Impromptu 5E3 while I wait for my my ODS chassis

[professormudd]

I came upon a transformer and turret board for a 5e3 at a moving sale, so I went ahead and built it out to pass some time whil I wait for a chassis from Amplified Nation for my ODS build.

I decided to go cakepan and use as many parts I already had on hand to avoid spending too much money towards this amp. I already have a 5e3 as well as aTweedle Dee, so I built this as more of an exercise in assembly and creativity. I'll probably give this away or barter it once I've put a few hours of play on it.

Modifications to the 5e3 include removal of both low inputs, pre-pi master volume, and a high gain channel in place of normal, which also cascades into the bright channel.

[xtian]

I decided to go cakepan and use as many parts I already had on hand to avoid spending too much money towards this amp

Very good! I call using what's on hand a "beaks and feet" build.

The position of our PT and OT makes me think you'll get induced hum. Did you use the headphone trick?

[professormudd]

The position of our PT and OT makes me think you'll get induced hum. Did you use the headphone trick?

Please enlighten me on this trick.

[xtian]

For background, realize that the PT is a big EM radiator at 60Hz, and the OT is an antenna. If you place the two transformers side-by-side, you can end of with a loud hum at the output. However, by positioning the two cores with their axes at 90 degrees, you can avoid most of the hum. The hum we're talking about is audible at the output jack even with NO power tubes installed, as long as PT is energized, so it's a mandatory step in making amps that run quietly at idle.

The best way to position transformers is by setting them on the chassis BEFORE you drill any holes, and listen to the output of the OT with headphones (thus "headphone trick") or an o-scope. Tape off or otherwise secure all PT secondaries for this test so you don't short anything or get shocked. Connect PT primaries to 120vAC. Connect OT secondaries (if you have multiple secondary taps, it doesn't matter which) to your headphones. Listening to the hum in your headphones, you can rotate and move the OT around, and you will learn how the hum changes.

Remember that amp building is a 3D exercise. Sometimes you can find creative solutions by rotating transformers on other axes, or moving small OTs inside the chassis.

[professormudd]

Thank for all of that information. That's a procedure I'll have to use in the future.

When I get back home I'm going to power this on for the first time, so I'm hoping I offset the transformers enough to avoid the hum.

I'll report back with results. Thanks again.

[xtian]

Best to power on first with no tubes installed, preferably with a bulb limiter in-line to prevent anything from burning. When satisfied with that first look, remove the bulb limiter and power up again with no tubes, and evaluate the hum from the speaker.

[professormudd]

Best to power on first with no tubes installed, preferably with a bulb limiter in-line to prevent anything from burning. When satisfied with that first look, remove the bulb limiter and power up again with no tubes, and evaluate the hum from the speaker.

I did as you suggested and I think I lucked out. I did not hear any humming. Hopefully that means the positions of the transformers are fine. I am certainly glad too, as learning that the orientation of the transformers can cause issues after I'd already gotten this far was a bit of a bummer. After work I'll probably pillage the tubes from my other 5e3 and give it a whirl.

Thanks again for the wisdom.

[xtian]

So far, so good!

[professormudd]

I popped some tubes in it and no sound.

So I go checking voltages and it looks good from the power transformer but there is very little voltage at all at the filter caps. Then I finally notice that I mis-wired the rectifier tube. I had the yellow leads from the power transformer shifted one pin in the wrong direction.

I felt dumb, but that is what I get for doing wiring at 03:30 in the morning in a 27 degree (f) garage. I resolved that issue and it seems to be a lucky build. No hums, hisses, or other unwanted sounds, and the amp sounds and acts like a 5e3. So it turned out to be a nice and easy build despite dumb luck being what saved me from the hum from the trannies.

Now to decide the fate of this amp.

Thanks for the wisdom.

[xtian]

Good job!

Picky point for future builds: Get some rubber grommets for the holes that the OT and PT wires pass through. Given enough rubbing, the sheet metal will cut through the insulation.

[professormudd]

Good job!

Picky point for future builds: Get some rubber grommets for the holes that the OT and PT wires pass through. Given enough rubbing, the sheet metal will cut through the insulation.

Noted. Thanks again.

[professormudd]

Hastily recorded YouTube demo: https://youtu.be/FT2bX2ty-kw

[thetragichero]

you can pick up a multi pack of rubber grommets (various sizes) for like three bucks at home depot. i try to find as many things locally and at hardware/surplus stores (I've been known to secure radial electro caps to the chassis with conduit clamps)