noise from reverb tank in amplifier head

[pula58]

I have a noise (hum) problem in an AB763 (Fender blackface/silverface)type of head I built. When the reverb mix is turned-up I get the hum. If the send to the reverb tank is disconnected (from the back of the chassis (RCA plug)) it make no difference whatsoever. If I unplug the reverb tank return cable from the RCA jack on the back of the chassis the hum goes away. If I bring the tank out of the head and move it far from the chassis(as the cable length will allow) the noise diminishes. The reverb cables are the vintage (but new) type of reverb cable, RCA jacks, outer conductor is the shield.

The RCA jacks on the tank are facing forward, towards the front of the cab. The chassis is a re-worked Fender Bassman chassis that I gutted and completely re-wired to be a single channel Blackface type of circuit with reverb. See post:

https://tubeamparchive.com/viewtopic.ph ... highlight=

So., O.K. reverb tank close to transformers..right? Well, what can ya do about it? Your thoughts/techniques ?

[txbluesboy]

Try rotating the tank left to right. You want the return jack farthest away from the power transformer.

[brentm]

It sounds like the output (send) jack on the reverb tank is as far as it can be if the RCA jacks are facing toward the front of the cabinet.

Standard disclaimer. I'm no expert, others will have better ideas for you and more experience. But I've been chasing this exact problem on an amp. Here's what I've done with some success.

Use aluminum tape to shield the bottom of the board (or cardboard) protecting the reverb tank springs. Ensure continuity between the shield and the reverb tank (I used an additional screw).

On my particular tank, only the output RCA jack is grounded. Do you have a possible ground loop (input is grounded too)?

I've also purchased heavier duty shielded RCA cables, which seemed to have helped the most with reducing the hum. Shielding the bottom of the tank didn't seem to do much for me. Neither did a ferrite bead.

Best R/A RCA cables I could find in-town were HOSA cables. Next steps for me will be to make some custom cables. I'd thought about using some quality instrument cable first and then some RG6 quad shield. Obviously the magnetic interference inside the amp is strong enough to hit the signal wire either through the cable or the through the tank itself.

[selloutrr]

I have a noise (hum) problem in an AB763 (Fender blackface/silverface)type of head I built. When the reverb mix is turned-up I get the hum. If the send to the reverb tank is disconnected (fro mthe back of the chassis (RCS plug)) it make no difference whatsoever. If I unplug the reverb tank return cable from the RCA jack on the back of the chassis the hum goes away. If I bring the tank out of the head and move it far from the chassis(as the cable length wil allow) the noise diminishes. The reverb cables are the vintage (but new) type of reverb cable, RCA jacks, outer conductor is the shield.

The RCA jack on the tank are facing forward, towards the fron of the cab. Th echassis is a re-worked Fender Bassman chassis that I gutted and completely re-wired to be a single channel Blackface type of circuit with reverb. See post:

https://tubeamparchive.com/viewtopic.ph ... highlight=

So., O.K. reverb tank close to transformers..right? Well, what can ya do about it? Your thoughts/techniques ?

do you have the reverb tank mounted inside the bag? this will help shield it from the amp. what style of RCA cables are you using? have you confirmed they are not shorted or faulty. a possibly break in the shield that gets better when the tank is out and the cable is at a certain angle?
have to tried a different tube, some are quieter then others. Are you driving the tank to hard? is it the correct impedance for your application?
does your amp have a ground loop? how is the amp with the reverb out of the circuit?.. breaking the return would cause the amp to be quiet as you have broken the signal chain from completing to the speaker. has the amp ever worked properly? if so what have you done since to create the problem?, work backwards and find the solution. Have you checked continuity to ground? are you sure your grounds to chassis are solid. have you metered them for Ohms? Are you sure your power source is not dirty. i.e. Florencent lights, microwaves, ceiling fans, etc.

[pula58]

the tank is in a bag (but bag is vinyl, I doubt that would affect susceptability to hum at all).

the tank is a 4AB3C1B - so, the tank chassis is connected to the shield of the output jack of the tank. I checked this with an ometer to verify (plus I looked inside the tank and you can see that the tanks chassis is connected to the output jack shield connection).
Because of the tanks jacks facing forward, the output jack is as far away from the power tranny as possible.

