[TV MAN]

Construction has started! So far, I've gotten the power supply done and filament wiring run. The major layout is finished, have a few sketches of where things should go. I've made a few changes and moved things around. Hopefully I can start on the high signal parts, then low signal stuff. Also have the bias supply together. Anyway, here's a pic of the progress so far.
   

[morzh]

It's getting along!

[radiorich]

Hello Tim,
It looks very nice so far !

Sincerely Richard

[TV MAN]

I got things pretty much in place, double checked, waited a couple days, did some resistance checks and powered up through a variac and dim bulb. B+ seemed pretty close to what I wanted so I brought it up enough to get something. Yup, I could get a signal if I touched the input, so I plugged in a CD player and ... AND.../// it sounded .... AWFUL! Almost no volume and was badly distorted. Did a few voltage checks and plate voltage was really high on the inverter. I could hit either side of the coupling caps with a signal and it sounded clean, hit the grid of the inverter and - nothing. It was dead. Tried another tube, same. The inverter was dead. So I studied the diagram (that I drew), everything looked ok, that is, until I rechecked the driver tube diagram. OooOooohhhh - wrong! I had the triode grid and cathode pins reversed. Well, that'd do it. I have a feeling I just looked at the wrong pinout and drew it wrong. 

So I switched it tonight and brought it back up - and ... AND ... SQUEEEEAAAALLLLL SQUEAL SQUEAL! Well, shucks. Been there, so I flipped the ground and feedback and it came on quite well. So, yeah, oops! So anyway, some of the same stuff I ran into before. I don't know if flipping the plate leads would change it or if the tranny is backwards. Anyway, I haven't really done a thorough sound check, but I did note it has the same lousy 60 cycle hum as my last kit. Not bad, just kinda ... there. So I know it isn't a filtering issue. Gotta either be coming the heater or ground loop. Since this is an experiment, I'm going to try a few things. I set this up so I can isolate the ground with a bus I may try some different heater arrangements, like DC, or maybe a hum balance. But all in all, the fact that it works is an accomplishment!  And, no smoke!

   

[morzh]

Tim

It's be interesting if you put your schematic here.
Who knows, you could get some advice from a few folks here.

That yellow tubular cap may sag over time. Long leads, perpendicular to the axis with reasonably heavy part.

[TV MAN]

Well, this was a real "Hum-Dinger" ! This will be kinda long, but hopefully it will help others having the same problem I was. Yes, I DID find the culprit - AND the solution! So, this should be some good reading. I really ought to start a separate thread on this, and may do so. 

Ok, so here we go. In my past, I've built a couple of the Chi-Fi tube amp kits. I have a couple or 3 threads here on these. While the kits seemed to work ok, there were ALWAYS hum issues! I could usually reduce it through some tube changes and/or wiring changes. I made some mistakes, like not grounding the filament center taps or one side of the filament circuit, as in my 1st kit build. One I had trouble with a 60 cycle hum/buzz was a push pull kit using overseas 6BQ5 type tubes and basically a 6GH8 driver/phase inverter. I could NEVER qet that annoying hum out, trying different grounds, thinking it might be a ground loop problem. It was NOT a filtering issue as it was not a 120 cycle hum, but more of a type of hum/buzz you would get from a pa system mic or guitar pickup. I tried shielding tubes to no avail, subbing different types. Yes, in my 1st build, I DID have some HK leakage on one of the preamp tubes, and replaced them with a good USA made set and pretty well took care of the rest of the hum.

Well, in my PP amp, I never did get the hum issue resolved. I can still hear the 60cycle hum in my speakers sitting across the room, which has made me dislike this amp. To clarify things - this amp would produce a hum in a speaker - even with the tubes pulled! It used a PCB circuit, so I always thought the fault lay in that board. I couldn't move things around, heater wiring, etc. I had honestly thought of throwing the board away and use point to point to redo. I even disconnected the B+ and powered it from my bench power supply, still had the hum. I killed grids if the output tubes and grounded the drivers to no avail. Anytime I had power applied, it would hum! DC filament supply? Nope, still hummed. So, it was what it was.

