[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