As a kid, I started working with vacuum tube audio in the 1950s' and never really stopped. After all these years, I have a weird 5U4 that baffles me. I seem to have a 5U4 rectifier that puts out a brief hum just as the filament starts to glow, but before any DC is produced. It is a PSVANE classic series 5U4 I bought for the classic look. I have never had a rectifier tube produce a brief hum before any DC output appears. How it gets to the speaker before any tubes conduct baffles me. It is a very low level hum barely audible. If I plug in another 5U4, there is no brief hum before DC output. Both output transformers are three inches away from the power transformer. I doubt there is any inductive coupling going on between the transformers. The 5 volt filament winding does have a center tap from which I pull the DC output. Filter cap at the 5U4 is 33uF at 500V. After a 6HY choke there is a 100uF at 500V cap. After the choke, there are 120 ohm power resistors that feed each channel B+ with a 470uF at 500 volt cap. None of that should matter since I can plug in a different 5U4 and no brief hum. As far as I can tell, the PSVANE 5U4 is doing no harm so I put it back in.
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That cap is most likely a little on the big side for your Psvane tube. You could drop it down a bit to 20uf, or use a 5AR4 instead, or do the yellow sheet mod that @rayma suggests.
jeff
After thinking about it, I decided there was no way any hum could come out of the speakers before the output tubes are conducting. This morning, I pulled the output tubes and the brief hum was still there. It's the power transformer that hums. The speakers are in close proximity to the amplifier and fooled me into thinking the hum was coming out of the speakers. The funny thing is that I kept thinking, this sounds like an old radio with a field coil in the speaker.
I have bad news for you. The Hammond 200 series power trannies have a habit of vibrating. I had to replace two of them in a pair of monoblocks I built for a friend. I will not use them anymore. If you want a good replacement at the same price as the 300 series, here's a very similar 760V CT @ 200mA tranny I had made by Heyboer. 5V @ 3A, 6V @ 6A. They run cool and quiet in my PP monoblocks. About $150 each.
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I wonder if using a silicon sheet under the transformer would help, like the ones that Edcor sells?
I noticed the 200 series are wound with 115v primaries. If your line voltage is 125v, that's not gonna help either.
jeff
The 200 series all now have 125VAC primary taps. The noise was coming from the windings. It wasn't DC offset either. Nothing we did would get rid of it. My friend has very acute hearing and could hear it across the room. I replaced them with Heyboers. They're sitting in the shed now.
This is not a continuous hum and it does not do it with a different 5U4 plugged in. But you are right, I can feel the transformer vibrate for that brief moment on warm-up. Not sure why I bought that transformer, and I actually have two of them. Probably planned projects never started. These do have the 115/125 tap. I once had bad luck with Hammond transformers shipped with messed up covers. They did send me new covers, but they really should not have been assembled or shipped like that. I've also had issues with Hammond chassis bottom plates that sometimes are slightly too big for the chassis. The edges on the bottom plate sticking out looks sloppy. I'm leaving things as is. If the vibration ever gets continuous, I'll deal with it then.
Good to know, thanks.
So the issue is really the Psvane tube, and not the transformer. I had a similar experience with my old HK El84 amp. I decided to try an old stock 5U4GA tube in it, instead of the stock 5AR4. The PT really didn't like that tube, and hummed quite loudly at startup. So I put the old 5AR4 back in.
jeff
There is another thing about this Hammond transformer. Why did they run the high voltage secondary wires out the same side as the AC primary wires? It created a quandary for me because I wanted the primary wires near the back of the chassis closer to the AC input and fuse. But that places the high voltage wires quite a ways from the rectifier. Not well planned. Thank goodness, they used long wires. The high voltage wires just made it to the rectifier.
This amplifier has been running daily since I finished it. It sounds great as is. However, I have a couple of notes on the build and mod in mind to try. The amplifier was actually built as a design and assembly example for my current version of Vacuum Tube Amplifier Basics. I used on-hand output transformers that were targeted for another project. Although the design is based on 6L6 output tubes, the output transformers have 3,500-ohm primaries (what I had on hand). The amplifier sounds fine on speakers. When the speakers are unplugged and high-quality headphones are used, the amplifier's sound is fantastic. I suspect that the higher load resistance placed on the output transformer's eight-ohm output increases the transformer's primary impedance quite a bit higher. Without the load variations of connected speakers and a lighter plate load, output tube performance is probably improved. At some point, I'll try a pair of KT66 output tubes. The output transformers should be a better match for the KT66. The mod I am going to try is to add negative feedback from the output transformer's 8-ohm output back to the stage before the output tube. Beforehand, I will do a quick frequency response test with speakers connected and again after negative feedback is in place. I'll be particularly interested in low-end performance.
I can see two plausible reasons for the hum. The first is initial arcing within that 5U4 that causes excessive peak winding currents. I suggest that is not a matter to leave for another day, and as rayma suggests do the series ss diode mod as that should give immediate awareness if that was the root cause of your problem, as well as avoid some future rectifier tube problems and possible collateral damage to the power transformer.
The other reason could be a very low cold resistance of the JJ filament - you could quickly see if there was anything obvious between the 5U4's from a filament resistance measurement.