#124 Questions

[rootz]

That sounds very much like coupling between the power transformer and one of the transducers of the reverb pan. Some guy over at diyaudio greatly reduced the coupling by adding steel shielding around the power transformer. Your practice of mounting the return side furthest away from the PT is already optimal. I think copper foil didn't help because it does not shield against magnetic fields affectively, like mu metal or permalloy would.

I'm pretty new to magnetic shielding, but google returned countless possible solutions. Some Blues Juniors even have a kind of zinc coated metal wing next to the PT.

I see mu metal sheets are available at eBay. https://www.ebay.nl/itm/151912090773 Not cheap, but $22,95 might be well spend. Anyone got experience with shielding with mu metal tape?

[Bombacaototal]

Thanks for the suggestion of MuMetal Mark and Rootz.

I will order some. Should I wrap the reverb pan with it or the PT?

I did some further experiments and noticed that the noise is greatly reduced if I make the pan vertical but unfortunately there is no space to mount it like this.

Another thing that is happening is an issue on the Deep Switch. I wired exactly like AN Wonderland (attached). The On position is fine but on the off position, when I touch the switch with my hands there is an introduction of noise. The switch is as tight as it gets so I don’t think it is an issue with the switch grounding itself. I wonder if it is because the circuit is “open” on the down position of the switch. What can I do to address this?

[rootz]

There are some interesting things I’ve read about shielding of magnetic fields and how and when they emerge. It seems M6 transformers have less of a stray magnetic field. Need to confirm that. But if it is the case, it would be wise to always build reverb heads with M6 PT’s or use a transformer that is not in the same orientation as the inductors of the reverb pan. Might an upright transformer (like I use in my head shell) prevent this problem?
I’ve also read that in some cases simple sheet metal wil work better than mumetal. Again, not exactly sure why or when that is the case, but that is an easy and cheap experiment. If sheet metal around the PT or the most sensitive part of the pan does the job, you are done.
I think you don’t need to wrap the whole pan with mumetal if you use that. Quite possibly you only need to do a small part of the pan where it is most sensitive, near the output transducer.

[Bombacaototal]

I found a supplier in the UK. Which thickness would you recommend?

[martin manning]

There are some interesting things I’ve read about shielding of magnetic fields and how and when they emerge. It seems M6 transformers have less of a stray magnetic field. Need to confirm that. But if it is the case, it would be wise to always build reverb heads with M6 PT’s or use a transformer that is not in the same orientation as the inductors of the reverb pan. Might an upright transformer (like I use in my head shell) prevent this problem?

With a high permeability (M6) core, leakage of the magnetic field is likely to be lower. Upright or lay-down, the orientation of the PT with respect to the pan is probably important. Assuming the PT and OT are in the correct orientation with respect to each other, rotating both of them 90 degrees might help with the reverb transducer coupling. The hum frequency is definitely 50Hz, so the PT is likely to be the source.

I’ve also read that in some cases simple sheet metal wil work better than mumetal. Again, not exactly sure why or when that is the case, but that is an easy and cheap experiment. If sheet metal around the PT or the most sensitive part of the pan does the job, you are done.
I think you don’t need to wrap the whole pan with mumetal if you use that. Quite possibly you only need to do a small part of the pan where it is most sensitive, near the output transducer.

For sure placing a piece of ferrous sheet metal above the pan is worth a try.

[Bombacaototal]

Thanks for chiming in Martin.

Would this ferrous sheet be a suitable candidate?

https://www.amazon.co.uk/adhesive-plain ... B00753M2F6

[martin manning]

I was thinking any old piece of sheet steel you might have handy to see if there is any effect.

[rootz]

I was thinking any old piece of sheet steel you might have handy to see if there is any effect.

Yeah, cheap and readily available. Make a cover to encapsulate the PT, you could bend something like a u-shape.

Not sure about the deep switch. Sounds like there is some continuity between the toggle and the middle lugs of the switch. Strange.

[Bombacaototal]

I was thinking any old piece of sheet steel you might have handy to see if there is any effect.

I unfortunately don't have anything handy. Decided to buy a Sheet of Flexible Sintered Ferrite (EMI Shielding, Square) 120 mm x 120 mm x 0.2 mm. Will be here tomorrow. I will report back.

I am also planning on taking the MuMetal to try to make it as quiet as I can. Just awaiting for the UK supplier to reply.

As far as the Deep switch I will convert it to Dumble wiring.

[rootz]

Still don’t see anything wrong with the deep switch wiring. Schematics and layout of the pre 1984 #124 differ. I wire it the way you did: no problems at all. Hence, I think it’s the switch that is the problem.

[Guy77]

I have also wired many of these Deep switches as shown in your layout and have not had any issues with noise. Maybe a switch issue or a solder needs re-flowing somewhere in the tone stack or a grounding issue or maybe some of the components are moving slightly when the switch is getting flipped.

Otherwise nice looking build.

Happy Holidays!

Guy

[Bombacaototal]

That sounds very much like coupling between the power transformer and one of the transducers of the reverb pan. Some guy over at diyaudio greatly reduced the coupling by adding steel shielding around the power transformer. Your practice of mounting the return side furthest away from the PT is already optimal. I think copper foil didn't help because it does not shield against magnetic fields affectively, like mu metal or permalloy would.

I'm pretty new to magnetic shielding, but google returned countless possible solutions. Some Blues Juniors even have a kind of zinc coated metal wing next to the PT.

I see mu metal sheets are available at eBay. https://www.ebay.nl/itm/151912090773 Not cheap, but $22,95 might be well spend. Anyone got experience with shielding with mu metal tape?

bought this mu metal sheet as well...lets hope either will do the trick

[Bombacaototal]

The amp is picking up some weird interference from time to time. I have the reverb at zero, so this is something else. Could it be a decoupling cap leaking DC, a bad choke, bad Power Tubes?

It seems to stop after the amp is on for a few minutes. Then it will settle but maybe 20min after it will happen again...

[erwin_ve]

The amp is picking up some weird interference from time to time. I have the reverb at zero, so this is something else. Could it be a decoupling cap leaking DC, a bad choke, bad Power Tubes?

It seems to stop after the amp is on for a few minutes. Then it will settle but maybe 20min after it will happen again...

Could be a lot of things, did you go with a chopstick through the connections?
When it happens, check the B+ voltage. When it goes up during the interference it might be a bad powertube.

[Bombacaototal]

The amp is picking up some weird interference from time to time. I have the reverb at zero, so this is something else. Could it be a decoupling cap leaking DC, a bad choke, bad Power Tubes?

It seems to stop after the amp is on for a few minutes. Then it will settle but maybe 20min after it will happen again...

Could be a lot of things, did you go with a chopstick through the connections?
When it happens, check the B+ voltage. When it goes up during the interference it might be a bad powertube.

Thanks Erwin. I am very inclined to think it is a power tube.

I will remove a couple from another amp and try replacing them one by one on this one to see the results.

I am using 6L6WGC-STR Blackplate GE-Style from TAD. Does anyone have a current production favourite?