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Hello and questions about Magnavox SE amp

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hoff70

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Joined 2014
#1
Hi all! I've recently started getting into audio gear and I've always been a DIY'er with experience in general electronics, microcontrollers, etc. but I've been wanting to fool with tube gear for a long time.

I recently picked up an amp from one of these: https://app.box.com/s/87rlvqtizmauyr3qcgeo

I didn't cannibalize it, just picked it up off ebay.

Not much info on these so I had a couple of questions.

1- Why use 2 different types of power tubes in the same amp? One channel uses 50CS and the other uses 50EH5. The circuits are identical other than the balance pot and a couple of carbon resistors of different values.

2- Would it hurt anything to make both channels identical? I have some tested 50EH5's on the way as well as all new caps and an isolation transformer.

3- What would be a good speaker fit for this little amp? I was going to run it into 2 separate speakers.

Thanks in advance! I have tested it the way it sits with an MP3 player as an input and hooked to some BA A-40's and was pretty pleased with the sound! Not great but not too bad either. I'm just planning on running it as a power amp.

How it sits now:

An externally hosted image should be here but it was not working when we last tested it.


Just fooling with aesthetics while I wait on parts.
 
#2
Try PMing Tom Bavis. He is quite familiar with these old "maggot boxes".

Cross compare the 50EH5 and 50C5 data sheets. Perhaps little in the way of electrical difference exists between the 2 types.

Another thing to look into is the possibility of a biamped mono console. A number of products, from several vendors, exhibit that sort of thing.
 
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#3
Thanks! I've looked at the spec sheets and the main differences are:

50EH5:

grid 1 to plate..: .65 uuf
input..............: .17 uuf
output............: 9 uuf

50C5:

grid 1 to plate..: .55 uuf
input..............: .13 uuf
output............: 9 uuf

When fooling with it I swapped tubes around and decided that I liked the sound of the 50EH5's in channel 2 the best. Not very scientific I know 😛😛

I also bypassed the "balance" pot as well.

Hopefully Mr. Bavis will weigh in.

I'll look into the biamped mono amp but for now I just plan on getting this one into shape and learning in the process!
 
#4
From the look of the schematic, both amps are run parallel to the internal speakers so that both channels can be heard from them as a sudo mono or an optional external speaker can be used for stereo. The 50c5 and 50eh5 are quite different when comparing their plate curves. Notice the voltages given at the cathodes, 8.5v for 50c5 and 4v for 50eh5, swapping them around will either over or under bias them. Putting the 50c5 in the 50eh5 circuit could burn them out. Liking the 50eh5's in channel 2 would be correct since that is the channel they are supposed to be in to be biased correctly.Notice the feedback networks are also different between the 2 channels. The 50c5's appear to be biased for 35mA each and the 50eh5's for 42mA each. As for the reason to use different power tubes for each channel, well I,m a bit stumped on that one.
 
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#5
jerluwoo said:
From the look of the schematic, both amps are run parallel to the internal speakers so that both channels can be heard from them as a sudo mono or an optional external speaker can be used for stereo. The 50c5 and 50eh5 are quite different when comparing their plate curves. Notice the voltages given at the cathodes, 8.5v for 50c5 and 4v for 50eh5, swapping them around will either over or under bias them. Putting the 50c5 in the 50eh5 circuit could burn them out. Liking the 50eh5's in channel 2 would be correct since that is the channel they are supposed to be in to be biased correctly.Notice the feedback networks are also different between the 2 channels. The 50c5's appear to be biased for 35mA each and the 50eh5's for 42mA each. As for the reason to use different power tubes for each channel, well I,m a bit stumped on that one.
Click to expand...

Thanks! I guess that explains the resistor differences between #1 pins on the output tubes? 50C5 has 120 ohm (R23) and 50EH5 has 47 ohm(R34). I ordered some 47 ohm resistors to replace the one in channel 1 (50C5 side) in order to run 4 50EH5's. Am I on the right track here or completely lost 😱

There are also differences between C10/C14 and C9/C13 as well as R14/R26 from side to side?

Being my first foray into tube electronics I'm a little baffled by some of the asymmetry...
 
#7
The 50c5's are actually capable of making slightly more power than the eh's. About 4 watts for the 50c5 pair and 3 watts for the eh's. I would remove the balance pot and give each input tube their own grid resistor, it looks as though channel 1 grid was grounded through the tone control section which I take is no longer present, leaving me wondering if it's left floating or if a resistor was already added. There's a large number of mods that I personally would make but that's just me.
 
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#8
jerluwoo said:
The 50c5's are actually capable of making slightly more power than the eh's. About 4 watts for the 50c5 pair and 3 watts for the eh's. I would remove the balance pot and give each input tube their own grid resistor, it looks as though channel 1 grid was grounded through the tone control section which I take is no longer present, leaving me wondering if it's left floating or if a resistor was already added. There's a large number of mods that I personally would make but that's just me.
Click to expand...

