micro 5E3 build

[Beezerboy]

I'm interested in building the small watt Micro 5E3M that Rob has on his site. He has a lot of stuff about it and several variations. One variant has a toroidal power transformer. I would love to try that tranny but it's in the most complex version. its essentially a full on 5E3 Delux with smaller stuff. I want to stick with the simpler, 2 tube version. the trouble is the voltages.... the toroidal tranny makes 200 volts. Its about 290-300 after rectification. the first power node on the 5E3M goes to the output tranny and is supposed to be 390. is that gonna work? well, I know it will work but will it be crap? I really don't want to build the push/pull version.

[bmccowan]

I have never built one of the micro amps. But I've researched them a bit with the intent to try one. I think many 12AU7 designs use the voltage range that you would get. I would search the Forum threads for discussion on micro, 1 watt, firefly etc. and see what you come up with. And reach out to Rob Rob for his thoughts.

[DummyLoad]

At first glance, what stands out for me: That pre-amp out as drawn could smoke some SS gear. May want to revisit plan with a divider network and possibly a level control as well.

Better yet, move the pre-amp out tap to the OT secondary.

Respectfully,



--Pete

[roarshock]

Antek has many toriodal transformers to choose from.  You can get a PT with a 250 or a 300v secondary for $32 plus shipping. That should fix your power problem. Hope that helps! Sidenote: I built this circuit as my first amp and it rips! Have fun!

[Beezerboy]

ha ha,,,, yeah, but I already got the tranny. I didn't look that close at it.   it's not a 200-0-200.... it's just 200. its actually for a completely different amp that is a shrunken push/pull, 4 input version of the full sized 5E3

thinkin' that a different tranny is in order. not sure what to do with this one... may have to build that other things some day.

and Mr Dummyload...   that part about smokin the SS gear... tell me more?

At first glance, what stands out for me: That pre-amp out as drawn could smoke some SS gear. 

Better yet, move the pre-amp out tap to the OT secondary.

Respectfully,

--Pete

[shooter]

tell me more

what bothers me, when you jack into the pre out, the PA grid floats with no reference to ground, will the world end, not sure, i just don't like a PA grid left to it's own devices

[Beezerboy]

forgot to add that there will be no preamp out or headphone jack. I wasn't even thinking about effects aa pre out might have. yeah, I can see where that might be a thing

[PRR]

> it's just 200

Through a full-wave Bridge to a good cap that's probably 270V of DC which is close to the 12AU7's safe zone. Yes, it may survive 390V for a while but is it really necessary?

[Beezerboy]

yeah thanks! I tested it with a diode bridge and a 20uf... the open circuit voltage was just under 300. I've pretty much decided to use it. the question is which dropping resistors to use. looking at the diagram above there seems to be no point in using the first one, the rectifier can go straight to B+1. Somewhere on Rob's site he talks about lowering the value of the B+2's 27k dropping resistor to 22K.  should work right?

[shooter]

the dropping R also forms and RC network with the filter cap, by taping the PA straight to the cap you might get some sag. if/when the PA shorts the R acts as the 1st line of defense against the PT/rectifier taking all the hit.

[bmccowan]

I'm glad you are moving ahead with that - as I'm interested to see how it goes, and I expect it'll go fine. With B+2 only feeding 1 12Ay7, at that amperage I do not think you would see much voltage difference between 22K or 27K resistor. I think either would be fine.

[Beezerboy]

it might take a while... the summer here is short. I have a cab from an old Singer sewing machine that was my mothers'... after I cut, shape it, and recover it, that will be the cab.  I'll make my own chassis out of 2024 T3... I've done it 4 times already & know what works for me. got a 10" Warehouse Veteran for a speaker....  have a few more parts to scavenge yet, but its underway