Finally

[surfsup]

Funny!

[RJ Guitars]

That is a beautiful build. I will be watching with great interest to see what you need to get the hum/buzz/vibration out of it.

I have found in at least one recent instance that it's really critical to get the center of the transformers on the same center line to eliminate magnetic coupling between the two transformers. In my test case I was using an Edcor power transformer and found the same results with Edcor, Heyboer, and Pacific output transformers. In the "official" chassis drawings, it shows both transformers centered on a line 14.25 inches from one end of the chassis. In my own "headphone" checks on this there is an extremely small window where the crosstalk doesn't occur. In my test case pulling the output transformer off of that center-line allowed magnetic coupling between the two transformers that was audible in the headphones. I tried this with Heyboer, Edcor, and Pacifc output transformers and the results were generally the same in each case. In some cases it never went away 100% but was always minimal right on that center-line location. It would seem that the folks that did the drawing and/or Ken Fischer did their homework in selecting the transformer locations. One interesting observation was that the physical closeness of the transformers made very little difference in reducing hum although the further away the greater the "quiet window" seemed to be. For the record - I have seen some cases where the transformers were not aligned and no hum occurred - YMMV.

Having said all that, I have also seen a random occurrence of physical vibration - sounds like maybe that is what you have here. It's not the first time I've heard of this and I'm aware of it occurring on most all of the major transformer brands. Phil at Heyboer gave me some help on this before. His council was that mechanical vibration is typically due to something that is mechanically loose and coupling with the magnetic fields from the power transformer. One test is to push on the humm band, both bottom and top while its buzzing to see if that's it. If that's not it then it may be a loose lamination plate or keeper. If that seems to be the case then you can try to tighten up the screws on the laminations. If you can find whats buzzing then of course you can tighten things up or use silicon or O'rings to dampen the problem.

Both magnetic coupling and physical vibration will occur immediately on power up - As soon as you hit the "On switch" regardless of what position the "Standby switch" is in. This is going to be different than the hum you can get due to a circuit problem. If you have AC coupling into the circuit from the heaters, you would not hear that until the tubes are hot and amplifying the noise. One other indicator of magnetic coupling is that the hum will typically be at the same volume level and does not go up and down with the amplifier volume.

As I said previously, YMMV.

rj

PS - I have talked to Heyboer a few times about getting those Humm bands tight against the top of the transformer. It hasn't really caused me any problems but I don't like the look of it or the potential for a buzz either way. I will typically pull the end bells off and put some glue under the copper then flatten it down snug against the top of the tranny.

[v846]

Hi RJ,

Thanks it was really fun to build and this might sound nuts but I like the challenge.
I did remove the OT bolts and moved the tranny around again today, the sweet spot is about a 1/8” to the left looking at the back of the amp. That eliminates the coupling hum in the speakers but actually isn’t a big deal as it’s not bad in its original location.
I could have done better in that regard but again it’s a very slight hum in standby where it now resides & not worth hacking the chassis IMO.

I do have hum which I believe is 60 Hz that increases with volume particularly with the bass around noon so it’s getting into the circuit. I did try a hum pot on the heaters with no big benefit so I might try running just the AX7’s on a battery. BTW moving the OT had no bearing on this issue.


I did push in the top band and will take a gander at the bottom one to optimized things as much as possible and try the other damping tips you suggest.

Again it might very well be that I don’t have good tubes in the amp or I have something wrong with my wiring layout I haven’t picked up on yet.

Thanks,
David

[RJ Guitars]

David,

Yeah there are a ton of things happening so you may find more than one thing in the solution and what works for you may or may not work for the next guy. transformers are complex beasts. The cool thing is that the amp sounds good. Impressive build for sure.

rj

[v846]

Applied the 6V battery to the 12AX7 heaters and the hum dropped nicely but I didn’t want to risk my precious Teles & Bugle Boys for the test so I used some old GE & RCA's in V1 & 2.

At this point I’m thinking the heaters must be the culprit, to confirm I wire them back up expecting to hear hum so I fire up the amp, flip the standby and there’s the familiar hiss but lo and behold something is different, where the hell did the hum go?
I had left the GE in V1 & RCA in V2! So the Tele goes back into V1 and hum returns. That old GE 12AX7 I generally use to try out new amps happens to not hum in this amp! Also stable enough, whew..

I had selected 15 of what I thought were the pick of the litter only to find they all exhibit too much hum in this built which is particularly embarrassing considering how many have already stated keep trying more tubes in V1...

Now I must write 100 times V1, V1,V1 ……and find more tubes..

Still need to tackle the vibrating PT chassis thing though, simmering on that one..

That’s enough for today

[MCK]

Great news!!!

[Reeltarded]

hah! Excellent!!

[RJ Guitars]

Congrats - it's always interesting to see how many different variables one of these tube circuits can throw at you... would love to hear some sound clips.

rj

[v846]

Yeah I’m pleased with the result, regarding the magnetic coupling I did give the OT a little bump & twist which minimized the magnetic coupling a bit more, just a slight hum in standby. Also considering putting the heater balance pot back in, there is some hum reduction as the heaters aren’t perfectly balanced.

Found an acceptable Tunggram for V1 but that’s only two good tubes, probably going to raid my other amps for potential candidates.

Be a while for clips; still want to optimize my tube selection.

Thanks much for the assistance getting this amp in line…

David