Reverb tank hum help please :)

[jimboyogi]

Hi again!

I'm still wrapping up a build (you folks have already helped me out with a trem switching problem thanks).
But now my final issue with the build is related to the reverb.

I breadboarded the amp, with the tank a distance away from all of the electrified things on my workbench. In this format the reverb added very little noise, some hiss and hum at full setting (especially when the soldering iron was switched on). But a reasonable amount of noise I thought due to the gain required on the recovery side.

But now with the tank in the cab, the hum level is too loud. I have tried moving the tank around in relation to the amp chassis and speaker, and the noise level is definitely related to the proximity of the tank output to the PT especially. The PT is a toroidal from Antek. I assumed that a toroid would have less magnetic leakage than a EI, but even with the tank output side at the diagonally opposite side of the cab (40 cm away), the noise pickup is still too great.

I have searched previous threads on this issue on this forum, and that gave me a few things to test.
First I used another tank to cover the open side of the plugged in tank, to completely shield the transducers in steel. This didn't appear to change the noise level at all.
I used aluminium foil to shield the underside of the tank, and made sure that the foil was electrically connected to the tank ground. This didn't help either.
I have unplugged the tank, and switched the reverb on and off, to check if the noise is coming from elsewhere. It's almost silent with tank unplugged.
I have tried touching the tank to the chassis, to make sure that the tank is properly grounded. No change.

The only way I have found to reduce the noise is to move the tank away from the amp.

Do I need to look at mu metals, or is there more things that I can try first?

Thanks for any all assistance. I really want to get this amp finished!

[g1]

I've sometimes seen amps with a round metal shielding band around the toroid.
"With proper design, many of the stray magnetic fields can be reduced, or eliminated. In these cases where proper design does not reduce the fields enough, magnetic shielding can be applied around the toroid in the form of a high-permeability metal band. This can be Silicon Steel for the majority of cases or Mu Metal for more sensitive applications. For extremely sensitive circuits, total encapsulation in a steel can or case may be the only option." (Power Toroid Checklist | Precision, Inc.).
You have been trying to shield the tank, but I think you will have to shield the transformer.

[jimboyogi]

Thanks g-one for the advice. I had a look at the Antek website, and they do sell steel cases to suit their toroids. So that could def. be something to try.

After reading your post, I went back and tried some things again. The house is much quieter today than yesterday, so I was able to listen more carefully to what was affecting the noise pickup.

I tried using another reverb pan to cover the open side of the live pan again. It did make a small difference to the hum/buzz.
I also tried moving the tank around some more. Unfortunately the PT and OT are near to each other on the chassis, so it is very difficult to tell if there was noise pickup from the OT as well as the PT. But def. the noise is coming from one/both of the transformers. No pickup from the speaker.

Assuming the noise is being caused by magnetic interference from the transformer/s coupling into the tank output transducer. The reverb pan is made from bent steel. Steel offers some magnetic shielding.
If I cover the open side of the pan with a steel plate, then the transducers would be completely surrounded by steel. How would this differ from enclosing the PT within a steel cover?

[Steve Conner]

Have you tried turning the PT around? Toroidal transformers leak more flux from the spot where the wires come out, because the symmetry of the windings is upset there. Douglas Self recommended leaving extra wire length so the PT could be rotated to find the point of minimum hum.

Also make sure the heater circuit wires are tightly twisted. They carry a heavy AC current and any loop area will radiate a strong magnetic field.

What you're seeing isn't that out of the ordinary, the noise floor of the reverb is often limited by hum. Can you drive it any harder?

[jimboyogi]

Thanks for your suggestions and questions Steve,

First up there isn't enough wire length to rotate the toroid, the B+ wiring just reaches as it is. My fault for spacing everything out so much inside the chassis.

The heater windings have a tight twist both outside and inside the chassis.

Re. driving the tank harder. This driver circuit is a cap coupled transformer-less one. Uses 1/2 of a 12AU7 to drive. I am at the limit of how much signal I can hit the 12AU7 with before it starts to distort heavily, which doesn't sound very good.
I have tried modifying the circuit to use both 12AU7 triodes in parallel, so as to supply twice the drive current to the tank. But doing this just seemed to overload the tank, making the reverb more splashy rather than louder. And at full reverb using just 1/2 the 12AU7, it is quite splashy as well. So I have assumed that I am getting sufficient drive current to saturate the input transducer using just the one 12AU7 triode. Could be wrong though!

I'm still curious about the steel shielding around the reverb pan, and whether there is any benefit to shield the PT if the pan is completely surrounded in steel (steel pan plus steel plate over the open side). Especially when the only hum pickup in the amp is via the reverb.

