? the coupling caps are the black ones underneath ,i think they are three? If I new how to upload pics I would !. the rectifier tube was replaced with a nos RS 5u4-GB. One of the output tubes is a Zenith and its much brighter also. The sound is like rumble it make the driver go nuts ! low frequency hum. If the amp is playing fineIll thump the amp and there she goes or if I crank it ! It seem that that the bottom end end will get it going or it the vibration that gets it going . Also does anyone know what the hum knob is for?? I will try to post some pics . Thanks for everyone's help!
 
The color of the caps is a manufacturing option, not an inherent trait. The coupling caps go from the plates of the driver tubes to the grid of the output tubes. You can get the diagrams of the tubes at triodeelectronics.com, also a careful vendor of tube amp parts. The tall can is probably an electrolytic cap. Looking under the deck, are there no cardboard tubes with plus on one end? Hammonds are full of cathode bypass electrolytic caps and the Dynakit has a couple of axial bias caps under the deck too. These are the most likely suspects, after the tall can filtering the rectifier tube output. I've never had a paper coupler cap fail, although I replaced the paper ones in the ST70 in the forty eighth year just because plastic film ones are supposed to sound better. Guitar amp techs replace coupler caps every time the amp hits the shop, it seems. The coupler caps in my Hammond organs are all original, although they were plastic film originally. Coupler caps tend to be 0.22 uf, 0.47 uf or 1.0 uf, 600 VDC. Note modern vendors use uf for microfarad, whereas the old caps say MF.
The brightness of the tubes is a construction trait, not a sign of quality. I determine tube quality by measuring idle current with fixed bias (which indicates leakage of air if too high) or AC voltage out with maximum signal in, which indicates gain. There is probably not a working tube tester within 250 miles of here.
You would have to draw the schematic for us to determine what the potentiometer does. I have some Hammond amps with a "tone" pot that cuts high frequencies, and another with a "factory use only" pot that balances the idle current on the push pull 6BQ5's. The dynakit has "bias" pots that sets the idle current on the output tubes.
Sometimes carbon comp resistors on the filter cap go bad, particularly if they look burnt or bubbled. Check their value after you take them off. The plate resistors go bad on the tubes, too, some. I buy metal film ones where I can get them, but buy 2 or 3 watt versions because the 1/2 watt versions are too short to hold off 450 VDC. Multicomp brand from Newark and Vishay brand from mouser have been quiet high quality parts. Carbon comp resistors are noisier than metal film, which is why they are not stocked much anymore. Above 1 megohm I have to buy wirewound, there don't appear to be any metal film ones stocked.
 
Last edited:
Under the quick reply, click the go advanced box. Then when you get there, type something in the text box, then click the manage attachments button. Then you can browse your home computer to get them off your hard drive. The pictures can't be huge, something under a megabyte, I think. The details are in the fine print on "upload attachements" window. I think you have to have cookies enabled.
Or you can load pictures on photobucket.com or someplace and paste a link in your text.
 
9304 schematic
 

Attachments

  • Magnavox_93xx console amp series.jpg
    Magnavox_93xx console amp series.jpg
    646.8 KB · Views: 720
THANKS for the schematic, "Gimpy".

C20, which is undoubtedly electrolytic, and R116 form a cathode bias network that's shared by all 4 O/P tubes. That means a quad of tubes that have been tightly matched for both cathode current and transconductance (gm) is necessary.

The aluminum can is a triple section 'lytic shown as C2A, C2B, and C2C on the schematic. You might have to use a 2X 40 μF. part in that opening and a separate 10 μF. part under the chassis, as replacements.

C104, C106, C204, and C206 are the interstage coupling capacitors.

Talk to Jim McShane about tube and other parts needs.

Does anyone care to comment about the asymmetries in the paraphase setup Magnavox used? I don't get a warm and fuzzy feeling.
 
Administrator
Joined 2004
Paid Member
<snip>
Or you can load pictures on photobucket.com or someplace and paste a link in your text.

Please, please, please use the forum manage attachment option if at all possible. Inline links are a real pain to deal with for us mods, and unfortunately over time they are not persistent. Loading times can range up to 30 seconds or more per picture. I recommend posting pix in the 800 x 600 to 1024 x 800 range directly on the forum where someone can see them 5 yrs from now if they are interested in the subject of this thread...
 
Ok I hope this helps.. As you can see i have put in a main fuse for the power and power switch. Now the cap that i was talking about is the tall silver one, should that be replaced and under the hood I hope I dont have to replace all that? I do see two back looking cap should I replace them? The knob on there is one that I just put on for looks but there is a pot there and it says hum. I do have the top for the transformer and I was wondering if I should put two more fuses one for each output transformers?

Thanks again Gimp and the others
 

Attachments

  • PB290244.jpg
    PB290244.jpg
    422.9 KB · Views: 350
  • PB290245.jpg
    PB290245.jpg
    532 KB · Views: 330
Last edited:
You could possibly put an IEC power connector in the back left where the cutout is in back of the output transformer. However this requires you to run power up to the front left, across to the right and all the way back to the power transformer diagonal to the cutout.

The disadvantage of the fuse location is that you will route the power across the majority of the circuitry exposing it to potential hum.

I would recommend pulling the fuse from the current location and moving it to the right side where power comes in and you have the power switch. Change to a three prong power cord and tie earth to chassis through a 10R 2W resistor with anti-parallel 5A Schottky diodes in parallel with the resistor to minimize hum.
 
Your amp doesn't have the balance control shown on the schematic, so you should install two 390 Ohm resistors to ground to replace it (OR you could try to find a 750 Ohm pot that would fit...). They can go on the terminal strip at the top center of your picture. Gain will go up and your oscillation will probably not be heard from again.

The Sprague black caps with red letting are almost always good. The ones with yellow lettering (or multiple color stripes) are always bad. As far as rewiring for 12AX7, yes, it's more common, but 6EU7s cost less... _I_ wouldn't do it, and I have a few hundred 12AX7s in my basement...
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.