I have the amp chassis on an amp bench. So I can move the tank around: I can move it under the chassis (like it is when the head is all put together with tank, chassis, etc) and move it awy as far as a foot or so. Huge difference in reverb noise. I could see how a combo style amp would be probably fine...but...how to silence this as a head is something I really wanna do..

I tried moving the ground of the reverb return 200k resistor to the ground point of the reverb return triode's cathode bypass capacitor. It had no effect. If I short the reverb return jack on the back of the amp (with tank disconnected) the amp gets much quieter.

[selloutrr]

Have you tried it mounted or only on the bench? Take a piece of plywood or scrap wood wrap aluminum foil over the surface and put it over the exposed springs of the reverb tank. It should quiet down.

[TUBEDUDE]

You might try isolating the signal wires from the transducer(s), then making custom cables from sheilded twinlead. The two internal conductors to the hot and return sides of the transducers to the in/out jacks, and the braid/foil from the chassis to the reverb pan as the sheild to ground. This isn't too hard if your reverb pan connections are already isolated. It might require a little drilling and a screw and insulator otherwise.

[renshen1957]

the tank is in a bag (but bag is vinyl, I doubt that would affect susceptability to hum at all).

the tank is a 4AB3C1B - so, the tank chassis is connected to the shield of the output jack of the tank. I checked this with an ometer to verify (plus I looked inside the tank and you can see that the tanks chassis is connected to the output jack shield connection).
Because of the tanks jacks facing forward, the output jack is as far away from the power tranny as possible.

I have the amp chassis on an amp bench. So I can move the tank around: I can move it under the chassis (like it is when the head is all put together with tank, chassis, etc) and move it awy as far as a foot or so. Huge difference in reverb noise. I could see how a combo style amp would be probably fine...but...how to silence this as a head is something I really wanna do..

I tried moving the ground of the reverb return 200k resistor to the ground point of the reverb return triode's cathode bypass capacitor. It had no effect. If I short the reverb return jack on the back of the amp (with tank disconnected) the amp gets much quieter.

Hi,

Have you considered using Mu metal to shield the ends and experiment elsewhere?

Are you using a faux-center tap (100 ohm resitors) on the 6.3 volt winding? (If the resistors are open, hum would be most obvious on the Reverb return.)

Best Regards,

Steve

PS FLeaBAy sells Mumetal in sheets, This material excels at screening out magnetic fields and static interference. Sheets have a peel-and-stick adhesive backing to facilitate easy installation. http://www.ebay.com/itm/Ultraperm-80-Me ... 25654e9aa2

[pula58]

the tank is in a bag (but bag is vinyl, I doubt that would affect susceptability to hum at all).

the tank is a 4AB3C1B - so, the tank chassis is connected to the shield of the output jack of the tank. I checked this with an ometer to verify (plus I looked inside the tank and you can see that the tanks chassis is connected to the output jack shield connection).
Because of the tanks jacks facing forward, the output jack is as far away from the power tranny as possible.

I have the amp chassis on an amp bench. So I can move the tank around: I can move it under the chassis (like it is when the head is all put together with tank, chassis, etc) and move it awy as far as a foot or so. Huge difference in reverb noise. I could see how a combo style amp would be probably fine...but...how to silence this as a head is something I really wanna do..

I tried moving the ground of the reverb return 200k resistor to the ground point of the reverb return triode's cathode bypass capacitor. It had no effect. If I short the reverb return jack on the back of the amp (with tank disconnected) the amp gets much quieter.

Hi,

Have you considered using Mu metal to shield the ends and experiment elsewhere?

Are you using a faux-center tap (100 ohm resitors) on the 6.3 volt winding? (If the resistors are open, hum would be most obvious on the Reverb return.)

Best Regards,

Steve

PS FLeaBAy sells Mumetal in sheets, This material excels at screening out magnetic fields and static interference. Sheets have a peel-and-stick adhesive backing to facilitate easy installation. http://www.ebay.com/itm/Ultraperm-80-Me ... 25654e9aa2

Nice idea to check the heater resistors that make a fake center tap. I measure 49.7ohms to chassis for one of them, and 49.9ohms for the other. That sounds about right...unless..is such a small imbalance going to make the reverb noisey?

But the thing is, if I disconnect the tank the noise goes way way down....so, that mu metal is sounding like it might be an interesting thing to try. Maybe a piece on top of the tank?

[selloutrr]

Just to weed out the tank. Have you tried another reverb tank? Or noise tested the reverb tubes? How much reverb comes through the signal versus noise? Is it a 60/120hz or something else?

