Converting a stand up PT to a laydown

[mikepukmel]

Hi All.
Not being able to leave well enough alone, Im talking myself out of taking the end bells off a standup PT I bought, having my friend dremel the feet off, and making it into a laydown. Worrying about having both PT and OT standup, is keeping me up at nights worrying about hum problems.
Is there anything else I need to worry about besides just cutting the feet off and laying the PT down 90 degrees? I assume I should just remove the other end bell and let the side of the PT hang down into the transformer like the Marshall's. THe Fender style Deluxe PT I have has bells on both sides, even though its a laydown.
THanks
MP

[loudthud]

Does the transformer have wires on both sides ? How are you going to deal with that ?

[Enzo]

You want the transformers at 90 degrees from one another. That is why you see a lie down power transformer but a stand up output transformer. It won't hurt to have both transformers standing up, just mount them so one is left right, and the other is front rear. They are still at right angles either way. I sure wouldn't bother.

[Chuck H]

Is there a hum problem now? And has it been positively diagnosed as a transformer proximity issue?

[mikepukmel]

Does the transformer have wires on both sides ? How are you going to deal with that ?

Hi Loud, nope, all the wires come out on the bottom of one bell. So if I take that bell off, the bundle would fall into the inside of the chassis.

[mikepukmel]

You want the transformers at 90 degrees from one another. That is why you see a lie down power transformer but a stand up output transformer. It won't hurt to have both transformers standing up, just mount them so one is left right, and the other is front rear. They are still at right angles either way. I sure wouldn't bother.

Cool, thanks Enzo! That would save a lot of hacking and dremeling, none of which Im particularly good at. I had started a discussion about the orientation of the cores, and got lost in the technical detail. Easy enough to both 90 degrees from each other.

[mikepukmel]

Is there a hum problem now? And has it been positively diagnosed as a transformer proximity issue?

Hi Chuck! Don't know. I have the board built, parts soldered in and some wiring hooked to that, but not bolted into the standoffs. The last task before putting stuff on the chassis is to get the transformers bolted on. I don't have the holes drilled yet, for the transformers, waking up with nightmares about what will happen if I don't mount the transformers the way millions of Fender, Marshall, etc. I got a tip on this blog someplace, someone said to hook up (power) the PT, but don't hook up the secondary wiring. Put earphones on the outputs of the OT, then move the output transformer around to see if it picks up hum. Didn't seem very scientific, since most nothing is hooked up at that point.

So, this is a completely home made thing, I don't have anything to compare it to, i.e. before hand.

[Mick Bailey]

Plenty of amps have both transformers as stand up - Traynor springs to mind as one manufacturer.

[mikepukmel]

Its a JTM45 chassis, I had made, with fewer pot and tube holes, for this bastardized Supro Thunderbolt 6420 circuit. I hacked up a layout on a fiberglass turret board, got some standoffs in the chassis.

[mikepukmel]

Plenty of amps have both transformers as stand up - Traynor springs to mind as one manufacturer.

Thanks Mick, didn't know that! I will mount 90 degrees and put the OT as far as I can get it from the PT.

[Chuck H]

Thanks Mick, didn't know that! I will mount 90 degrees and put the OT as far as I can get it from the PT.

But... You also don't want the OT or it's leads too close to the preamp circuitry. There are exception examples that have the OT mounted at the preamp end of the chassis. This can be done as long as any interaction tends to be more antiphase I think. Still not ideal IMO because that only introduces another tone affecting consideration and it could take several attempts at lead dress to get it stable. And...

In this light, assuming the power and output transformers will be perpendicular to each other, which orientation is best for which transformer!?! Fender tended toward having the OT lams perpendiculat to the chassis while Marshall's have the lams parallel with the chassis. By inference the PT lams would be opposite. So which way is best? I have two cases where it seemed to matter. One was a combo amp that I put a reverb pan in and mounting the pan on it's side was the only way to eliminate hum induced from the PT. I have to assume if the PT lams had been oriented the other way that things could have been different. In the other case I had an amp that sounded just ok, but if I rotated the OT 180° the overdrive tone took on some real nice hair and liveliness without making the amp unstable at all. I have to assume this was due to some interaction with the preamp. So...

Which transformer goes which way? I'm sure it can be different for every design, but how can we possibly know ahead of time what any given amps overall layout, lead dress and component placement will mean WRT transformer orientation.?.

Now you've got THIS to worry about instead.

[Mick Bailey]

Interestingly, Marshall went to stand-up transformers for some amps (100W plexis) in '69.

[Helmholtz]

Interestingly, Marshall went to stand-up transformers for some amps (100W plexis) in '69.

This was probably an improvement as the earlier lay-down PTs had no lower endbell.
Big 100W stand-up transformers may be somewhat critical with an aluminum chassis (as used by Marshall until 1966) for mechanical reasons, though.

[mikepukmel]

But... You also don't want the OT or it's leads too close to the preamp circuitry. There are exception examples that have the OT mounted at the preamp end of the chassis. This can be done as long as any interaction tends to be more antiphase I think. Still not ideal IMO because that only introduces another tone affecting consideration and it could take several attempts at lead dress to get it stable. And...

In this light, assuming the power and output transformers will be perpendicular to each other, which orientation is best for which transformer!?! Fender tended toward having the OT lams perpendiculat to the chassis while Marshall's have the lams parallel with the chassis. By inference the PT lams would be opposite. So which way is best? I have two cases where it seemed to matter. One was a combo amp that I put a reverb pan in and mounting the pan on it's side was the only way to eliminate hum induced from the PT. I have to assume if the PT lams had been oriented the other way that things could have been different. In the other case I had an amp that sounded just ok, but if I rotated the OT 180° the overdrive tone took on some real nice hair and liveliness without making the amp unstable at all. I have to assume this was due to some interaction with the preamp. So...

Which transformer goes which way? I'm sure it can be different for every design, but how can we possibly know ahead of time what any given amps overall layout, lead dress and component placement will mean WRT transformer orientation.?.

Now you've got THIS to worry about instead.

Thanks Chuck. OK yeah, a little knowledge ( or in search of a little ) is a dangerous thing. its a hodge podge amp, so I don't even know where the parts will be relative to the transformers. Most certainly not where Supro designed them. I guess the best I can do, is make this one off prototype, taking into account all of your suggestions, and see how it turns out. Mu metal mu metal mu metal !!!! Oook, ook!

[mikepukmel]

This was probably an improvement as the earlier lay-down PTs had no lower endbell.
Big 100W stand-up transformers may be somewhat critical with an aluminum chassis (as used by Marshall until 1966) for mechanical reasons, though.

I wondered how those AL chassis amps fared over the years, getting banged around the way they do when a band is traveling. I like steel! (can even solder to it). The supplier I got for this chassis didn't do steel, but he would customize the hole layout.