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Eico HF12 Hum

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I have finished restoring a pair of Eico HF12s. I plan to use them as mono blocks, i.e., use the Tape Out jack as a Preamp In. However, I restored them so they can also be used as stand alone mono amps, if desired.

As part of the restoration I replaced all of the resistors and capacitors, except the ceramic capacitors. I replaced wiring and tube sockets as needed, but kept most of the original sockets and wiring. I added a three-prong grounded power cord and a fuse. I replaced all of the rca and speaker jacks. I installed new JJ tubes. I installed a new bulb. I also installed a new "slide switch" style on-off switch, i.e., I moved the power switch off the treble tone pot. After bringing them up on a variac, I tested the voltages and resistance on both units. Everything tested within range.

I tested the units using a CD player run through the B1 NuTube preamp I recently built. (Love the preamp, by the way. 6L6's build guide and all of the great info I found here was very helpful in completing the build.) The units sound great. One is dead silent. The other has a mild 60hz hum. You cannot hear it when music is playing. In fact, you have to be within a few feet of the speaker to hear it at all. I could probably live with that. However, given how nice the units turned out, and the fact that the one is silent, I would like to try and address the hum.

Here is more info about the hum:

1. The hum occurs when the preamp is connected through the Tape Out/Preamp In jack. There is no hum when nothing is connected. There is no hum when a component is connected and run through one of the input jacks, i.e., when things are running through the onboard preamp.
2. The hum fades in upon startup after the tubes have warmed up.
3. The hum is not affected by the volume knob or any other knob on the unit, or on the preamp when I use the unit as a mono block.

I have taken the following troubleshooting steps:

1. Swapped tubes in and out with the quiet. It does not appear to be a tube issue.
2. Poked around with a non-conducted probe to check for loose connections, bad solder joints, or trouble spots. None found.
3. Checked the ground connection points to make sure they are tight.
4. Tried plugging the unit into various different outlets and power strips.
5. Tested the unit with and without the metal cage and the shield on V1.
6. Tried a number of different cables for connecting the unit to the preamp. Shorter cables produced less hum. Of course, the shorter cables were also shielded, whereas I am not sure if the longer cables were shielded. So, I am not sure if the issue was cable length or shielding.
7. Connected my iPhone directly to the Tape Out/Preamp In jack. No hum.
8. Connected the preamp to the quiet unit to confirm it is quiet, then switched the same cable to the unit with the hum. The hum was there.
9. Spent time looking at the undersides of the two units to see if I could spot any obvious differences that might explain the hum. I will discuss one of my discoveries below. In addition, I did see some component leads that were dressed on the quiet unit, but not on the one with the hum, and visa versa. Poking around the undressed leads did not impact the hum.

When comparing the wiring of the two units, I did find what appears to be a wiring error on the unit with the hum. This gets a bit into the weeds. However, I think it is worth mentioning. It has to do with the wiring of the V4 and V5 tubes, the EL84 power tubes. There are three wires, blue, red, and yellow/brown, that run between these tubes to the output transformer. I noticed that the blue and yellow/brown wires were reversed on the two units. The quiet unit had the blue wire connected to pin 7 of V5, and the yellow/brown wire connected to pin 7 of V4. That is what the build instructions say. The two wires were reversed on the unit with the hum. I thought, AHA! I reversed the wires on the unit with the hum. Apparently, that was not a good thing, because when I fired it back up it emitted a loud shrieking noise, like feedback or microphonics. I was prepared to quickly power down, and did so. Not sure what that was all about. I have not had the chance yet to switch the wires back, fire it up again, and make sure things are still ok. I cannot explain the reversed wires. I wonder if there were different versions of the circuitry and/or build instructions.

So, at this point, I think I can say that the problem is not the external preamp or the cable, since they work fine with the quiet unit. It is not the unit generally, since a device run through the onboard preamp does not produce a hum. The problem must be occurring after the preamp stage, in the Tape Out/Preamp In circuit. Not sure what to think about the reversed wires on the V4 and V5 tubes. Hope I didn't fry anything. Not exactly sure what to do next in terms of troubleshooting. I am an enthusiastic hobbyist, but certainly no tech. Your thoughts and suggestions are appreciated.
I sorted it out. It ended up being one of two things (or maybe both, I did not test in between). First, I noticed that I had run the wires a little differently when installing the new power cord and fuse. I corrected this to make sure everything on the humming unit was the same as the quiet unit. While doing this, I found there was a problem with the V6 tube socket for the EZ81 rectifier tube. I swapped it out with a new one. The hum is now almost entirely gone. If you put your ear right next to the speaker, you can still hear just a little bit of it in the background, which is good enough for me.

When I have restored units like this in the past, I have usually stripped them all the way down, and installed all new wiring, tube sockets, etc. In this case, I thought I would save some time by keeping the original wiring and tube sockets. Next time, I think I will go back to the full strip down and rewire approach. I will take some photos of the finished units and post them. I will now be turning my attention to the HF85 that will complete the setup.
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