SERIAL TONE KILLER - II Le Retour

Reprise du projet - nearly one year later !

I was somewhat fed up by the too numerous issues, and having other things to do...





For those who are still interested in ... More adventures and pics tomorrow !

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Exactly, Ricochet : you got it = noise, or more precisely hum due to magnetic induction. And it's still the issue that I have to cure...

I used a toroidal transformer because - among its significant lightweight - I read everywhere that they provided much less 50Hz magnetic radiation than the classic EI transformers but I'm experiencing that this is not true at all !

I will probably have to displace the reverb unit, because with already shortened cable, the improvement is good, but I want silence : I am hum allergic...

Well, I use the (excellent) Spring preset only, so...

I made some displacement test to judge the hum attenuation :



I draw the conclusion that it was the wires... So I shortened the IN / OUT connections to the reverb. Neat improvement, indeed... But not perfect !



That's why I think that a displacement may be mandatory... It's not what I would, but...

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Thanks dafreeze !

Well, The Great Hum Song isn't my preference at all - I much prefer That Great Gretsch Sound - without hum !

Yes, Ricochet : these are the screen serial resistors of the 6L6GC power tubes (1K/5W) - usually 470R on Fenders and 1K on Marshalls. This allow me to install 6L6 or EL34.

Today I made a power transformer displacement tests - quite conclusive :



An EI type transformer of the same power may finally radiate less hum , or at least an orientated one , moreover I can find one giving 2 plate voltages allowing HI power - FIXED bias / LO power - CATHODE bias.

But the drawback is the weight increase : +3kg ! I tried to design this amp to be lightweight... I'm somewhat puzzled...

Wait and See...

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Latest news for today...

Thanks to a tech-minded member on another forum, I placed the toroidal transformer with its axe horizontal (90° rotation) :



And It eradicated the hum in the reverb circuit ! As simple as stupid as that...

That said, the toroidal transformer is nonetheless a widespread magnetic hum generator : don't expect to approach the amp at less than 2 meters with a single-coil pickups guitar in overdrive mode, and in a chosen precise position, at moderate overdrive and gentle volume.

I just imagine playing loud with a hi drive level with a strat ...

Tentatively, I'll install the transformer on a bracket in order to go further in the building / setting of the amp, and make other tests to see if an EI transformer would be worth to install.

Despite that, I can say that the short sound test I mad this evening were very promising, both in clean and overdriven mode... It's encouraging, finally !

Wait and See, then.

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Thanks for the suggestion, but this grounded shielding is already in the cab since its construction, making a Faraday cage, you guess it, wabash slim...



But this has no efficiency against magnetic field hum radiations. To be truly efficient, the chassis and the upper shielding would have been made from µ metal, an absolute pain to work and costing an eye !

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During this times, I made several test of magnetic and electric induction directions finding, comparisons with EI transformers radiations, comparisons between different shielding materials... It would be too long to relate it all, and I'm not sure that it would interest much people !

To sum up :

- Toroidal Xfo is an omnidirectional disturber, but the radiated field is more electric than magnetic : more buzzz than humm, if I can say so. The Xfo has an external metallic ribbon ring of unknown composition.

- EI Xfo is much more directional, the direction being the axe of the core where it is the strongest, the plane of the laminiations being the area of secondary perturbations. Conversely, the radiated fiels is more magnetic than electric (I mention that the Xfo winding have covers).

- On the EI transformer, lead screen (thickness 3mm, added mass 750g) demonstrated absolutely NO efficiency against magnetic field, and is easily beaten by... Aluminium screen (thickness 2mm) : I would have bet the contrary...

In the meantime, I discovered and bought on eBay a cylindrical cap shielding cover of the suitable size for my Toroidal Xfo . It's made of annealed iron . So I'll made a test - I hope conclusive !

For the moment, I just surelevated the Toroidal Xfo of about 20mm with a wooden shim, and it proved to be enough to diminish tentatively the hum to an acceptable level :



In waiting, I finished the Footswitch - minus the design and engraving of the top plate :



This FSW controls : Overdrive, Booster, Reverb and FX Loop. Note the possibility of connecting the FSW cord at the right or the left, depending on stage placement...

Unlike the power transformer issues, it worked perfectly at the 1st time : no adverse noises nor clicks...

If you put the FXs (for the FX loop insertion) simply on the top of the amp, there is 4X 9VDC regulated outputs allowing to power up to 4 stompboxes directly from the amp itself...



Wait and See !

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Continuation...

I added a 3mm Aluminium folded shielding screen underneath the reverb and FX loop circuit :



It proved efficient enough to cut the remaining direct power transformer radiation to these circuits , but - unsurprisingly indeed - it's not enough at all...

I'm waiting impatientely for the shielding cover cap for the Toroidal Xfo, and for some Mumetal sheets... I really hope that this will do the trick ...

Without that power transformer hum issue , this amp would work and sound wonderfully...

So the Hum War is not finished - Wait and See !

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Thanks @Outlaw !

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Thanks @pmac11 !

Well, ça va pas pas vraiment top notch...

I received and installed the cover on the toroidal power transformer :



The cover is reputedly made from iron (softer than steel to cut) and I made a baseplate from 3mm aluminium.

No hum induction in the chassis circuits - due to the presence of the 3mm alu baseplate , but the cover itself proves virtually ineffective against hum radiation : the guitar pickups are still induced by hum as if there were no cover, or nearly. An aluminium cover would have been better but it doesn't exists...

Add to this that this hum radiation is omnidirectional...

Moreover, reading litterature about toroidal Xfo, I learnt that they are very sensitive to AC mains phase imbalance distorted waveforms, DC offset superimposed to AC mains, prone to saturation way more quickly than conventional EI Xfo, all this resulting in a widespread hum and parasitic impulses, interferences, noises radiation...

OK. I think that the trial of the toroidal transformer is done, this time.

I could try to recover the cap with very costy Mumetal/Permalloy shielding, but I'm not confident at all in the result : the iron cover should have at least already shown some efficiency against hum radiation, even moderate, that would have been finally complemented by µmetal recovering... But not at all.

So status quo : I'll shift for an EI, low loss, power transformer.

https://www.tube-town.net/ttstore/tt-powertransformer-100-watt-for-marshall-jcm-800-plexi.html

Obviously, the amp's weight will increase by 2kg... That's probably the price to pay for silence !

Wait and See...

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Thanks @pmac11 !

Yes, you're right : an alu cover of 3mm thickness may have done something... The problem is that there would be no place for the extra-thickness of the cover on the diameter.

Sure I'll post details and soundclips of the beast ASAP !

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