[renshen1957]

the tank is in a bag (but bag is vinyl, I doubt that would affect susceptability to hum at all).

the tank is a 4AB3C1B - so, the tank chassis is connected to the shield of the output jack of the tank. I checked this with an ometer to verify (plus I looked inside the tank and you can see that the tanks chassis is connected to the output jack shield connection).
Because of the tanks jacks facing forward, the output jack is as far away from the power tranny as possible.

I have the amp chassis on an amp bench. So I can move the tank around: I can move it under the chassis (like it is when the head is all put together with tank, chassis, etc) and move it awy as far as a foot or so. Huge difference in reverb noise. I could see how a combo style amp would be probably fine...but...how to silence this as a head is something I really wanna do..

I tried moving the ground of the reverb return 200k resistor to the ground point of the reverb return triode's cathode bypass capacitor. It had no effect. If I short the reverb return jack on the back of the amp (with tank disconnected) the amp gets much quieter.

Hi,

Have you considered using Mu metal to shield the ends and experiment elsewhere?

Are you using a faux-center tap (100 ohm resitors) on the 6.3 volt winding? (If the resistors are open, hum would be most obvious on the Reverb return.)

Best Regards,

Steve

PS FLeaBAy sells Mumetal in sheets, This material excels at screening out magnetic fields and static interference. Sheets have a peel-and-stick adhesive backing to facilitate easy installation. http://www.ebay.com/itm/Ultraperm-80-Me ... 25654e9aa2

Nice idea to check the heater resistors that make a fake center tap. I measure 49.7ohms to chassis for one of them, and 49.9ohms for the other. That sounds about right...unless..is such a small imbalance going to make the reverb noisey?

But the thing is, if I disconnect the tank the noise goes way way down....so, that mu metal is sounding like it might be an interesting thing to try. Maybe a piece on top of the tank?

Hi,

There would be hum if one or both of the resistors had opened up, which is not the case.

Usually both of the ends have Mu Metal attached makes an impact one end more so than the other, but your mileage will vary. Other placements on the tank have merit; it is basically trial and success. In some DIY Hi Fi builds one finds the transformers (PT and OT) are encased in mu metal.

selloutrr recommendations are worth investigation. Hum can be induced from a variety of sources. Since your amp is based on a AB763 (Fender blackface/silverface) multiple capacitors in cap cans aren't a potential problem.

Best Regards,

Steve

PS, Your test area isn't anywhere near a fluorescent lamp (induced dirt in the line) or a bunch of electronics plugged into the same circuit (ground loops) I have worked on amps and guitars plugged into amps that an old ballast would induce hum when the amp (or guitar) was turned up. Moving the amp or unplugging other equipment solved the hum problem.

[pula58]

I tried two other tanks - same problem.

No flourescent lights. And, the hum shows up whenever and wherever I play the amp.

it's 60 Hz....

If I short the reverb return jack (the one that feeds the grid of the reverb recovery circuit) all the hum goes away.

If is definately the tank picking-up noise.

I also shorted the end of the reverb return cable at the tank and moved it around. All the hum went away.

The power tranny, BTW, is a hammond 291EX (Blackface/Silverface Bassman).

[selloutrr]

Does it get any better if you run a ground wire from the tank to a grounding point on the chassis? Everything hooked up as it would be during correct operation.

[pula58]

I did some experiments.....If I orient the reverb tank so that the bottom of the tank faces forwards, towards the front of the cab, and is mounted on the inside of the front of the cab. The noise diminishes substantially! That is, instead of mounting it horizonbtally, I mount it vertically, with the RCA jacks facing upwards.

Ah yes, now I remeber....magnetic fields...if the time-varying magnetic flux is perpendicular to the loop of conductor, the current in the conductor (induced by the magnetic flux passing through it) is maximized. So, by turning the tank 90 degrees I made the part of the field that is normal (perpendicular) to the tank almost nil!

So, the noise is much better. I have not had time yet to see if the reverb still sounds good. The tank I have is a 4AB3C1B, designed to be mounted with the bottm facing donwards....by having the bottom (open side) of the tank facing forwards I am not sure how the reverb itself might, or might not be affected...but the hum is sure alot better!

Eureka!

[selloutrr]

epoxy the spring clips together at the middle and ends it will help make the signal stronger.