Parallel 50B5 homemade amp?

knockbill

knockbill

Lunatic Member
I grabbed this little chassis for the PT, but decided to see what it was or if it worked before stripping it,,, looks like its triode strapped, parallel 50B5...Correct??

I copied the schematic from the chassis, and figure it uses a 5Y3 rectifier, and 12--7 driver,,, Needs another 50B5, and an OPT, guessing it needs 2.5K, as that's what the manual calls for, if it's the same for 2 tubes...

It has 4 binding posts instead of jacks,,, looks like an input and grd... and filament and grd (didn't power it up to get readings)... also has a bypass switch, maybe it was intended as a guitar amp?

Any thoughts? the learning continues!!

Regards,
John
 

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Not sure what you've got there but I don't see any output transformers. A type of power supply maybe? What are the front controls?
 
I think its parallel 50B5s triode strapped,,, I think the OPT was taken off, there are holes on the side of the chassis where it could have been,,,
I can hook a SE tx on it to try out,,,

Regards,
John
 
I had a open heater in a 50B5, so I rewired one tube for a 50C5, and got the heaters and B+... the chassis wiring looks like the OPT should go where the two 47ohm resistors are, from B+ to the two plates of the output tubes... But the primary of teh OPT should be 2.5K for that tube, and the resistors are just 100 ohm or so,,,
Sound reasonable?
I'll try it DBT and variac, try to keep teh smoke to a minimum!!

Regards,
John
 
Hooked an OPT and speaker, got a little sound thru it,,, til one of teh can caps gave out,,,everything I've read about changing old caps is true, just a matter of time, sometimes minutes!!!
I'll update teh schem I drew, and shelve it... Couldn't find a schem for a triode strapped, 50B(C)5 parallel SE amp... Think I'll revisit it rather than gut it right now...

Regards,
John
 
That unit is much more likely a regulated power supply. Check the connections to terminal 2 of the power tubes, to be absolutely certain that it doesn't -- or didn't at one time -- connect to one of the binding posts. Also, see if the terminal of the "volume" control that currently goes to a binding post didn't originally maybe go to the output of the B+ supply.

If your drawing is correct, then likely someone before you thought it was an amplifier, and tried to make it work as such as well, and gave up for someone else to have a try at it.

There is no doubt that it could be reworked to operate as such, but significant changes would have to be made. For starters, no amplifier employing push-pull, or even paralleled 50B5s (in either pentode or triode mode) would ever employ a 28.7K cathode resistor.

It could no doubt be made into something neat. But the likelihood is that you will need to rip out whats there, and start fresh if you want it to really become an audio amplifier.

Good luck with it!

Dave
 
Got this thing for a buck,,, just wanted the big, vertical PT, figured it would be useful to save room on some build or project down the road...
But, out of curiosity, I wanted to try it out first...Even with that PT and 6V available, it uses 50B5 tubes, with the heaters wired in series on teh line side, so it is in effect still a hot chassis amp, of sorts... I really don't know what the OP was thinking...
One set of binding posts is tapped from the 6V winding,, the other is input to the driver... The cap is indeed 28.7K I measured it... I think it is pretty cool, and figured maybe a 4-5W guitar amp, or who knows...Didn't gut it yet, maybe it'll become a project, if I don't find a use for the PT first!!

Regards,
John
 
That would make one hell of a nice power transformer for a tube preamp project. You would have all kinds of options for tube choice (plenty of heater current).

Regards,
Gordon.
 
If that PS can put out anything close to 300-350v DC (maybe with a voltage doubler?), that'd be great for a Marantz 7 clone... hard to think of many other things that I'd be inclined to do, over that.

Of course, you'd need to verify you have a 6.3v winding for heaters, or use a separate 6.3v heater transformer, if not... or, you MIGHT be able to use low-loss solid state diodes (low voltage drop, as in Shottkys) and DC rectify the 5v winding for the 5Y3 to get 6v for heaters. That would require a solid-state B+ rectifier (which would be OK, since that's what the Marantz 7 originally used).

Regards,
Gordon.
 
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Just make sure that the transformer has a Faraday Shield on it, or you'll run into all the problems I just did on the HF-81 project! (Or, you can always remote the power supply of course......)

Dave
 
Good thought, and it doesn't... I liked it because it has a small footprint, but large stack,,,
I built a remote PS for teh Heathkit preamps,, really works out well...

But, how about this Maggy? got it from teh same guy for a buck... Hell of a lot better deal then lottery tix!!!!!!!

Regards,
John
 

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If I was building something as complex and sophisticated as a Marantz 7, I would seriously think of a separate power supply chassis.

Regards,
Gordon.
 
knockbill said:
Good thought, and it doesn't... I liked it because it has a small footprint, but large stack,,,
I built a remote PS for teh Heathkit preamps,, really works out well...

But, how about this Maggy? got it from teh same guy for a buck... Hell of a lot better deal then lottery tix!!!!!!!

Regards,
John
Click to expand...

What are those, 5U4s? If so, that's probably a similar PT to that used on the AMP150 or the Concert Grand amps. You could run a 6L6PP or EL34PP stereo amp on that, if so.

Regards,
Gordon.
 
Yep,,, two GE 5U4/5AS4, same date code....
Figured I'd build a bench PS with this, just put the caps and dropping resistors on a different chassis, and connect em with the umbilical,,, can use the variac to dial it in...
But, PP 6L6s sound like a plan, down the road...
I was on a transformer hunt Friday, and did pretty well!!

Regards,
John
 
bit scary to have male pins as the output, unless that is also the 120v input. I had a Williamson 2 chassis amp that someone had mixed up the sockets, and you could actually unplug the umbilical and get some 550 vdc across 2 of those exposed pins. I had always wanted to replace that with an octal socket and loop the 120v power through it to shut down the supply in the event that it got unplugged, but I sold it before I ever got around to doing that.

Twin rectifiers = lots o current capacity. Should have no issues running pretty much anything you can imagine off of that.
 
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