• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

DIY 50C5 mini amp...

nige838

Member
2009-12-12 9:07 pm
Hello all,
Just wanted to share my newest project...

It's tubes are as follows:
35w4
12AT7
12AT7
50c5

It's built into a 6" x 4" x 1" enclosure, quite small...
Sounds amazing, very bluesy, even with a crap 1970s 8" that I dumpster dived for a few weeks ago...ever so slight of a hum, barely noticeable,

All tubes are pulled out of 2 1950s radios,

Total Cost: $35
Tell me what you think...
 

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nige838

Member
2009-12-12 9:07 pm
Well, just to update...

I left it on for 12 hours to make sure that nothing melted and everything is A OK. I'm about to start working on another design that uses the popular 50L6GT and the 12AX7 (Although I may substitute 2 12AT6 instead)

Took it to the pawnshop and they gave me $100, not bad for a $35 investment...:)
 

nige838

Member
2009-12-12 9:07 pm
Yes, I know that I need to add a fuse, maybe 1 Amp? The chassis is grounded to the negative lead of the capacitor, and all the other grounds ground there too...

It doesn't have a bad hum at all, but next time I will run the output transformer perpendicular.

This is my first amp, so any feedback will be more than welcome. One thing that I didn't count on was the heat generated by these tubes. Now I understand why you can't just pack all of them onto a small chassis. Live and learn I guess.
 
Also, should I ground the center tap from the AC in? I used the transfo. backwards to get the dual primaries to run the heaters and the other to run the B+ and B++.

Thanks

HUH? Im talking about grounding the chassis to the green wire of a 3 wire plug.

AKA its a safety thing were if your neutral happens to loose contact with the chassis, your finger doesnt complete the circuit to hot. Thats all ground is, a non current carrying backup to neutral. ALL metal chassis's should have it.

Ya a 1amp fuse is fine.

To de hum you can try the following:

Lift the potential of heaters 60-100v
Convert Heaters to DC
keep signal wires away from power wires
Make all the wires cross at 90 degrees.
 

nige838

Member
2009-12-12 9:07 pm
HUH? Im talking about grounding the chassis to the green wire of a 3 wire plug.

AKA its a safety thing were if your neutral happens to loose contact with the chassis, your finger doesnt complete the circuit to hot. Thats all ground is, a non current carrying backup to neutral. ALL metal chassis's should have it.

Ya a 1amp fuse is fine.

To de hum you can try the following:

Lift the potential of heaters 60-100v
Convert Heaters to DC
keep signal wires away from power wires
Make all the wires cross at 90 degrees.

I only have a 2 prong cord that isn't polarized...I thought that I would be ok if i used a isolation transformer...
 
Which is fine if it is an isolation transformer in a SEPARATED box.

You have raw AC going into a metal box. You do not have isolated AC going into a metal box.

If your transformer develops a short, or the cable frays your in trouble. Want to know worse case? The Hot touches the chassis. Nothing happens, the amp works great. However now the amps chassis has been lifted to 120 volts. So the next person that goes to shut it off, in bare feet, while walking on a concrete floor, gets dropped like a rock.

Isolation transformers are there so that the equipment you're working on doesnt have a potential to ground so you dont get shocked, but they cant be in the same metal chassis.
 

nige838

Member
2009-12-12 9:07 pm
Which is fine if it is an isolation transformer in a SEPARATED box.

You have raw AC going into a metal box. You do not have isolated AC going into a metal box.

If your transformer develops a short, or the cable frays your in trouble. Want to know worse case? The Hot touches the chassis. Nothing happens, the amp works great. However now the amps chassis has been lifted to 120 volts. So the next person that goes to shut it off, in bare feet, while walking on a concrete floor, gets dropped like a rock.

Isolation transformers are there so that the equipment you're working on doesnt have a potential to ground so you dont get shocked, but they cant be in the same metal chassis.

I'm sorry, are you saying that a 3-prong cord is a MUST? If this is the case, perhaps it's worth the extra $5 investment.

Thank you
 

benb

Member
2010-04-24 1:52 am
Hello all,
Just wanted to share my newest project...

It's tubes are as follows:
35w4
12AT7
12AT7
50c5

It's built into a 6" x 4" x 1" enclosure, quite small...
Sounds amazing, very bluesy, even with a crap 1970s 8" that I dumpster dived for a few weeks ago...ever so slight of a hum, barely noticeable,

All tubes are pulled out of 2 1950s radios,

Total Cost: $35
Tell me what you think...
I really think those two smaller tubes look like 12AV6's or 12AT6's. :)

I don't do much with tubes, but I sure do remember what the ones from the old All-American 5 miniature tube radio design look like.
 

nige838

Member
2009-12-12 9:07 pm
OK, I'm going to go by the thrift shops today to look for a cord...

I guess that everything else is OK, just need that cord. Next time I'm going to do a better job grounding too, use a bigger box, and fit a power switch inside...

Thanks for the input. I'm going to try a 50L6GT next, 2 channel, using solid state diodes and a diode dc step up. I'll post the schematic when I'm done with it...