The other thing is that I may have unreasonable expectations about how little hum the reverb circuit can provide. It's just that with the reverb knob at say 1/2 rotation, when switching the reverb on the hum noise floor increases substantially.

[Bruce / Mission Amps]

What is the reverb tank number? You might be using the wrong with with respect to the tank's chassis.... there is a difference in the way the RCA jacks are grounded.

[jimboyogi]

Thanks Bruce,

The tank is an Accutronics/Belton, #N9FB3C1B. Only the output transducer RCA jack is grounded to the tank chassis. In the amp chassis the reverb tank and lead shield is grounded to the main pre-amp ground, at the V1 PS cap ground. The reverb recovery triode is 1/2 of V1 as well.

So the reverb tank chassis and lead (both send and return shield) are grounded only at the recovery triode PS ground node.

[loudthud]

There was a thread recently where the tank did'nt match the numbers stamped on it. Check the DC resistance of the coils and compare to what specifications you can find.

[jimboyogi]

Hi loudthud,

I measured ~250 ohms dc resistance on each coil, input and output. The Accutronics website lists the dc resistance of type F input coil as 200 ohm, and of type B output coil as 200 ohm also.

[jimboyogi]

Just now I finally got back to the amp build to do some more testing.

First up I put my scope on the speaker output. No input to the amp (so V1a grid grounded). Tone controls at 1/2, gain and master volume at full, reverb at full. I wanted to test what frequency the noise signal being picked up by the reverb output is. The signal being picked up by the scope had a range of different frequencies in it, and the scope had trouble triggering on just 1 of these. The best freq. measure I could get was jumping between 50 Hz and 100Hz, which is very logical, as I am in Australia where the power is 50Hz. So this noise signal could be coming from the mains (50 Hz), the heaters (50Hz), the rectified HT (100 Hz), either inside the chassis, or radiated from the PT.

I tried moving the reverb pan around in relation to the chassis and PT. The noise was def. stronger when closer to the PT, but even when moving the pan to the end of the reverb cable outside of the chassis, the noise was still present. The noise sounds more like a 'buzz' than a pure 'hum' also.

Next I tried unplugging the reverb input from the pan. The noise signal level measured on the scope was essentially unchanged. So the hum is not coming from the reverb send circuit.

Next I tried using a wire loop to short the pan output at the pan RCA jack. When shorted, the noise was gone. So if I have got this correctly, that means that the noise is not coming from a grounding issue, as using this ground scheme and cabling, there is no noise when the output transducer is shorted?

I also tried unplugging the reverb cable from the chassis jacks, and switching the reverb footswitch on and off. There is a very small amount of hiss present when the reverb is switched on, but fairly negligible. So I also think that this tells me that moving the reverb switching from the reverb return stage grid grounding, to grounding the output of this stage, could only remove at most this hiss.

When the reverb was switched off, I took a screen shot of the scope. I'm curious as to what the big, high frequency spikes are, what's causing them, and if they are a problem that need fixing.

[loudthud]

Pulses that short are probably solid state diode snap-off. If you are using 1N4007's try to find some UF4007's.

[jimboyogi]

Thanks loudthud.

Just ordered some UF4007s. Will be interesting to compare the scope shot after swapping them in place of the 1N4007s in the amp ATM.

[loudthud]

The torroid transformer probably exacerbates the situation. One typical cure is a small damped inductor in series with each diode. Typically made with several turns of wire wound around a low value resistor, or a magic ferrite bead slipped over the diode's lead. These things can actually be tuned to the transformer. John Camille (now deceased) wrote an article on how to do it in Sound Practices magizine issue #7. The article was not copyrighted so it should be online somewhere.

[jimboyogi]

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.


I have also bolted a steel plate onto the pan to completely cover the open side. Not sure how much difference it makes, but I figure it can't hurt.
So now the reverb pan output transducer is completely surrounded by plate steel, which is electrically grounded solely to the recovery triode PS ground.

Will there be any further advantage to getting a PT steel cover?

[ReadyTeddy]

I was able to get rid of reverb hum in one of my builds (solid-state amp with toroidal PT, BTW) by playing around with placing some fairly small pieces of mu metal on the reverb tank itself. By small, I mean quite a bit smaller than the tank itself. The mu metal was pretty thin--not like foil, but no more than .020"--I could cut it with scissors. If you can scare up some mu metal try moving some pieces around by hand, placing them here and there--you may just find a couple of spots that make a big difference. In general, I think one can do a better job of driving the send coil in the tank with solid-state devices rather than tubes, but that's not to say a good implementation can't be done with tubes--it's just bigger and heavier. This site has some good info on reverb-tank drive and recovery circuitry, spring reverb info page