Reverb hum - pedal related

[osing]

Howdy,

I have a Hoffman AB763 Super Reverb that I built after gutting my reissue SR. There is a hum present when the reverb/vibrato pedal is connected. The hum disappears when the reverb is turned to zero and increases as I turn the reverb dial up. The hum also disappears if I disconnect the pedal. Further, the hum also goes away if I hold/touch the metal casing of the reverb/vibrato pedal, so I assume it must be grounding related. I opened the pedal and the plug, and connections seem ok (see photos). The pedal jack itself is simply grounded by being mechanically connected to the chassis. Could the hum somehow be related to the jack? Do I need to isolate the jack and connect it to the preamp ground? Has anyone else experienced this?

Before I realized this hum was somehow related to the pedal, I moved the ground of the 220k resistor that used to go to ground at the pedal jack to the cathode of V4a (I believe I had read on the forum that in some cases that might eliminate reverb related hum). You'll see this wire in the photo below if you locate the 220k resistor.

Thoughts?

[sluckey]

Remove that 220KΩ resistor from the FS jack and mount it directly on the reverb jack between hot and ground terminals. That should leave you with just 2 white wires on the FS jack, one for reverb and one for tremolo. Now switch those two white wires. Is that better?

Your footswitch cable has one shielded wire and one unshielded wire. The reverb must use the shielded wire. Trem doesn't have to be shielded. See pic...

[osing]

Thanks Steve! I moved the resistor and that seems to have helped quite a bit (see photo). There is perhaps a little hum still, but at the reverb level I play at (less than 3) it seems to be insignificant. I checked the shielded wire in the pedal, and it was properly wired for reverb whereas the unshielded one was for the vibrato as you had indicated. Just out of curiosity, are there any logical further steps to eliminate the noise altogether, such as isolating the reverb jacks?

On a separate note, when the reverb/vibrato pedal is disconnected, the reverb defaults to being on as does the vibrato. I prefer that being the case for the reverb, but not for the vibrato. Is there a way to wire the vibrato so that it defaults to being off when there is no pedal connected?

Thanks!

[eleventeen]

"Is there a way to wire the vibrato so that it defaults to being off when there is no pedal connected?"

Can't do that with an RCA jack.

You could do it with a switching 1/4" phone jack wired much like an input jack: No plug inserted, hot is shorted to ground. On the cable coming from the pedal, you either change the connector on the trem side or add a female-RCA to male 1/4" adapter on the RCA male plug.

You could also do this by making an RCA shorting plug that stays in the chassis-mount jack until you are ready to switch the trem on. That would be my choice.

[sluckey]

Is there a way to wire the vibrato so that it defaults to being off when there is no pedal connected?

Yes. Just wire the tremolo oscillator footswitch like a real AB763. Look at the pic below. The original circuit is the way your amp is wired now. The modified circuit footswitch operates like the AB763. The switch must be closed for the oscillator to work. Open the switch or unplug the footswitch and the oscillator is biased off by the negative bias voltage applied thru the 2.2M and 1M to the grid of the oscillator. This simple mod will work with any kind of footswitch jack.

Perhaps an even simpler method would be to replace your stereo 1/4" phone jack with a Cliff style stereo jack and wire the switch to ground the trem wire when the plug is removed. Look at Doug's "6 Solder lug Marshall jack" on this page...

      http://www.hoffmanamps.com/MyStore/perlshop.cgi?ACTION=enter&template&thispage=JacksPlugs&ORDER_ID=!ORDERID!

[silverfox]

I don't know if in this case the shielded wire should be connected at both ends. From what I recall the shielded wire should only be connected at one end or you end up with a ground loop.

Unless the shield is being used as a circuit ground also. If so, perhaps a cap on the shield to earth ground should be used to filter AC hum to ground. Need other opinion on this...

Silverfox.

[HotBluePlates]

I don't know if in this case the shielded wire should be connected at both ends. From what I recall the shielded wire should only be connected at one end or you end up with a ground loop.

The footswitch don't work like that. It doesn't carry a signal, per se.

Instead, the trem switch is connecting a wire to ground (originally to enable the oscillator to work). On the reverb side, the switch kills the reverb signal by grounding the grid of the reverb recovery amp. But since that wire is connected to a live tube grid when the switch is open (not grounding), the wire needs to be shielded to avoid hum pickup.

It's fine to attach the shield wire to the case of the pedal and the plug, because these should be viewed not as "ground" but as continuation of the metal shield.

It just so happens the shield is also grounded at the chassis end. The metal switch housing doesn't count, because it is only "grounded" by virute of the connection through the cord shield back to the chassis and "real ground".

But doesn't the footswitch sit on "ground"?

Yes, but is has a plastic bottom, and "ground" of the floor might be a slightly different potential than wherever the ground spike is that is ultimately conencted to the ground buss at the service panel, and on to the amp's 3rd power cord wire.

[osing]

Thanks everyone for your suggestions and input! I might go with the new jack for the foot switch option but will need to order one from Doug first. Since the 6 Lug Marshall jack is plastic and is isolated from the chassis, would it make sense to run a wire from it to ground by the cathode of the reverb recovery tube (V4a), or is that not the optimal location for ground?

[sluckey]

I would first ground the jack to the reverb jack just because it's so close. If that's not satisfactory then move the ground connection to V4A ground point.

[osing]

Happy New Year folks!

Sluckey - in thinking some more about the 6 lug jack, I am trying to envision how I would wire it so that reverb would be on and vibrato off when the foot switch is not plugged in.  I have never seen one of the 6 lug jacks in person, but I found the following switching diagram which I assume also represents that switch in Doug's store as it is also for a 6 lug Cliff jack:

http://www.tubesandmore.com/sites/default/files/assoc_files/s-h554_switching_diagram.png

Here's what I think would need to be done if I am understanding everything correctly:

1. Solder sleeve/ground lug to reverb jack ground since the 6 lug jack is insulated from the chassis. No need to solder anything to the switched sleeve lug
2. Solder vibrato wire to ring lug. Solder a wire from the switching lug of the ring to the ground/sleeve lug. This would short the vibrato when nothing is plugged in and thus vibrato would be off, which is what I want.
3. Solder reverb wire to tip lug. No need to solder anything to the switched lug. This way, reverb will be on when the foot switch is not plugged in.

Does this sound right?

Thanks! 

[sluckey]

Does this sound right?

Yes. Kinda like this...

[sluckey]

I'm curious as to why you want the tremolo to default to being off when the footswitch is unplugged? I've always preferred it to default to on and thought it was slightly annoying to have to plug in the footswitch on an AB763 (or similar) just to use tremolo.

[HotBluePlates]

Even more so, I always thought the Trem Depth pot on the first-version Tremolux made the most sense. Like a Champ volume pot, when you turn it all the way to zero, you flip a switch that kills the trem oscillator.

Dunno why they changed to having that switch on the Speed pot in later versions of that amp, and then dropped it altogether.

[osing]

To answer your question as to why I want the tremolo to default to being off...I don't find myself using the tremolo all too often, whereas I do pretty much always play with reverb. By negating the need to have the foot switch plugged in in order to turn off the trem, I can thus leave my amp set up without the pedal always plugged in.

Having a pot with a built in switch to turn off the trem when turned to zero would probably be the best of both worlds!