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Magnavox stereo PP console amps and the mysterious balance pot

mhardy6647

mhardy6647

Lunatic Member
I thought it might be helpful to someone out there to document an interesting and fairly well-known "feature" of some of the Magnavox tube stereo power amps that some of us are fond of finding/collecting/rehab'bing. If everyone knew about this already, I apologize for the redundancy.

These amps are generally on separate chassis than the tuner/preamp sections in the consoles that contain them, but are somewhat tightly integrated to the preamp by an umbilicus and a multi-pin Molex connector. AK member Cabinover had gotten such an amp which had been modified for standalone use, but he noted that the gain in the amp was very low. He asked me to take a look at it.

Fortunately, there is a good "Magnavoxfriends" group on Yahoo, and a few Magnavox experts kicking around (e.g., Gabe V., who has a site of his own, as well). This particular amp seems to be the "88-01-00" chassis. It turns out that some of these stereo amps use the (preamp's) balance pot as part of the feedback loop from the outputs. Bypassing the umbilicus open circuits both channel's feedback loops. It further turns out that all one needs to do to "fix" the problem and restore the amp's gain is to replace the balance control with a pair of 390 ohm resistors (one for each channel) tied to ground.

I tried to take advantage of the bright AM sunshine today, but only succeeded in demonstrating yet again my lack of photography skills!
Here's das Ding an sich:

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Here are the underpinnings in toto. Note the replaced coupling caps from the paraphase splitters to the PP 6V6 outputs:

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Here is a close up of the 390-ohm carbon film 1/4 watt resistors added to "replace" the balance pot and close the two channels' feedback loops.

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Reference: http://db.audioasylum.com/cgi/m.mpl?forum=tubediy&n=112480

These are nice little 10-watt stereo amplifiers and if found cheap or free well worth simple restoration or complete rebuilding/modification, depending on one's level of motivation. I hope this is helpful to someone.
 
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magnavox88-01-00.jpg


Here's part of the Sams schematic for this particular power amp... note the "balance" control. This is what needs to be replaced by the 390-ish ohm resistors (or, I guess, a 750-ohm pot with wiper to ground!) to make the standalone power amp whole.
 
wow that is the same amp i picked up about a month ago with a hum problem mine still has the balance and hum hooked up so i need to remove the balance pot from the amp ??? to make it function completly thanks for the pics do i have a problem with the extra 6ue7 tube still being in the amp circuits ??? any help would be appreciated
 
No, you can leave it in... I think that some of the amps had the balance pot outboarded (on the preamp, I suppose). This one had open feedback loops; yours should be fine.

"Extra 6EU7"? Do you have one of them with three 'front end' tubes? I think/guess/assume the extra tube was a mag phono preamp. This particular amp has a blank socket hole where such a tube could've been installed; note the "6EU7" embossed by the hole in the top view photo.

EDIT: I'd guess your hum is due to a P/S electrolytic, the coupling caps, or poor lead dress/routing somewhere or a ground loop.

I have now shared everything I know about these amps :-) I'd recommend for anyone who is interested in 'em to join the "Magnavoxfriends" Yahoo group and/or peruse http://gabevee.tripod.com/
 
The 9300 series 6BQ5 PP I have doesn't have the balance pot on the chassis, it was the tuner/pre-amp or whatever.

It's been a few years, and it's kinda hard to recall... I brought the amp to a local friend for bench test.
I seem to remember that one side of the amp was overdriving the output stage, or one side of the output stage into distortion.

We both pondered, and then he figured the balance pot or a tone compensation pot was tied in with the feedback loop.

He then placed 1/4 watt 1K resistors from each feedback loop to ground, and that solved the problem. A 1000 picofarad silver mica was also bypassed across the 3.9K resistor in the feedback loop.

Paraphase inversion, quasi bias.....the Magnavox amps do have some esoteric circuits, like you mentioned in another post. Or strange circuits....

The quasi-bias with the cathode bypass cap tied to the center tap of the hum pot. I always thought that was interesting. From what I understand,(barely) it works like a diode...

I tried the Dynaco paraphase inverter circuit using 12AX7 in the front stage of a Admiral 6BQ5 console amp. It was 12AU7 with the (concertina?) driver stage. (Plate and cathode driving the output stage...)

When the amp was stock, it was ratty and almost sibilant sounding. And would come unglued when pushed rather easy. Stereo imaging was non-exsistant. The stock driver circuit did measure out good, resistors, etc.... The caps were not leaking.
When I went to the Dynaco paraphase driver circuit, it made quite a bit of difference. I dunno.....I don't think paraphase inversion is that bad.
 
