Fender Blues Jr reverb noise.

[MLW9110]

I have two Fender Blues Jr tube amps. Both are the green board USA made version from about 2000. ss

Amp #1 is stock except it had a newer Accutron spring reverb tank in it. The reverb was very noisy when turned up. I discovered that unscrewing the tank from the cabinet and moving it away reduced the hum a lot. I put a new MOD reverb tank in and the reverb noise is minimal now.

Amp #2 I bought non-functioning. It was blowing fuses. I found the power transformer was bad. I replaced it with a new OEM type transformer and the amp works good. The reverb tank was bad, one of the coils had fallen off inside. I put the reverb tank from Amp #1 in and it was too noisy. I've got a new MOD tank coming for Amp #2.

I assume the reverb tank noise is caused by it picking up RFI generated by the amp. I'm curious about why the MOD tank is less affected than the Accutronics tank though.

Has anyone encountered a similar situation?

 

[RoadrunnerOZ]

This might be whether the input is insulated or grounded on the tank. The Blues Junior should have insulated input tank according to the schematic. What models of tanks are you using?

 

[MLW9110]

This might be whether the input is insulated or grounded on the tank. The Blues Junior should have insulated input tank according to the schematic. What models of tanks are you using?

The noisy tank is Accutronics/Belton model 28055.
The bad tank is Accutronics 8EB2C1B (Fender OEM)
Both have an insulated input jack, output jack is grounded to tank chassis.
The MOD tank is model P-RMOD-8EB2C1B (Fender equivalent).
Mike

 

[RoadrunnerOZ]

The noisy tank is Accutronics/Belton model 28055.
The bad tank is Accutronics 8EB2C1B (Fender OEM)
Both have an insulated input jack, output jack is grounded to tank chassis.
The MOD tank is model P-RMOD-8EB2C1B (Fender equivalent).
Mike

Is there grounded shield foil under the tank base mount?

 

[MLW9110]

No.

 

[CJ]

Try and eliminate outsider issues, like cables or oxidised RCA jacks,

Orientation can make a difference but I bet you installed them the same way,

New power transformer might radiate more stuff. Did you hook up the mains the same way? Sometimes swapping the primary wires will make a difference as unbalanced capacitance is a possibility. But that is usually a subtle difference.

 

[RoadrunnerOZ]

Are the leads long enough to reverse the orientation of the tank?
RFI does not sound like your problem if you have hum. Most likely induced magnetic field in the tank case from your transformer, feeding thru to the pickup coil.

 

[MLW9110]

I've replaced the Accutronics tank with a new MOD tank in Amp #2. The MOD tank is quieter.
Both Blues Jr amps have the same hum. Hum level is directly affected by the Reverb knob. It does not get louder with changes in the Master or Volume knobs. It would be nice if the hum could be filtered out somehow.

 

[RoadrunnerOZ]

I've replaced the Accutronics tank with a new MOD tank in Amp #2. The MOD tank is quieter.
Both Blues Jr amps have the same hum. Hum level is directly affected by the Reverb knob. It does not get louder with changes in the Master or Volume knobs. It would be nice if the hum could be filtered out somehow.

In the later Blues Junior Rev D, the Reverb is fed to the input of the 12AX7 V3 from the reverb level control after the Master level control. In earlier version it is fed in before the Master.
The hum comes from the tank - it’s worth moving it around to see if you can find a quieter spot. A separate MU metal shield mounted over the top of the tank (or the transformer) may help.

 

[RoadrunnerOZ]

Note: In the later editions the reverb input comes from post-Master level control and has no Master control over return level - only the Reverb control itself, hence master having no control over hum if you alter that control.
If you put a high pass filter you will lose bottom end from the reverb but it may be a solution. Check your cable routing as well as tank position - quite often there’s a null point position wise where the hum is minimised. Also make sure the RCA connectors are not those nasty little plastic jacketed connectors, but have full metal body shield.
I have had success with metal shielding over the top of the tank - just a folded mild steel or MU metal plate over the top and grounded to the amp chassis.