5E3 Tweed hum induced from PT to OT

[1rebmem]

Just finished up a 5E3 build (my first 5E3). Used a Hoffman chassis, mojotone OT and a NOS Triad PT that is a perfect fit into the chassis.
I have a relativity low hum from the speaker with NO tubes installed. This hum is louder than my previous amp builds and could almost be considered acceptable I suppose.

When I dismount the OT and move it around, the hum changes dramatically of course and actually disappears with the OT rotated 45 degrees and one mounting ear lifted about an inch off the chassis, or something like that.  Not a permanent mounting config.

Wondering if this hum is normal with this chassis layout?
If not, could the PT be the cause? And if so what is a recommended PT?

Any thoughts/advice?
Thanks.

[eleventeen]

These things can be fairly sensitive to relative orientation between the PT & the OT. You'll note, if you've looked at numbers of them, that virtually *all* hi-fi tube amps have the laminations of the PT and OT oriented 90 degrees apart. This was for anti-hum reasons, of course. Is either tranny open-frame, eg; not enclosed in a metal case? Most Triad low-wattage output trannies I can recall are that way, though most Triad PTs are enclosed. I have numbers of those open-frame output trannies (or similar) in the junkbox that are destined for small amp builds but I am generally reluctant to use open-frames for just this reason.

The short answer is, "do whatcha gotta do". You've apparently located the source of your hum, and so, you know that *nothing else* will cure it. Sometimes hum can be reduced by placing a grounded metal shield between the PT & OT, or, enclosing the OT in a metal box of some kind. I appreciate that you'd like neither since additional drilling is required, but as I said, you've found the problem and demonstrated the cure to yourself so thinking that it's something else is kind of fooling yourself. Perhaps it would be possible to buy an aluminum LMB-type metal box of the type used for small projects with the two "U" shaped sections that fit together to form a 6-sided enclosure and mount same using the same holes used by your OT so that no add'l drilling would be required.  

Incidentally, the one absolute test for this is to substitute a SS rectifier. If the hum starts immediately on turn-on, eg; with no rectifier tube warmup, that's the culprit, no questions asked.

[1rebmem]

Eleventeen........thanks for the thoughts and ideas. Good logical thinking!!
I was aware of transformer lamination orientation concerns. I assumed the 5E3 transformer layout would have been quieter.
On scratch builds, I do the headphone test for trans placement.

The OT is open frame so the layout looks like this: http://www.tweeddeluxe.com/builds/1960-ftdr-04252010/photos/1960-ftdr-04252010%20020.jpg

So you think shielding the OT with a LMB box would help?
I may give that a try by rigging it to mount using the existing screw holes as you suggest.
Interesting about the open v.s. enclosed frame OT concept.

Have other 5E3 builders/owners experienced low level hum that couldn't be tracked down or was demonstrated to be induced?
 

[Raybob]

Best to have the transformers mounted so that the coils run at 90 degrees to one another, for least hum.

[eleventeen]

That (your pix) is pretty close PT <> OT, in my book. Even if a stock 5E3 tranny is open-frame, it may just be that the particular one you have is more (incurably) sensitive. The other approach is to fabricate some kind of adapter plate of 1/4" or 3/8" aluminum that would allow you to orient the OT at whatever favorable angle you determine. Two screws would mount through the existing tranny holes, one through the plate; one through the plate and one ear of the OT. You find another hole further down the chassis you can machine-screw through (you'd probably want at least 3 screws through any such adapter plate) and then you swing the OT one way or another to defeat the hum. You then mark the odd tranny-mount hole and either drill and tap it or counterbore or countersink (from the underside) a hole at that location so you can have a machine screw "threads up" and mount the odd ear of the OT with a nut and lockwasher. Not the prettiest thing in the world but you'll never see it once installed.

I note the "depth" of a Hoffman 5E3 chassis is 4-1/8". I am thinking of a square plate, 4" x 4" or maybe 4" x 5". I would suspect the LMB box would work and be less work (unless you have reasonable machine tools at your disposal) but of course you never know until you buy it and try it. If you kill the hum by swinging the OT away at an odd angle, you at least know you have the solution in hand even if the fab process is possibly more tedious than hacking up a < $10 chassis box. 

Or you could just blow the money for a metal-enclosed OT.

[1rebmem]

What I've done is unsoldered the OT and set it on the chassis in it's original location. Then I connected the secondary wires to a speaker and left the primary wires disconnected. Powered up the PT and listened to the hum. I had hum unless I moved the OT to the far end of the chassis (preamp end). 

When I re-oriented the OT from this: http://store.triodestore.com/5e3twdeoutr.html
To this: http://store.triodestore.com/40-18090.html  The hum was 99% gone even when located one inch from the PT like the original mounting had it.
So I need my WINDINGS re-oriented to eliminate the hum.
I have some end bells that fit perfectly so I removed the 2 screw mounting bracket and installed the end bells to look like this: http://store.triodestore.com/outrforhotro.html

I'll just position it for least hum and drill four holes. I'll trade quiet for stock looking.

I will try the hum test again, with and without the end bells to see if they provide any significant shielding, and report back.

[1rebmem]

Best to have the transformers mounted so that the coils run at 90 degrees to one another, for least hum.

As raybob said..... the coils need to be at 90 degrees.
In my case, the end bells being on or off didn't make any difference either, hum was the same.  I'll mount it using the end bells because I have them but 4 "L" brackets would be just as good.

Seems to be that the original 5E3 Tweed design is flawed in this respect. I have always read that they were noisy amps but I figured with simple but proper/modern grounding schemes it would be fine........... 

[sluckey]

Seems to be that the original 5E3 Tweed design is flawed in this respect.

I don't have any problems with mine and the transformers are mounted just like the original.

[1rebmem]

Seems to be that the original 5E3 Tweed design is flawed in this respect.

I don't have any problems with mine and the transformers are mounted just like the original.

Sluckey......so what is my problem (with the amp...lol)?
I gotta wonder if the PT is my issue. It is a NOS Triad. Ever heard of a given PT generating more hum than another?

What PT and OT does yours have in it??

With my OT re-orientated the amp is dead quiet aside for slight hiss with volumes full on.

[sluckey]

I don't know that you have a problem. You said "I have a relativity low hum from the speaker with NO tubes installed." Is the hum objectionable with the amp actually working?

My transformers came from a Deluxe Reverb Reissue (DRRI).

With my OT re-orientated the amp is dead quiet aside for slight hiss with volumes full on.

Well, there you go. Live long and prosper.   

[eleventeen]

It's entirely possible that two apparently similar transformers, particularly open-frame style, might be wound differently and have differences in their susceptibility to hum.

[1rebmem]

Thanks guy's.
The hum is not objectionable.......till I stop playing.

My curiosity is killing me since I plan to build another 5E3 sometime with a $30 mojotone OT I recently picked up.

[sluckey]

My curiosity is killing me since I plan to build another 5E3 sometime with a $30 mojotone OT I recently picked up.

I would temporarily install that OT in the existing 5E3. How does it sound?

[eleventeen]

"I would temporarily install that OT in the existing 5E3. How does it sound?"

There's the good idea of the day.

[1rebmem]

"I would temporarily install that OT in the existing 5E3. How does it sound?"

There's the good idea of the day.

Great idea...but I've re-oriented, mounted and wired the OT. I still could try the spare OT but the mounted one is in the stock location.
See what I can do............. 

[Willabe]

Only 5 wires and 2 bolts?

 
             Brad     

[1rebmem]

OK............tried the other OT.
Same exact result.
Suggests the PT is the cause..........