In my disgust with this amp (and also to fulfill something on my "bucket list"), I started this amp build. A simple monoblock (X2), using largely my own design. Yea, a pretty common old school design, but the old stuff worked well and sounded great (most of it). After carefully laying out, studying some of the greats, like Dynaco MK3, Eico HF-22 (thanks Mike), and some others, I started the build. When I finished and began testing, I found this amp had the EXACT SAME HUM that my previous amp had! EXACTLY!  So - HOW??? WHY? I could hear the hum across the room during a silence. No, it didn't affect the sound when playing music like a bad filter cap. Now, I know full well that ALL tube amps will exhibit "some" hum, except maybe the really high end stuff like the Macs. But you still shouldn't be able to hear it from your speakers across the room, right? 

I decided that I am going to find the cause of this hum if I have to rip that thing apart wire by wires! It too would hum without even having any tubes installed. I thought maybe it was the bridge making switching noises or defective, - out it came! Still had the hum. Ok, must be the filament wiring. Snip snip, off it came. Still had the hum! Tried changing chassis ground, line cord ground, cleaning connections on terminal strip grounds, even the transformer bolts. Alright, I think I know what it is. I failed to twist the input wires from the power plug to the power trans. Snip snip, twist twist, solder. The hum was STILL there! What the ****! HOW? I could actually connect my headphones to the speaker winding on a separate output transformer and hear that hum when I would hold the tranny close to the chassis! No, not near the power trans, but the chassis - and it was WORSE farther away from the power trans! As a test, I connected the amp to my bench power supply and it would be perfect in that time between flipping the power switch off and before the tubes cooled off. As soon as I flipped the power switch, the hum was back. I finally loosened the bolts that held the power trans to the chassis and pulled it away - and the HUM WENT AWAY. Bad trans - right? So I tried another, the hum came back. Well, maybe the trans bolts are loose or not grounding to the chassis. So I took things apart and cleaned all the varnish, paint and everything from the bolt area and put back on. Still had the hum. Worse even. I found the tighter I put the bolts, the worse it was. By slightly loosening the bolts, the hum seemed to diminish. Hmmm...  So, I did the opposite, and SPACED the power trans, and found the hum greatly diminished. After looking at a several other amps that used the same design transformer I noted that they all had the nuts between the trans and chassis. So, I did the same. Yea, the hum was diminished, but the tighter I put the nuts, the more the hum came back. So, I found some thin plastic poly spacers and used those between the chassis and nuts on both top and bottom. Hum was diminished! I could still hear a little, but very little. I went a step further and made a spacer out of aluminum and using longer bolts, put the spacer between the bottom of the trans and nut, then used the poly washers between the nuts and chassis and COMPLETLEY eliminated the hum!  

Again, this amp build is an "experiment" and learning experience, but is tremendous in helping to solve problems with other amps, builds. I really hope this helps someone fighting the same problem. I did a LOT of experiments around this, which I will start another thread to share those, since this is an amp build thread. This really puzzled me, since this chassis is stainless (non magnetic), but it sure carried the hum from that transformer throughout the chassis! Then with my design using a pentode driver, was amplifying it! Possibly though the feedback circuit. Yes, I tried clipping a lead from the trans bolt to the chassis to see if the hum would return, it didn't, so it has to be magnetic somehow! I've since reverted to using only the poly washers and restored the wiring to test. It's totally quiet! Even with my ear to the speaker, no hum whatsoever. I'm going to do this to my PP amp kit and see if I get the same results. Sorry for the "story" like long post, but this is how I express myself.  Now, I am at the point that I can start experimenting with the amp and share my results.

Mike, I am still working the schematic. I have a sketch, but have changed several things, and will do more of that until I'm satisfied. Anyway, stay tuned. More to come.

In this 1st pic, you will note how I had the power trans mounted. Just like the kit had pictured.
   
The poly washers I used. Came from hardware store.
   
Aluminum spacers I made for the trans, but didn't use.
   
This is the final install using the poly washers
   

[radiorich]

Tim,
nice write up and thanks for sharing !!

Sincerely Richard

[morzh]

Tim

Would mounting the transformer vertically change the hum, if used without additional parts?

Also, in these amps, Macs, Eicos, they use outer shells for their transformers.
So do all the Philco radios.

Of course, mechanical vibration, couple with various capacitive/inductive/EMI microphonics, also could result in humming.