Thanks again! Are you referring to the grid (section 2/pin2) on the 7025 ? I don't have the tone control panel. Just the amp section and it doesn't appear to have been modified.

Are the bias resistors R23 and R34? If so I guess I'll need to change R23 to a 47 ohm resistor?

I'm also a bit confused about the different values in the control loop (I guess that's what it's called...) Is that because of the tone circuit as well?

I have new caps on the way and plan on studying the components in detail as I install them. I seem to learn better that way 🙄
 
#9
The control/feedback loop differences are as baffling as the reason they used different power tubes. The 50eh5 requires less voltage to drive to full output, yet, they chose a lower feedback/higher total gain for that channel. The balance control would let you compensate for this, but, it seems an odd way round about. The capacitors in the loop reduce feedback and the ends of the audio spectrum. C9/C13 reduce feedback in the bass end while C10/C14 reduce it in the treble end. The BOM shows the same transformer for both channels, so, there's no technical reason for these to be different between the two channels. The ones in the 50eh5 side are smaller giving a "more bass less treble" effect and vise versa for the 50c5 side. The only real idea I have is that they were designing for a specific external speaker to compensate between it and the internal ones. This is still such an odd way to go about it, but, the engineers had some kind of design goal in mind and this is how they reached it.
 
#10
Thanks again for the input jerl! So do you have any recommendations for cap values to make the 2 channels the same? I have ordered some .033, .047 and .022 PIO caps as well as some .01's for coupling. Also what resistor would be proper for biasing the eh5's?

Sorry for all the questions, I'm working through all this!
 
#11
First, I hope you're using an isolation transformer? Triad make an inexpensive one that will power this. Part number N-68X is about $15. Will run hot, as this close to its 50VA rating. (58 watts including phono motor).

50EH5 is similar to the 50C5 but has higher transconductance (gain). The two channels are different in gain so that the balance control has some range in both directions - channels are equal with pot centered. But you probably want both channels the same. Use four 50C5 or 50EH5 and use the corresponding bias resistor and 22K feedback feedback resistors. The caps in the feedback loop should be removed if you want a flat response. Jumper in place of C9, C13, clip out C10, C14. They were used to EQ the cartridge, simple tone controls, speakers in they original console. Instead of making each section "flat", they adjusted the amp's response to compensate for the rest.
 
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#12
Thanks Tom! I have an N68X on the way as well as some other components plus 4 tested 50EH5's.

I am also replacing the electrolytics in the Multi cap with individual caps. I'm waiting for them to arrive. I added some jumpers where the multi cap was installed:

An externally hosted image should be here but it was not working when we last tested it.


An externally hosted image should be here but it was not working when we last tested it.


I guess with using all 50EH5's then R23 and R34 should be 47 ohms for proper bias?
 
#13
Thank you Mr. Bavis 🙂 I removed the feedback loop caps and changed the resistor and it made a WORLD of difference!

I got the multi cap replacements installed without needing a fire extinguisher as well 😀

An externally hosted image should be here but it was not working when we last tested it.


I'm now looking into a volume circuit and investigating input resistors. Any input?

Thanks!
 
#14
I'm now looking into a volume circuit and investigating input resistors. Any input?
Click to expand...

The O/P "iron" is on the puny side. Use high pass filtration at the I/Ps to prevent core saturation. Follow the example shown in "El Cheapo". Commercial CDPs must be able to drive 10,000 Ω loads, hence the 10,000 Ω control. The 0.056 μF. part shown "corners" at approx. 28 Hz. Decreasing the value of the cap. increases the "corner" freq. and vice/versa.

Oh yeah, Xicon brand 1% metal film resistors from Mouser are quite adequate in the 100,000 Ω grid to ground role. If you want better quality, go for Vishay/Dale RN65 series parts. Please avoid ceramic caps. in the signal path.
 

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#16
Here's where I'm at in case anyone else is fooling with one of these:

An externally hosted image should be here but it was not working when we last tested it.


I got the isolation transformer and electrolytics from the multi cap worked out plus I put a full wave rectifier in. The yellow traces show how the heaters were re-routed. I also pulled/bypassed the feedback caps like Tom Bavis suggested. The silver coupling caps are .022uf that are just bodged in for experimenting. The originals were .01uf.

I picked up a set of vintage Coral bookshelf speakers and it is sounding pretty good!
 
#19
Figured I'd update this since it's almost finished.

An externally hosted image should be here but it was not working when we last tested it.


An externally hosted image should be here but it was not working when we last tested it.


I'm still waiting on some .01 PIO caps plus I need to mount a fuse block and figure out the power cord mount. Plus some cooling slots...

The 6e2 tube isn't as sensitive as I'd like but it seems that plenty of folks have battled that one...

https://www.youtube.com/watch?v=4HkBhdErNMI
 
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