Home Forums Design and Building 2 Stroke Amplifier Design and Building New Build, OT placement question

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    Hey guys. Got my kit a few weeks ago. Just finished most of the soldering on the board.

    I’m wondering about the OT. Andy, I saw that photo you posted (copied here); that’s where you typically mount it? Sort of in the middle of the chassis, a few inches from the input jack?

    I saw that Robin said: “Regarding the output transformer; it’s important to keep it toward the “power supply” end of the chassis, away from the input jack and to position it so the coil winding are perpendicular to the power transformer as much as possible.” Not sure what perpendicular means in this context…would having the OT at a diagonal be a problem?

    Thanks! Mike


    Hey guys, Here is my tentative placement of the OT. Anyone see anything that might cause a problem later? There are some predrilled holes that line up nicely. The arrangement of holes where the wires enter the chassis is a little different than in DH’s book, but this looks to my (completely novice) eyes like it might work.


    That might work, to test it, build the amp and leave one side of the OT not screwed to the chassis, then, with the amp on, rotate the OT to find the most quiet “sweet spot”. The rule is to have the windings of the OT and PT at opposing directions, so if the PT is laying down, stand the PT up, or if the OT is going front to back on the chassis, then put the PT side to side. Here are two photos of how I did my first Two Stroke (which is super quiet). Because the 5F2a (tweed Princeton) chassis has holes already drilled, I made offset mounting brackets to use the existing holes for the tranny wiring. I’ve also built similar circuits with the OT going side to side and they are quiet too. As long as you keep the transformers away from the input end of the chassis and at dissimilar angles, you will probably be ok.



    Hey guys, new builder here. Thanks for all the work you guys have put into helping guys like me get started!

    I too am messing with the location & orientation of the OT, and I found the “headphone trick” to test various locations before you build. I’ll paste the procedure below.

    So I set this up: ran power through the PT (and fuse and on/off switch) using the primaries and attached the secondaries from the OT to a headphone jack, and listened. It seems to be a cool way to try out various OT locations.

    I found though that all of the OT locations I see in photos here produce a fairly loud hum in the headphones. The location that seems most feasible and has a fairly quiet hum is in the photo. (There is also nearly no hum when the OT is placed “on its side”.) I have the Heyboer PT, if that changes things from the usual Fender in the kit?.

    Maybe this technique amplifies the hum you can expect “in the wild”? Are there issues that I’m not seeing that might arise from mounting the OT like this?

    Pasted from a board at ax84.com:

    Author : joe b
    Date : Tue, Jun 01st, 2004 @ 17:45

    i’m going to include the key phrase “headphone positioning trick” in this post, so people can find it easily. :) I do it similar to that, but without initially dedicating the PT to a spot on the chassis…

    -Use electrical tape or heat shrink on the PT secondary wires to insulate them.

    -Use electrical tape or heat shrink on the OT primary wires, and all but one tap and ground on the secondary (to insulate them as well).

    The important part here is that there is no chance you’re going to have the PT’s secondary wires connecting to anything, and the OT will only connect two wires to headphones (and have no other connections). There is no electrical connection between the PT and OT for this test.

    -Hook the OT’s secondary tap and ground to headphones (make a wiring harness if you have to–me, I just use an old busted set of $10 walkman headphones).

    -Wire the PT’s primary to a power cable.

    -Put the headphones on, and plug in the power cable. Move the PT and OT around on the chassis and find a spot which gives the least amount of hum. Orienting the cores 90 degrees to one another will help a lot. But a certain amount of space is required still, or some hum may get induced from the PT to the OT.

    -If your amp uses a choke, put that on the chassis for this test (isolate its wires too). Although it’s not electrically connected to the PT or OT, it can actually help induce hum from the PT to the OT if it’s placed between them. That’s why on a lot of amps, you’ll see the choke way over near the preamp section, as far from the PT as possible (within reason). Some people assume the choke will help shield the OT from induced hum, but the opposite is generally true.

    This is only going to help a person determine how far apart, and what orientation the PT, OT, and choke will have with respect to one another for induced hum consideration. This has nothing to do with feedback, hiss, oscillations, heater hum, or any other noise issue. The rest of your layout, some with respect to the PT and OT positions, will tell that tale.


    That’s a new treatment for sure. Remember you need to support it somehow when you mount it in the chassis. I would still move the OT closer to the PT, but you might get away with it there. The more you can move it away from the input jack, the better off you’ll be. The headphone test works well, you can also leave the leads long and move it to find the sweet spot after you’ve finished the build.
    Good luck


    Thanks Robin. Yeah, I was not expecting that to be the sweet spot, that’s for sure. But it isn’t even a close call. The hum is clearly quieter in that orientation (it’s zero there). I was wondering whether I was getting too close to the input, and of course the headphones don’t help with that. From looking at your photos and Andy’s, it looks like the midpoint of the chassis, or maybe slightly closer to the PT, is ideal?

    I hear you about perfecting the position after I fire up the Amp. I’ll try to do that. But I guess I need an approximate location/orientation to start. Thanks!


    I have a number of Two Stroke variants with the OT in slightly different locations and orientations. They are all pretty quiet. SE amps have a reputation for being noisy and, in fact, I’ve built a non-Two Stoke SE with a lot more hum than any Two Stroke I’ve heard. DH did a great job designing the amp.


    I’m certainly not that detailed. I put it on the diagonal and never have issues. The amp is so quiet for a SE. Personally, I do it on the diagonal since the tabs make it the easiest to do. I start with one of the predrilled holes near the center of the chassis and angle it away.

    If there is a lot of noise, it is probably not the placement of the OT. I’d troubleshoot everything else first. Then, if you think you can get it quieter by adjusting the OT to a perpendicular orientation, let that be the last resort.

    Thats my 2c.


    Thanks Andy. I was really originally just trying to figure out what Robin meant by “having the OT and PT be rotated 90 degrees”, since you could do that on different axes.

    I wound up mounting the OT on L brackets, which was the other silent position through headphones and much easier to do. The brackets only required me to drill one hole in the chassis.

    Right now I have been playing the amp for about a month, and it is fantastic. I absolutely love it. It is dead quiet. As soon as the weather warms up a bit more I will glue the tolex to the cabinet, and when I disassemble it I will post pictures of the OT here. (And I’ll post shots of the amp once I have it finished.)

    Thanks for all your help Andy, & Robin!


    Yes, orienting the OT turned 90 degrees from my photo example works well too.
    It’s a little harder on the chassis as it is supported sideways but it’s just as quiet and sometimes allows for more space between the transformer and speaker if you build a small cabinet (which is how I tried it the first time).
    Congratulations on your build, please be sure to post photos of the finish product.


    I have a number of Two Stroke variants with the OT in slightly different locations and orientations. They are all pretty quiet. SE amps have a reputation for being noisy and, in fact, I’ve built a non-Two Stoke SE with a lot more hum than any Two Stroke I’ve heard. DH did a great job designing the amp.


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