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Old 27th November 2011, 01:18 AM   #1
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Default Vintage Magnavox tube amp

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 .

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Old 27th November 2011, 02:15 AM   #2
TheGimp is offline TheGimp  United States
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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.
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Old 27th November 2011, 02:49 AM   #3
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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.
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Last edited by indianajo; 27th November 2011 at 03:02 AM.
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Old 27th November 2011, 02:55 AM   #4
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"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.
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Old 27th November 2011, 06:34 AM   #5
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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!
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Old 27th November 2011, 06:51 AM   #6
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Thanks Gimp, there are numbers on the amp in black they are A9304-00 there is 102283 and stamped in the amp is 636826-8. It had the pre tuner with it also and took the 12" drivers .

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Old 27th November 2011, 11:56 AM   #7
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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.
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Last edited by indianajo; 27th November 2011 at 12:06 PM.
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Old 27th November 2011, 02:08 PM   #8
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Does anyone know how to upload some pics ????
Thanks
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Old 27th November 2011, 04:36 PM   #9
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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.
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Old 27th November 2011, 06:48 PM   #10
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9304 schematic
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