Reverb tank hum help please :)

[loudthud]

Is the 'small damped inductor' fix a better one than using fast recovery diodes? Sounds like more work anyway

The UF4007s were ~ 20c each in a pack of 50, home delivered. So still pretty cheap for a homebrew.

Both fixes were intended to be used at the same time to make the best possible audiophile power supply. I can't predict what will be good enough in your amp. I looked online for the article and didn't find it, so I scanned it and will arrempt to post it here. On the last page I deleted two adverttisements in the right column and pasted the publishers statement that it's ok to reprint.


Looks like it worked!!!

[jimboyogi]

Thanks ReadyTeddy,

This coming weekend I'm taking the amp build over to a friends place, who has had similar troubles to me in the past, and has a collection of mu metal. Will report back as to whether the mu metal can help solve the noise pickup.
And thanks for the link - interesting reading!

[jimboyogi]

Thanks for posting the article loudthud. Very interesting.
I should get the UF4007s into the amp this week, and will be informative (for me) to see what difference they make to the PS and speaker output hash signal.
If necessary I can then try implementing some of the tactics in the article.
I'm also curious to see if the hash is at all involved in the reverb tank noise problem.

[jimboyogi]

New fast diodes not here yet, but I did try another test on the reverb, to make sure the hum/buzz was actually coming from the tank, and not from the recovery gain stage.
The reverb tank output transducer (#B) is listed at nominal 2,575 ohm impedance (at 1 kHz). I put a 2.7 K ohm resistor across a RCA jack. I then unplugged the reverb return cable plug from the tank output, and plugged it into the RCA jack/2K7 resistor. All hum/buzz was gone, just a slight white noise from the gain stage, at a completely acceptable level.

With the tank plugged back in, I also played around with the tank position again, listening. The hum/buzz is definitely radiating from the PT only.

[g1]

The hum/buzz is definitely radiating from the PT only.

Shielding the tank is dealing with the symptom. Shielding the PT is dealing with the cause. The hum from the PT may also be causing low level noise issues elsewhere, which you may not be hearing right now. Shielding the PT will also help with that.

[jimboyogi]

Hi g-one. I understand what you're saying. I'll probably end up purchasing a steel PT cover from Antek. Shipping them from the USA to Australia at least doubles the price, so i am holding off until i have tried some other things first.

I am curious (hopeful to see if cleaning up the PS diode noise will reduce the radiated hum/buzz, as it does seem unusual to be having such a problem otherwise, with ~40 cm distance between the PT and the tank output transducer.

[g1]

I'm not sure what the Antek cover is like, but suggest you try a band around the toroid. You had mentioned experimenting with some mu-metal, so why not try a band around the toroid. Either silicon steel or mu-metal is suggested.
G.O.S.S. Band - Custom Designed Toroidal Transformers from Nuvotem Talema

[jimboyogi]

Does the band need to be earthed/grounded?

Also, here is a link to the Antek covers. I could't work out how to just copy and attach the photo

[Steve Conner]

The main purpose of the steel band is to channel magnetic flux into itself in preference to the air around the the transformer, and it'll do that regardless of whether it's grounded or floating.

When working with mu-metal, bear in mind that cutting, bending or hammering it will destroy the special magnetic properties that you just paid so much money for. To get them back requires annealing at red heat in a hydrogen furnace. Best not to mess with it in the first place

I've also seen toroids enclosed in copper canisters. These function the same as the copper "hum strap" aka "belly band" sometimes seen on EI transformers. The can is highly conductive, so any magnetic flux that tries to pass through it induces eddy currents that tend to cancel the flux out. To be effective, the joint between the lid and the rest of the can needs to be highly conductive too.

These two forms of shielding complement each other. For audiophile-grade shielding, you can apply the steel band and then put the whole lot in a copper can.

[Mick Bailey]

You said you moved the pan around, but did you physically flip the tank around end-to-end?

I use toroids in all my standalone reverb builds without any magnetic screening and without hum problems, though I did one have a defective recovery tube that took a while to figure out (brand new tube, too). Tank position isn't so critical, but input/output orientation is as well as the grounding scheme.

[jimboyogi]

Hi Mick, yes I have tried rotating the transducers in the tank around relative to the chassis/PT. The closer the 'output' transducer is the the PT, the louder the buzz. When the tank is moved to the end of the cable, output transducer ~90 cm away from the PT, then the noise is effectively gone.
Mick what rectification did your toroid builds use?

Last weekend I did some basic trials with mu-metal that an amp building friend has. With the tank in position inside the chassis, we laid one sheet of mu-metal over the top of the tank on the output side. The buzz did reduce audibly, but was not completely gone. In my friends experience however, the only way to make the mu-metal shield fully effective was to completely surround the reverb tank with a mu-metal 'box'.

My UF4007s arrived yesterday, but they will have to wait until the weekend to be tried out, as I need to use the amp tomorrow.

[jimboyogi]

Ok some updates. I received the UF4007 rectifier diodes, and today swapped them in place of the existing 1N4007s.
Before and after I did the swap, I used my scope and took some screen shots. All shots with the same scope adjustments (ie. no change to scope between various shots)

1 - Using 1N4007s. Probe at speaker output jack. Gain and Master volumes on full, Treble and Bass at 1/2, nothing plugged into input jack. Reverb footswitched off (recovery triode grid grounded to it's PS node ground).


2 - As above but with Reverb control at full, and reverb footswitched on.


3 - Using UF4007s. All else as for #1.


4 - Using UF4007s. All else as for #2.


Make of the PS changes what you will

I was hoping that somehow the 'buzz' that I was hearing from the reverb tank was related to RF hash due to using 1N4007s, and that using faster diodes would solve both the PS hash AND the reverb noise. Well................ it didn't solve the reverb noise. Still there.
I have ordered the matched steel transformer cover from Antek to suit the toroid PT in my amp. Will take a while to ship across the world, but when it does, hopefully I'll have a quieter reverb!

[Jazz P Bass]

Reverb set on ten will be noisy.
Fact of life.

[Diablo]

Reverb set on ten will be noisy.
Fact of life.

Is this because the output inductor coil on a reverb tank is like a single-coil guitar pickup, and it's too close to the various AC magnetic fields produced in the rest of the amp? What if they used two coils on the output inductor and put them on opposite legs of the transformer and connected the two coils out of phase as humbuckers?