Chassis Layout/Transformer Placement (Lightning based build)

[theundeadelvis]

He guys! I'm building a 15 watt EF86 amp on a Matchless Lightning board, using actual Lightning transformers (both are the horizontal type that require a cutout in the chassis). I'm trying to get it all to fit in a chassis 16.5"x6.5"x2.5". The way I have it laid out, the output transformer will sit between power tubes and preamp tubes. I did this to keep it adequately away from the PT.  Is this acceptable, or am I likely to run into noise/hum problems? Unfortunately, I don't have the option of playing with transformer placement before fabricating the chassis. Thanks in advance!

[bnwitt]

how about a photo?

[PRR]

I'd put the OT next to the PT before I'd put it next to the preamp tubes.

[tubenit]

I would suggest looking at other successful builds to see what others are doing. Based on your description, I don't know what you're actually planning on attempting?

I have had successful quiet builds without oscillation problems with the PT and OT trannies next to each other and on opposite ends of the chassis (such as Matchless and HiWatt).

http://s28.photobucket.com/albums/c216/tubenit/?action=view&current=CarolinaBluesExpresschassisexterior.jpg

http://s28.photobucket.com/albums/c216/tubenit/?action=view&current=frontpanelchassis.jpg

With respect, Tubenit

[sluckey]

I'd put the OT next to the PT before I'd put it next to the preamp tubes.

me too

[HotBluePlates]

... (both are the horizontal type that require a cutout in the chassis). ... The way I have it laid out, the output transformer will sit between power tubes and preamp tubes.

OT near any preamp tube puts high voltage and/or high current wiring near low-level preamp wiring. This is an excellent recipe for oscillation. Avoid like monkey plague.

I would personally look for angle brackets to mount the OT upright Fender-style, and put it closer to the PT. Basically, right at the output tube sockets. Then drill holes in the chassis for primary and secondary wires to pass through, and use rubber grommets to keep the chassis from cutting wire insulation.

I know we can find examples of working amps with the transformers mounted other ways. We can also find video of a suspect fleeing police in a car with 4 blown out tires. Driving on bare rims still doesn't seem like a good long-term plan.

[theundeadelvis]

Thanks for all the suggestions! I will definitely change the layout. I will look for some brackets like HotBluePlates suggested, because not have to cutout the chassis would make for a roomier layout.

[eleventeen]

Two big cutouts right next to each other might also weaken the chassis or make it springy/bendy. The upright mounting brackets for one of the trannys are a good idea, IMO.

[theundeadelvis]

So here's the layout I came up with. I have a friend who's a metal worker, and he is going to build it (laser cut stainless, seamless welds). Does anything jump out as problematic? Thanks for all the input.

[HotBluePlates]

You have an L-shaped space between the transformers. What do you think about placing the rectifier there? Gives you a little more room.

I still hope you'll consider standing up the OT. Taking the side facing the present rectifier location and rotating that to the top would be about perfect as far as hum-reduction goes. If an uncovered side seems like a safety hazard (and I suppose it is), rotoating the OT on the vertical axis to face any bare lugs towards the front faceplate would be the second best position.

[theundeadelvis]

I really like your idea of moving the rectifier to the other side of the OT and being able to move the OT farther away from the PT. I like the idea of the OT bracket, but I'm somewhat hesitant for two reasons. One being what you mentioned about having the bottom exposed, and the other being that this is going in a Bluesbreaker style cab. I'm afraid mounting the OT on it's edge with a bracket might not be as sturdy, since the chassis will be mounted to the cabinet back plate (tubes horizontal to the ground).

[theundeadelvis]

Alright, here's a revised layout:

[TIMBO]

HI Guys, I agree with HBP , I had a problem with my 50w plexi build (first major build)I had the OT and PT at opposite  ends of the chassis. This meant that the wires from the OT ran past the preamp valves as well as being quite long.This caused all sorts of problems. With help from the HBP, the conclusion was a rebuild. With more research, the new build had the OT and PT next to each other (at right angles to each other and a gap of 3/8") I had enough room to place the rect valve between the PT and the front of the chassis (or move the PT further forward and place it at the back for easy access) . The final layout has the OT next to the the power valves (about 1") .When fired up there was still a problem. Further investigation found the schem for the OT was printed wrong.This sorted ,now have a great amp.So i don't know if having the tranys at either end of the chassis was OK, but the lesson learn't is that having the two tranys together with the rect valve at the front and the power valves next to the OT (the tight cluster keeps all wires shot)was the way to go.I employed this setup on "MY OPUS" build which had 9 Valves,OT,PT and Choke on a 22" x 8" chassis. The amp is dead quiet and sounds great thanks to the forum. Timbo

[HotBluePlates]

I wrote some detailed stuff, and the computer dumped it again.

Bottom line, I wouldn't move the PT or OT at all, just change the orientation of the OT in the spot it is shown. 2 laydown transformers = great coupling = hum. Distance reduces hum, but puts the OT over the preamp, which equals a great oscillator. The PT and OT could be nearly touching, yet still have very little coupling (low hum) if the OT were stood up on end, and possibly rotated 90 degrees once there.

L-brackets are either molded into most transformer endbells, or else sandwiched between the core and endbell. You can mount it on the outside of the endbell, just stick an L in each corner. Not sturdy? Make the L thicker.

Try our suggestion, or not. It might work out okay (but I wouldn't personally try it). It it hums and/or oscillates, you'll know why.

[theundeadelvis]

HBP - When you say you wouldn't move the PT or OT, are you referring to my first drawing or the second (is the OT in the second drawing too close to the preamp?)? I think what I may do, is go ahead and cutout the chassis for the laydown mount, and if it causes noise, I'll fabricate a bracket that covers the cutout and mounts the transformer upright like you suggest. I appreciate your input! Thanks!