[RodB]

And so does a barber shop quartet. mmmmmm

[TV MAN]

We need to start a singing radio group! I play the guitar, so maybe get a few vocals? Anyways, Mike, I did try mounting the trans vertically, and even a Thordarson vertical replacement I had on hand and it still transmitted some hum. Not as bad, but it really seems those poly spacers did the trick. There's something about contact that makes this happen. What's so weird is that this chassis is non magnetic. A magnet wont stick to it. It is super hard metal too. So how can that transmit this hum? I mean, if it's strong enough that I could pick up the hum holding an output tranny with my hand and putting it near the chassis, it would pick it up! Of course it would pick it up if I held it close to the power trans, but it was actually strongest if I held the tranny near the center of the chassis - right by the preamp/driver area! Oh, grounding the power trans didn't change anything, nor clipping that green mystery lead to any part of the chassis.

I'm prepping a little more powerful amp build using a similar SS chassis, but decided to go with a vertical power trans on this one. Oh for what this is worth, the hum seems to pick up more with a smaller output trans verses a larger, higher wattage transformer. I'll do a more thorough write up in a new thread once I get this figured out, but am a little puzzled that I haven't found any write ups on the 'net - anywhere. I did even experiment setting a power trans and output trans on a plain piece of steel connected to my headphones, and was quite surprised at how the two interacted. It wasn't what I would have expected and may be able to share some valuable information when it comes to home built amps.

[RodB]

Mike, Tim; you are about to enter the twilight zone. Doo doo doo doo, doo doo doo doo
This is something alright. I'm going to ask my grandson if the amp he built has any hum and if so how he might get rid of it.

[TV MAN]

I expected a challenge and this is just part of it. But, it's also supposed to be fun, entertaining, informative and enjoyable. It's an experiment, using today's readily available, reasonable priced parts than will work good, look nice and last awhile. Once I get it working and tuned in, I'm going to play with things and see the results, maybe some tube rolling, bias changes, even output tranny changes. Got several sitting around.

[morzh]

Tim

Well...a chassis is metal. Magnetic or not, it does not matter.
A transformer has some hum to it. It is natural. There are eddy currents, 120Hz vibrations etc, and so any power transformer will have some mechanical vibrations to it. As well as some leaking magnetic field, which bulges out just a tad out of the core.
The vibrations will make the chassis vibrate. Also the leaking field will create the eddies in the chassis (because it is conductive), and the latter will interact with that field by additionally vibrating.
Some of it (directly coupled mechanically transmitted noise) could be mitigated by what you did. Or grommets. Some cannot. With some the bell (encasing, as we see in many Hi-Fi amps) will help.
Then the chassis (metal, vibrating) will create some capacitive microphonics with something else.

I am not sure I understood this: are you saying that you pulled all the output tubes, and the speaker emitted the hum?
If yes, was the speaker connected to the output transformer, and was the latter powered (through primary centertap, I guess)?

[TV MAN]

It must be eddy currents or something. I'm surprised how the placement of transformers in proximity to each other affect each other. I didn't have to connect anything at all except power to the power transformer and head phones to the secondary of the output tranny. Moving or turning either transformer even 1/8" makes a huge difference. I have the amp working pretty good now and am starting to do some experiments with different bias, screen voltages, tube types. I have a pair of older RCA speakers that one sounds pretty good but tweeters on the other are dead. Probably a bad crossover or something. It has NO audible hum or any other noise with my changes in a quiet room. I'm getting ready to build the twin this weekend so I can have a baseline to directkly compare to. It seems the more I use it, the better it sounds.

Edit: Mike, when I did the test, I had only the primary of the power trans connected to 120v, then only the secondary of the output tranny to my headphones. Every other wire from both trannys were loose (not connected). I could hold the output tranny in my hand and move it around the chassis and hear the changes. Now, how much hum is too much? Well, I could still hear a very slight hum/buzz in my headphones but with the final assemble, the amp is completely silent.

[morzh]

I am not a transformer design specialist, so...I would look into how a known good transformer is designed.

Actually, in the pulse transformers, to kill the leakage field, they wrap a turn of foil around the transformer (not around the bobbin, but the whole thing): this will act as a shorted turn, but only for the leakage field.
Of course, this is a different animal.

But as I remember, even low-frequency transformer, if designed for high quality applications, ain't that straightforward.
They use sectioning, and interleaving, to minimize and optimize stray parameters.
There was a good book I saw at my ex-boss' library, by some old expert, an Army colonel, actually. Large book, very detailed.
Many things in it, other than "wind your primary and your secondary"

I also suspect that by making some strategic cuts in metal of the chassis around the transformer, one could possibly localize the eddies.
But this would require lots of experimentation, to prove or disprove any of the theories.
To me, the best way is to try to analyze the existing successful designs.