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Old 19th November 2011, 11:53 PM   #1
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Default Magnavox 50EH5 Stereo Amp

I purchased a old Magnavox Suitcase record player that came with a 50EH5 stereo amp. The amp has no transformer for isolation. I have been testing it with a variac and isolation transformer and the thing actually works fairly well. I traced the circuit for one channel, see attached picture.

I am not sure what the circled portions of the circuit do.

1. The .047 600V caps from RCA chassis to common ground and from common ground to chassis. Are these protection caps in lieu of a missing isolation transformer?

2. The 68K to the cathode of the 12ax7, is this Ripple NFB to minimize hum? There is very little hum in the amp.

3. The 220K in series with .002uf. This must be NFB. I am planning to install a permanent isolation transformer. Currently the speaker terminals of the output transformer are floating. Would GNFB from the output of the transformer be better?

If I wanted to bypass the tone controls, would I need to short the 47K in series with the .01uf blocking cap and lift the leg of the 1 meg pot connected to the pentode grid?

Thanks

Alfredo
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Old 20th November 2011, 12:06 AM   #2
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Here ismy 50eh5 amp... built from my parents fold-down stereo (and some links to others)

5-Buck Amp

Mine didn't have the luxury of an addiitonal voltage stage (12ax7 in yours). You couldlikely get away with less gain.

dave
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Old 20th November 2011, 12:23 AM   #3
ke4mcl is offline ke4mcl  United States
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the shortfall on those is the tiny output transformer. output power is about 1.5w or so. i'd leave the circuit alone other than recap it. mount it into some kind of enclosure with an isolation transformer and use it as a desk amp for the pc.
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Old 20th November 2011, 02:18 AM   #4
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Before doing any mods i would like to understand what the current circiut is doing. I have not seen this configuration before.
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Old 20th November 2011, 03:25 PM   #5
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I changed the 12ax7 for a 12au7 and the gain is better matched for a ipod input. Thanks for the suggestion Planet 10.

Anyone willing to give it a shot describing the feedback in the circuit?

Took a picture after sawing off the record player portion of the unit. Now i am left with two equally sized speaker boxes (one with amp and controls) that closes like a clamshell using the original harware.


Click the image to open in full size.
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Old 20th November 2011, 06:28 PM   #6
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The circled bits:

1/ (at the input) chassis ground
2/ power supply injecting residual hum into the negative feedback loop
3/ feedback from output stage to input stage (not around the OPT as is common)

dave
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Old 20th November 2011, 07:20 PM   #7
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There's good reason for the O/P trafo secondary to "float" and not be associated with a GNFB loop, which requires grounding. The "floating" secondary isolates the speaker from the power mains.
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Old 11th December 2011, 07:21 PM   #8
slor is offline slor  United States
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Any tips on reducing hum in this amp? Replaced the PS caps and coupling caps, but there's still a nasty hum no matter which was the AC cord is oriented....

Also, the balance control is different than that shown above, mine sends the outputs of the cathodes of the 12AX7 to the balance control.

Would the cap between PS ground and chassis ground have a noticeable effect on hum? I typically remove "death caps," but that's in more common transformer-isolated amps.

Last edited by slor; 11th December 2011 at 07:49 PM.
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Old 11th December 2011, 07:39 PM   #9
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I've made a similar amp, (mine uses an isolation transformer: 230AC mains /120AC secondary) Now I'm thinking to change to a SS bridge rectifier, and burn the excess B+ on a simple SS regulator, to feed everything, including plates. Regulator+ right cable layout sure will get rid of any hum.
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Old 12th December 2011, 04:34 AM   #10
slor is offline slor  United States
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Would an isolation transformer have any effect on hum? I can't believe this unit would have hummed as badly when new.... Swapped a different 12AX7 but no real change. I would swap the 50EH5s but currently have none
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