The previous videos on this project: • Do I Make Amps? | Yes,... For more info, Subscribe to this channel, join us at PsionicAudio, or email us at info@psionicaudio.com.
Ken Fischer also used an aluminum chassis in his Trainwreck amplifiers as he felt steel had a negative impact on the magnetic field of the output transformer. Next to Dumble... Trainwrecks are the most coveted amps ever built... so I trust my friend Kenny. I sure miss that goofball.
As a novice I am constantly learning and found your channel very helpful. For that I thank you very much. I would like to know how you wired the mercury transformers for the hum test. I for whatever reason cannot duplicate your test, if you can give instruction as I will be on the lookout. Once again thank you for contributing to my growing interest.
I have a Vox AC4 Handwired that has this exact issue. I need to drill new holes in the chassis and remount the OT because of how poorly it was positioned in the factory. Great amp otherwise. This video helped me understand the problem better, thanks!
I just put in an oversized Vibroverb OT into my 2000's Custom Vibrolux. I measured the foootprint before ordering and it barely fit - but I forgot to account for the extended width where the bells stick out. So I moved I mounted it w/the bells sideways (the way that increases hum). It's ~3" away though, w/the choke in between. The amp is pretty quiet now though (I did several other mods to cut down the ton of noise the stock amp had).
Nice informative video. I just must say that I don't like amp heads that are not weight balanced and tend to incline on one side while carrying it. But of course it is way better to have an amp with no hum. Cheers.
IME that becomes more of an issue with larger/heavier heads, at which point there is more real estate to play with and things can be farther apart. This 15W head will not be heavy, and the combo version will have the weight mostly centered (the chassis is not as wide as the cab and the speaker will be centered). So it will balance well.
@@PsionicAudioThat's true. Mostly those are heavy 50/100W heads. Combo can be centered well yes. But I always tend to prefer the head + cab combination.
Most good transformers have a copper shield on the outside of the steel they also sell copper tin tape I used it on a vintage amplifier and it actually worked
Magnetic coupling between coils in 3d space is a fairly lengthy, and rather ugly equation. I'll give y'all the coles notes: 1) the rate of fall off in field intensity follows the inverse cube law. Not inverse square, like electric or electromagnetic field. 2) The field created by a coil is not spherical. It is an oblong elipse. 3) Coils can couple in 2 ways, radially and tangentially. A quick read up on Biot-Savart's Law shows the 0.5sin/cos relationship. 4) There are 3 orthogonal orientations which help achieve cancellation. No surprise, they are planar. 5) Transformers in close proximity cannot achieve a zero coupling. As Maxwell describes, the B field vector radiates perpendicular to the surface of every single part of the winding coils. It is also received by every single part of the coil. So emission in one part of the radiating coil is bound to be non-perpendicular with some part of the receiving coil. It's a fact of life with real-world coil geometry, and the nature of the equation B=i*dL. Where L is the length of wire, not Inductance. So long story short, spacing is the most effective way to eliminate transformer crosstalk. Orientation certainly helps, but it is limited in its effectiveness.
Marshall must have an acceptable level of transformer induced hum, they still include it on their new amps. The Origin series 20 and 50 both have it. An aluminum shield on the 50 helps, relocating the OT would be best. I'm sure space and cost all come into play, but hum really shouldn't.
Lyle can you please explain some of the pros and cons of not just how an OT transformer is placed, but where on the chassis? Say Fender style of the transformers together vs Hiwatt where they are at each end of the chassis. Would be great to get your take on this and how it affects preamp placement etc. Thanks!
That kind of thing can either be really short and vague or chapter length. I'll try to be somewhere in the middle here. Each has pros/cons. You want enough distance (and to observe correct orientation) so no hum from the PT is picked up by the OT. You also don't want the OT wires too close to the input jacks and their wiring. This can cause noise and instability. You also have to consider weight balance (all the weight at one end vs a more even distribution) and speaker placement (making sure no transformer contacts a speaker). Either approach can be done well. Or poorly. Hiwatt made primarily heads, at least for the 50W and up amps. They had deep chassis, so they could get distance from the inputs and the OT. And they had huge iron, so if they placed them like on a Fender all the weight would be at one end, making it really hard to carry. Fender focused on combos, and while the weight distribution isn't even, the speakers somewhat balance that out. Leo was seemingly focused on keeping the OT away from the input jacks, and used much more shallow chassis. Vox was more like HiWatt, but didn't have as much distance inside the chassis, so sometimes the OT can interact with the input jacks as a result. It's not very common, but sometimes a shield between the input jacks and the OT can be very beneficial. Anyway, either can be done right, and both are possible to mess up. Neither is "better."
Another excellent presentation. I know this aspect of design already, but I have a concrete question: if the trannies are on opposite ends of a chassis (with about 16" between them) for a 15W amp, do you expect that this hum would still be a significant concern?
Thank you! I cannot go into real detail as the wiring will change with different transformers. But in this case I had the power transformer primaries connected to 120VAC with the AC safety ground to chassis. The HT center tap secondary was grounded. All other secondaries should be taped off for safety. The output transformer primaries were not connected and I connected the 8 ohm tap and common secondary wires to a speaker jack. Most headphones will work with the 8 ohm tap, though you can use 4, 8, or 16 as needed. The output is low, won't damage headphones. I used a speaker here so you guys could hear it but headphones are better.
This could work however, if you get the output transformer too close to the input jacks or input tube you'll potentially run into even worse issues. Usually 3-4 inches apart or more and you won't have hum coupling
Not an issue with the JMI layout, though interaction between the input jacks and output transformer can be an issue on those. Not too common but it can happen.
Just to make it clear your test setup was you only are using the primary side of the 120VAC power transformer with nothing connected to the secondary side and also the output transformer primary side has nothing connected but you have the speaker connected to the secondary side of the output transformer? This way you can move the output transformer orientation to reduce the 60hz hum
Exactly. I did connect the HT center tap secondary to ground just to get the coils working a little bit, but the was no load on any PT secondary and no primary connections on the OT. Just checking for induced hum.
@@PsionicAudio Thanks, Because this same problem happens with guitar pedal boards when its common using multiple different power supplies from different companies/brands that the linear transformers will cause HUM and other noises being to close together. Also overdrive and distortion pedal raise the noise level floor so you can't have them to close to the power supply transformer on the pedalboard. I'm not sure if you're going to make a video lesson like this on the orientation of hum and noise level on pedalboards.
On the DSL2000 Marshall placed the two transformers onto each side of the chassis and then used a shield around each once. Would that work on this amp? I know you don't want the OT near the input jacks, but if you used a shield around it would that reduce induced hum? I have installed MM's upgraded transformers and a choke into a few DSL2000's with no hun issues. Even my wife noticed the difference in tone.
Yes, but the issue is custom chassis and custom shields ala that Marshall vs working with an off-the-shelf chassis as economics forces me to do for this. The Marshall chassis is also much larger with more real estate. I'd love to design a custom chassis but it adds way too much cost unless you are doing many hundreds of an amplifier.
@@PsionicAudio I understand. Fabricators usually want a large minimum order to do custom work. I can see why you would not or could not order 100 units.
Bad, confusing and unfair explanation, one transformer is lying down and the other is standing up, put them in the same position, then you can talk about the interaction between the flux lines.
