K&K Audio / Lundahl Transformers

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Mounting ideas for IT / OT 's

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Posted on September 16, 2009 at 18:11:59
KurtP
Audiophile

Posts: 217
Joined: January 25, 2007
 Does anyone have suggestions or pictures of how they choose to mount their Lundahl transformers? In efforts to minimize vibration from the top panel, I have typically mounted the transformers to the wooden side plates of my amps. In retrospect, it would make for easier, more convenient mounting to drill four holes in the aluminum top plate and just use machine screws. Core automatically gets grounded (a good thing), and it's easier to remove for mods in the future. I think the only thing that concerns me is potential vibration. But since my power supply is in a separate chassis, I'm thinking vibration is a non-issue.

Ideas or experiences ?

Well,, posted on September 18, 2009 at 19:41:20
Ian L
Audiophile

Posts: 1207
Joined: July 5, 2002
Contributor
  Since: September 11, 2008



typically . . . .

Noice, posted on September 19, 2009 at 05:21:30
KurtP
Audiophile

Posts: 217
Joined: January 25, 2007
 A familiar site, to be sure. This stage is necessary to make sure you don't have magnetic pickup from other sources, as is quite possible when the power transformer and choke are in the same chassis.

Maybe a simpler question to ask is has anyone had problems of any sort when mounting the Lundahl cores directly to the underside of the chassis? Issues could be vibration, hum pickup, circulating current, etc?

RE: Noice, posted on September 19, 2009 at 06:00:51
KevinC
Manufacturer

Posts: 1999
Joined: April 19, 2001
Contributor
  Since: October 7, 2002
 I've mounted Lundahl interstage transformers and output transformers to the bottom and top sides of top panels for years and never have an inkling that this caused a problem.

Kevin Carter
K&K Audio
www.kandkaudio.com
RE: Mounting ideas for IT / OT 's, posted on September 17, 2009 at 13:18:36
Ed Sawyer
Audiophile

Posts: 2158
Joined: November 3, 2000
 I usually mount mine on sub-plates on the insides of the wooden chassis of the amps. Mostly for aesthetics reasons, and easy accessibility if possible. I generally avoid fasteners and such on the top plate if at all possible. Since the lundahls usually mount from the back side, I first mount them to a subplate, then mount that to the inside of the wooden chassis. That way they can be removed if need be, and all fasteners hidden.

Can post pics if need be, to illustrate the above.

-Ed
RE: Mounting ideas for IT / OT 's, posted on September 16, 2009 at 19:28:18
tuneful
Audiophile

Posts: 87
Location: New Mexico
Joined: October 29, 2004
 Great topic!

I have been thinking about this problem and have done a few crude measurements to try to evaluate the effects of vibration. I am going to rebuild my 32b P-P monoblocks (6SN7>2A3>32b) this fall to incorporate some power supply changes and also do a better job of wire routing. Right now, everything on each amp is screwed to a 5/16 aluminum plate. I can hear some mechanical vibration coming from the power supply transformers and chokes.

I used an oscilloscope to monitor one amp's idle signal across a 8R resistor dummy load; the amp sits on 5 racquet balls for isolation/dampening. This amp has pretty low noise, about 2 mv p-p ripple at roughly 240 hz. My VOM flickers between 0 and 1 mv. I think if vibration from the magnetics were a big problem, I'd expect to see evidence of a vibration-induced signal at 60 hz or 120 hz.

However, if you rap on the side of the amp, you see a signal that can go over 100 mv p-p; the characteristic vibration ringing frequency of the amp/tube structures is about 1 khz (the mechanical excitation shows up on the scope as an electrical signal at roughly 1 khz). Tapping the side of a 2A3 produces a much larger signal, the 32b is less (DHT's are sensitive to vibration), and the 6SN7 the least. I also observed that acoustic noise (voice) near the tubes produces an electrical output. (As an aside, I also tried some well-known tube vibrational dampers, and they had no effect.)

So my conclusion from these crude tests is that external noise and vibration produces much higher electrical output than amplifier self-vibration. It may be that the internal tube filament structures aren't excited much in the range of 60 to 120 hz, even if the vibration is present (from the transformers and chokes).

I plan to do a few more tests, where I run one powered speaker close to the other amplifier (at idle) and see if loud room sound causes significant signal in the idle amp.

Being your typical neurotic/anal amp builder, I do plan to use vibration dampening and isolation in the rebuild, both for internal and external sources. Having a separate power supply is probably a smart move, although I found that tapping the 6CJ3 damper diodes in the PS also caused output noise. I need to understand this vibration problem better and would really be interested to hear other ideas and experiences, as well.

It's possible that isolating the tube sockets could give the best payoff. For other components, grommets and isolators should help, for example as shown in this link, could be useful:

RE: Mounting ideas for IT / OT 's, posted on October 6, 2009 at 09:25:13
Tubelcain
Audiophile

Posts: 4
Location: Guernsey
Joined: January 6, 2008



Hi Kurt P.

Would whole hearted agree with Tuneful mains Iron/chokes & Tubes are going to be your major problem. As you can see I mounted my Lundahl signal transformers under chassis and pretty close together too. My aim was to keep the wiring lengths as short as possible. I personally don’t agree with potting Transformers and as Lundahl Tranies are more functional than pretty the best place for them is under chassis.

The only noise I have now is from the AC heaters to the 300b and that is very minimal. I would however highly recommend a separate PSU box ( Mains Transformer Buzz can be very Irritating) & some form of sub chassis which can be isolated with rubber grommets, great to hide undesirables & put your filament/control wiring above and away from HT & Ground wiring too.

Happy and safe Amp building.

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