Bizarre OPT reaction

[Colas LeGrippa]

hello, my friends !

New build: SE amplifier, 2 paralleled EL34, 250V ( supplied  with a little help of a VVR, otherwise too much V  ) 100mA each tube, OPT 1640 sea ( Hammond) 1250k, 200mA .
You get the picture. Nice tone, Eminence 12 in speaker ( Red Coat ). One volume and one tone knob. ASAT. When cranked up to 3/4, if I approach my hand to the OPT ( not the bells, the sheets, the metal layers.. ( or whatever you call them, sorry for my poor english vocabulary), the amp starts humming, and when my hand is at 1/8th of an inch next to it, it stops working ! I mean no damn sound at all . Back to normal if if take my hand away.
QUESTION: normal ? last preamp tube too close ? OPT at max operation limit ? there is a ghost inside of me ?

Colas 

[Colas LeGrippa]

sorry for the error but you would have understood  that the OPT primary is 1,250 ohm and not 1,250 kohm.

[TubeGeek]

Definitely not normal.

Share pics of the chassis so we can see the components layout...

I once had some weird volume drop when building an ac30 clone.  It turned out that moving the OT a little over toward the PT fixed the problem.  Placement was very key on my project.

[Willabe]

hello, my friends !

When cranked up to 3/4, if I approach my hand to the OPT
the amp starts humming, and when my hand is at 1/8th of an inch next to it, it stops working ! I mean no damn sound at all . Back to normal if if take my hand away.

Then don't put your hand there.          

Sorry, yeah I"d say somethings wrong.      


                   Brad                

[John]

Obviously, your OPT is a female.

You should kiss your wife instead! 

[Colas LeGrippa]

Hi gang ,

I kissed my girl friend again and again close to my amplifier : I noticed a strong magnetic field that made my needle point to the north.
I tried to keep my hands away, but I just couldn't, she's too much attractive.

What should I do next ?

[jojokeo]

Crazy behavior Colas...the amp, not you. 

Questions: are you holding your guitar or touching the amp chassis when doing this? Does this happen if the amp is only 1/3 or 1/2 way or fully turned up? Or is it only at the 3/4 up position? How far away are your PT & OT to each other? Is your output jack grounded to the star ground tranny bold or = ? Is the jack isolated or not? Is the OT new or a pull from something else? How are the OT wires ran/are they close to any other wiring?

[kagliostro]

May be there is some kind of auto oscillation ?

what are you touching with the other hand ? (I suppose you have one  )

K

[TubeGeek]

Hi gang ,

I kissed my girl friend again and again close to my amplifier : I noticed a strong magnetic field that made my needle point to the north.
I tried to keep my hands away, but I just couldn't, she's too much attractive.

What should I do next ?

Post pics maybe?

[Colas LeGrippa]

thanks for your serious answers. I love you. Concerning the other replies, those who think I will post pictures of my magnetic needle or anything else concerning the kisses ( and more ), you are on the wrong path. I am a VERY serious guy and so is this forum. ( YES I HAVE 2 HANDS, so far ).

Ok: it is a new HAMMOND SE opt, ( it is huge  ), less than 1 inch far away from the first power tube, the PT is an Avell Lindberg toroidal, 2 inches away. Laminations from the OPT are 90deg with respect to the choke.  Connections from the OPT: Power tubes plates and screens close to the OPT and speaker jack close too, so wires comes from the tranny, through the chassis and the connections are right there. No criss cross of signal wires around.
When I approach my hand close the OPT, my other hand is touching my guitar strings, volume from 1/2 to full. ( At this moment, my wife is not there, so she is not interfering with her two big irons ).

I'll take pics of that beast ( no, please, not my wife, but my amp.).

Colas

[kagliostro]

Have you an oscilloscope to look at the signal in the moment you put your hand near the OT ?

What happen if you don't touch with hands the string and you put the strings near to the OT ?

May be I'm wrong but I really think around auto oscillation, something like the Larsen Effect but at higher frequencies

few days ago HBP told to try to use a radio as to detect High Fequency Oscillation, if you haven't an Oscope may be you can try that way to detect for undesired high frequency presence

K

[Colas LeGrippa]

grazie Kaglostro, I don't have a scope no more, how could I use a radio to tame the oscillation ?

Colas

[kagliostro]

I was asking about here

http://www.el34world.com/Forum/index.php?topic=14086.0

read what HBP told

K

[HotBluePlates]

It would be interesting is a steel plate was placed between the OT and the nearest power tube, with its edge touching the chassis. If that stopped the strange reaction, it might point to your layout.

Any wiring from the output tube plates to the OT primary which run near the grid wiring of the output tubes, or near the grid wiring of earlier preamp stages could cause feedback problems.

[Colas LeGrippa]

HBP, you pointed it. It seems to be a feedback reaction, just like the wole signal was fed back into the amp, out of phase, thus cancelling everything to the point that NOTHING AT ALL comes out of the amp, strange, indeed ( dont talk about my wife, please ).

[Colas LeGrippa]

...........but there is no feedback until the damn OPT is touched by me...........I would carry the whole signal, from my guitar p/u's to my belly and finally my hands, and inject it back out of phase into the OPT to cancel everything ?

[jojokeo]

With oscillations, feedback, coupling, parastics , etc. - these become worse (usually) when the amp is turned up and more currents are flowing. I remember reading not to have too close to each other, too close to tranny, and for the plates to be positioned in a certain direction to the tranny. It may have been in either by Blencowe or Aiken? I forget as it's been a while and I can't get re-focused from the two big irons!? Still no comment on if this does this at different power levels?

[Colas LeGrippa]

you're right, I remember reading on a tube data chart that a certain position is recommended for power tubes. I have to try changing by 90degrees their position.

Colas

[HotBluePlates]

you're right, I remember reading on a tube data chart that a certain position is recommended for power tubes. I have to try changing by 90degrees their position.

Colas

That's only for when you mount the output tubes horizontally. The diagrams show how to orient that particular tube so that a plane passing through the grid support rods will be vertical. It helps insure gravity and overheating don't cause the grids to sag and short to another tube element.

Magnetically-induced issues redcue with the square of the distance change. If you move the centers of the tube and transformer from (say) 2 inches apart to 4 inches apart, the distance center-to-center will be doubled and the interference reduce to 1/4. If you move the centers from 2 inches apart to 6 inches apart, the intereference will be reduce to 1/9th its original value (tripled distance, 1/9th interference).

I'm not sure if the problems are fields inside of or outside of the chassis. But it does seem you might have tried to build to compact a package.

[Colas LeGrippa]

I must admit the new transformer with its huge dimensions is closer to the preamp and power tubes. I did rotate the power tubes by 90deg aligning the keys on the same axis without any result. I will now move the preamp tubes away from the opt, that will give me room to move a bit the opt away from the power tubes. Magnetic influence diminished by square the distance, just like light: twice the distance 4 times less bright.

Colas