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
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
Attachments
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
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
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.
Before doing any mods i would like to understand what the current circiut is doing. I have not seen this configuration before.
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.
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.
An externally hosted image should be here but it was not working when we last tested it.
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
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
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.
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.
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.
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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.
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
Mine has very little hum. Is your power supply filtering similar? Does it have the ripple feedback to the cathode of the 12ax7?
It does not. My circuit is slightly different, the balance control is fed from the two cathodes of the 12AX7. A little bit above my level of comprehension, that.
Worked a bit more on the power supply, implementing a somewhat better layout / routing. The nasty hum is gone!
However, one side sounds fairly dull and lifeless compared to the other, and if I turn the balance control all the way to that side, it goes into high-freq oscillation.
I swapped tubes but no change. Moving the .05 cap between input and ground with a chopstick can quell the oscillation momentarily, so I'll focus on that next. Obviously this is not a PS issue as it's confined to one side only....
However, one side sounds fairly dull and lifeless compared to the other, and if I turn the balance control all the way to that side, it goes into high-freq oscillation.
I swapped tubes but no change. Moving the .05 cap between input and ground with a chopstick can quell the oscillation momentarily, so I'll focus on that next. Obviously this is not a PS issue as it's confined to one side only....
And yes by the way, I was wrong: There IS a feedback loop back to the cathodes of the 12AX7. Really overall it's nearly identical to your circuit, other than the aforementioned balance control.
Sorry for the necropost, but just wanted to thank y'all for this thread! I found an old Magnavox stereo phono which matches the description of this one, and there's a wealth of good info here.
It's been a while since your post. Did you get your unit working okay? How's the hum on yours? Just picked up one myself and started recapping it. Weird circuit design. Wondering if I should change anything or keep it stock.
Kennyg
Kennyg
Hey, thanks for asking! Yes, I got it working and overall it sounds good, but the hum persists. So far as I can tell it's the "cheap potentiometer disease": It goes away at extreme low and high settings, but is present everywhere in the middle.
I would try something like a better-quality pot, but there's really no room in the chassis whatsoever. Think I bumped the pot value down from 1 meg to half that as well, but it's a while since I checked....
I would try something like a better-quality pot, but there's really no room in the chassis whatsoever. Think I bumped the pot value down from 1 meg to half that as well, but it's a while since I checked....
That's not "cheap pot". It could be an unsuitably high-value pot.
Can you use a lower value pot? But this is brute-force "fix". 10X lower Ohms is 10X lower buzz, which is not really "a lot" of reduction. And puts more load on the source.
But the real issue is that crap is being induced in the wiper wire. At either extreme, crap is sucked-out by ground or by source. In the middle is worst.
So where does your wiper wire wander? Past transformers and line cord wiring? Is it shielded?
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