Resurrecting an old thread...Balance pot resistor replacement

Picked up a Maggie 8802-00 yesterday, and HomerJSimpson and I have been comparing it to the underside of my 9302. When I bought it, I cringed as the seller fired it up, but no problems...

Anyway, we have figured out that the missing balance pot is dropping the gain as previously metioned. We have traced the connections back to the green and yellow wires coming off the 5 connector strip connected to the 6EU7's, and leading back to the big pre-amp plug.

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Our assumption is that we need to jump the green and yellow wires with a resistor of some value. Is this the right assumtion? If so, what value should we use? I measured the resistance of the 9302's pot, and it was in the 400-700 ohm range....
 

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Yeah, get two 390 ohm 1/4 watt or higher resistors, both leads get soldered to the grounded center tap of the terminal strip, the others ends get soldered to where each brown-green-red resistor meets the yellow & green wires. MHardy's picture in this thread shows it clearly.

After you do that, it's easy to cut out the wires that go to the moulex plug and get rid of that wiring. If you do, you will need to shrink-wrap or put on electricians tape on two leads coming out of the power transformer I think.
 
Thanks!

Nirky said:
Yeah, get two 390 ohm 1/4 watt or higher resistors, both leads get soldered to the grounded center tap of the terminal strip, the others ends get soldered to where each brown-green-red resistor meets the yellow & green wires. MHardy's picture in this thread shows it clearly.

After you do that, it's easy to cut out the wires that go to the moulex plug and get rid of that wiring. If you do, you will need to shrink-wrap or put on electricians tape on two leads coming out of the power transformer I think.
Click to expand...

When you say 390 ohm, 1/4 watt or higher, do you mean higher resistance or higher wattage????

Can I safely assume that what I'm trying to do is duplicate as closely as possible the resistance of the pot as it would be in the centered position?

Therefore, if I use 370 Ohm resistors, would that provide me with any more appreciable gain? How can I maximize the gain at this point?

Also, since it seems to be functioning OK, do I have anything to gain by bringing it up slowly on my Variac? Can I get better performance or longer life out of the PS caps by doing this reforming process at this point?

I figure I will just go ahead and replace the coupling caps, and anything else that looks dubious. How long should I wait for the caps to discharge before messing with it???

Thanks again.....:thmbsp:
 
1/4 watt or higher. The resistors here are taking the place of a balance pot that was on the pre-amp chassis. The 370's that you have are within 10%, close enough, don't know if you'd hear any difference if you had 390's.

Even though it is working, I'd consider replacing the cap can, any other electrolytics, and the coupling caps. Until then, bringing it up on the variac is safer to the components. If you want, you can discharge a cap with that 370 ohm resistor, connected to insulated alligator clips, or just simply short the cap out.
 
analog addict said:
When you say 390 ohm, 1/4 watt or higher, do you mean higher resistance or higher wattage????

Can I safely assume that what I'm trying to do is duplicate as closely as possible the resistance of the pot as it would be in the centered position?

Therefore, if I use 370 Ohm resistors, would that provide me with any more appreciable gain? How can I maximize the gain at this point?

Also, since it seems to be functioning OK, do I have anything to gain by bringing it up slowly on my Variac? Can I get better performance or longer life out of the PS caps by doing this reforming process at this point?

I figure I will just go ahead and replace the coupling caps, and anything else that looks dubious. How long should I wait for the caps to discharge before messing with it???

Thanks again.....:thmbsp:
Click to expand...

1/4 or 1/2 watt R's should be fine. Higher wattage is overkill, unless you have a higher wattage in that value on hand.

Yes, I'm pretty sure you are replicating the load of the balance pot that isn't in circuit.

370 ohms should work pretty much the same as 390 ohms. I am using 1k 1/4 watt resistors on my 9300 amp.
We just figured the pot was around 750 ohms, and went with a 1000 ohms.
I guess we never pondered both sides of the balance as 390, but it works.

Without the balance pot or a replicated load for it, one side was overdriving the output into distortion and slight oscillation. The 1k resistors cured the issue.
So I guess you can get too much gain....

You be better off to use a DC power supply to re-form capacitors than a variac. It can be done with a variac I suppose, just use it until you feel the amp's caps are formed.
If the amp doesn't buzz and hum in a objectionable way, the PS capacitor may hold up for awhile.

Like Tim says, it's good to discharge HV capacitors with a resistor. A 1K or so R of a decent wattage like 5 watts or better works best to me.

It saves from arcing, which is kinda harsh on a capacitors plates. The R sops it up I guess....

