Trainwreck Express build, PT hum on and off standby

[LarryLarry]

Howdy - I finally got around to building a Trainwreck Express using a huge PT I got off of eBay and a Weber WOT45LHR OT. The amp sounds wonderful and just working out the final kinks. The worst issue is a noticable hum for the PT with the standby off (and on). I have the transformers at a 90 degree angle, but I may try re-orienting the OT. I can't move the PT because it's so big it won't fit anywhere else. The PT weighs 14lbs by itself!

Here is a link to photos I put on my facebook page.
http://www.facebook.com/album.php?aid=2 ... ca92551b80

Any suggestions would be humbly appreciated!

BTW, I got the boards from JoeBob - thanks Dave!

[Noval_novice]

Well, obviously your PT has some sort of cancer. Good Lord!

[Travst]

My little transformer now feels inadequate.

[xtian]

PT hums in standby? Sounds like mechanical vibration. Whattayagonnado? That's a monster.

[Alexo]

Hyowza! That PT wants to take a lay-down!

Time for the headphone trick! You need to reorient those transformers, I would disconnect them from the circuit, tie off the HV leads, jumper the PT into mains and plug headphones (or the speaker) into the OT and VERY CAREFULLY slide them around until you find the magic spot where the PT hum is not magnetically induced into the OT> Just being 90 degrees is not enough, you have to have the *right* (as in correct) 90 degree orientation.

I would really try it with the PT laying down, I know it's not fun to cut a big square hole out of metal, but all those wires sticking out just does not bode

...btw, I love the WOT45LHR.

...aren't you the guy who won the JTM45 from 18watt? Welcome to TAG!

[LarryLarry]

Yep, I'm the one that won the JTM45 on 18watt.com. The whole reason I stand mounted the PT is because there is no room physically to mount it laying down and not have it hanging over the front of the chassis and leave enough room for the OT.

I'm not aware of the 'headphone' trick. I'll have to search around for some more details. Alexo - perhaps you can advise me with some more details?

[Alexo]

Hey Larry, the headphone trick is just as I described, you just hook the OT secondary up to a pair of headphones, and you can hear any hum that's being induced. This sort of hum comes in through electro-magnetic radiation from the power transformer, no circuitry required. You just have to hook the PT into your mains voltage and try different transformer orientations until you find one where the hum is minimal. Usually they do wind up being 90 degrees to each other, but there are several different 90 degree orientations, and not all of them avoid the hum. I do this for every single build now, after a few where I went by theory alone and wound up with the hums and had to re drill or sometimes replace the PT.

I hate to say it, but it looks like ou may be out of options and your best bet might be to get a new PT that gives you the flexibility to place it in the right spot.

[LarryLarry]

Thanks Alexo, you cleared up my concerns about the headphones deal. Yeah, I may have to go with a different PT. I'll have to take a good look and see if I have any other options, but I don't forsee being able to lay this one down.

I've heard Edcor makes one that is reasonable, I'll have to look into it...

[David Root]

I would think the hum would go away if you put the OT in the laydown position, but that is obviously not doable.

What are the specs on that PT? It looks like at least 100W. Way too big. At 14 lbs it is likely good for close to 150W! BF Twin PT is 11 lbs!

[LarryLarry]

Here is what the eBay auction text said about the specs:

This auction is for one 1962 Foster power transformer. The transformer is stamped 59811. 120V input. Windings are 302-0-302VAC, 408-0-408VAC, 6.7-0-6.7VAC, 5.3VAC, 5.3VAC, 45VAC, 6.3VAC. Very clean with no rust or corrosion. This large transformer used a two 5U4 rectifiers and powered 2 6L6's and various preamp tubes. The cloth covered wire leads are 6" long or longer. Mounting holes are 4 3/8 " x 3 1/2". 3 1/4" tall installed.

[Phil_S]

Sorry, it appears you painted yourself into a corner. Here's a wild hair idea. Try a vertical metal plate between the OT and the PT. Maybe it will act like a shield.

[LarryLarry]

I'll give the metal shield a try tonight. I did that on another amp I built (5E3x2) only it was a metal shield between the power filter caps and the pots. It worked though!

I think I'll be adopting the headphone trick from here on out...

[LarryLarry]

Well, I found no difference moving a metal plate between the transformers or any other internal location regarding the hum, so for now I'll live with it.

One thing I've noticed about this amp - there is no thump/surge/pop when taking it off of standby. It is noticably quieter when taken off of standby than any other tube amp I've played...

[Structo]

Does this amp have a choke filter?

I have read that if you have a choke input that it can make the PT buzz.

If you can't figure it out you may be better off with a different PT.

BTW, the thump on taking standby off is fairly common if you have your volume pot turned up.

[LarryLarry]

Nope, no choke on this one. I meant 'no' thump not 'now' thump - my bad!

BTW, I found a pair of Sylvania 6V6GTs and some EH EL34s that are perfectly biased with the switch I put in to add a 33k resistor to the bias circuit. (47k for the EL34s, 47+33 = 80k for the 6V6s)