Hang on to the body of the resistor or use pliers to hold it, then short the cap terminals to ground a few times.
 
How would you....

Mike Stehr said:
1/4 or 1/2 watt R's should be fine. Higher wattage is overkill, unless you have a higher wattage in that value on hand.

Yes, I'm pretty sure you are replicating the load of the balance pot that isn't in circuit.

370 ohms should work pretty much the same as 390 ohms. I am using 1k 1/4 watt resistors on my 9300 amp.
We just figured the pot was around 750 ohms, and went with a 1000 ohms.
I guess we never pondered both sides of the balance as 390, but it works.

Without the balance pot or a replicated load for it, one side was overdriving the output into distortion and slight oscillation. The 1k resistors cured the issue.
So I guess you can get too much gain....

You be better off to use a DC power supply to re-form capacitors than a variac. It can be done with a variac I suppose, just use it until you feel the amp's caps are formed.
If the amp doesn't buzz and hum in a objectionable way, the PS capacitor may hold up for awhile.

Like Tim says, it's good to discharge HV capacitors with a resistor. A 1K or so R of a decent wattage like 5 watts or better works best to me.

It saves from arcing, which is kinda harsh on a capacitors plates. The R sops it up I guess....

Hang on to the body of the resistor or used pliers to hold it, then short the cap terminals to ground a few times.
Click to expand...

...use the DC power supply? I have a couple of Kenwood 0-70VDC/0-1.0A power supplies sitting around, but how would I hook them into the circuit, and is that enough voltage?
 
You need like around 300-350VDC supply. 70 volts DC won't form them all the way to the spec voltage.

Just ramp the amp up using the variac until you feel confident the PS capacitor is formed enough to where you can just plug into the wall.
You can do that while you listen to the amp. But if you don't listen and decide to ramp it up with a variac, make sure to have proper load resistors to make up for the speaker load.

If the hum isn't objectionable, you may get away with the stock cap can for some time.

Using a DC supply of the right voltage, you just unhook or lift the the leads/resistors to the cap terminals.
Ground to ground, B+ to the terminals. Then you run up the voltage of the DC supply, monitoring current.
Too much current drawn usually equals fried cap.

This isn't the optimum way to re-form caps, as it doesn't indicate ESR, or DF factors. But it works, and is better than nothing.
 
An old school capacitor checker is useful for this (and also will indicate HV leakage of an electrolytic). You might want to pick one up sometime.

I bought a Heathkit at a local antique radio flea market for $20.

EDIT: An ESR tester is definitely a good investment, too. I keep trying to convince myself to invest in one.
 
Thanks....

I was figuring I needed to get up to B+/plate voltage on the DC power supply. I can link these in series, but I'm still not going to get 400 VDC out of them.:no:

I do have a plain cap checker, so I will check the uF values on the PS can. I would like an ESR meter, but so far have balked at the price tag....:thumbsdn:

Thanks again for the help gentlemen...:thmbsp:
 
Some of the old Maggie amps that had balance pots, the balance pot only affected one channel. If you dig up the schematic for those, you'll find that the input stages from one side to the next have different resistor values, and possibly one cap will be different also, can't remember exactly.

Some of the later 93 series (EL84 push-pull), the balance control was part of the preamp, and totally omitted from the amp chassis. Those will be the easiest to convert to stand-alone amps.
 
BrassTeacher said:
Some of the old Maggie amps that had balance pots, the balance pot only affected one channel. If you dig up the schematic for those, you'll find that the input stages from one side to the next have different resistor values, and possibly one cap will be different also, can't remember exactly.

Some of the later 93 series (EL84 push-pull), the balance control was part of the preamp, and totally omitted from the amp chassis. Those will be the easiest to convert to stand-alone amps.
Click to expand...

Oh yeah, nobody's mentioned this yet (I think), don't just leave the 390 Ohm resistors out, no matter how logical the schematic may look without them, or the amp will have a tendency to motorboat. Yes, learned this the hard way, and GordonW heard it right away.
 
Ok....

Soldered the two 390 ohm 1/4 watt resistors into place, and also changed out the two .047 uF coupling caps while I was underneath it.

I'm kinda tempted to change out some of the resistors too, especially the 20% tolerance ones. I did freshen up all the solder joints that looked dubious, and am still scratching my head as to why there are two different values for the coupling caps.

I had .047 uF caps in stock for just such a need, but have never seen the .0047 uF caps used before. Anyone know why they went with two different values?

I'll post some pix tonight or tomorrow....

Thanks for all the help!
 
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