Reverb hummmmmm

[jaret]

Hello,

I'm hoping (pleading?) that someone can help me with my build. It's a 2x6V6GT, 3x12AX7 build with reverb and tremolo and it's working fine except for a low, loud hum when the reverb knob gets turned up above zero. The hum is present even when the reverb tank and V2 (reverb driver & recovery) are disconnected. It gets louder as the reverb is turned up but the volume pot has no effect on it. The reverb signal itself sounds much quieter than it used to (before I rebuilt this amp from a Princeton Reverb-minus-tremolo clone).

Here's the schematic:
http://tinyurl.com/263aq26

I've chopsticked the lead dress with no change/improvement. I unmounted the reverb tranny and moved it around (as much as I could) with no change. Grounding out the grid of V1B or either reverb pot terminal kills the hum completely. I think the problem is DC on the grid of V1B. I'm measuring about 18mV on its grid, even with V1 & V2 unplugged. This voltage drops across the 220k to the reverb pot and is negligable across the 3.3M, so I'm sure it's originating on the V1B grid wire. I'm trying to understand how this is possible! The 4.7n coupling cap on the reverb out is not leaking (I replaced it just to be sure). I thought the grid wire was capacitatively coupling to a B+ line or something, so I replaced it with shielded wire (grounded only at one end)- same problem. The amp is DEAD quiet with the reverb turned all the way down so I don't think it's bad filtering (just tried adding an extra 40uf onto the stage- no luck).

Could it be a conductive tube socket?

That has exhausted all my ideas (and me) so I'm hoping someone with more experience (most of you, probably!) has a better idea.

Thanks for your help and apologies for the lengthy post!

Jaret

[statorvane]

Your link points to a Google Docs website?

[jaret]

Sorry! Try this one:

https://docs.google.com/leaf?id=0B5eUEw ... ey=CKut4jQ

[martin manning]

Can't see your set-up, but if you have chassis-mounted jacks for the reverb tank send-return, are they isolated from the chassis?

[jaret]

Yes, they're both isolated. I have both rings of the RCA jacks grounded to the chassis along with the -ve end of the RT secondary, though. Since the reverb tray I'm using has both rings connected (through the tray chassis, I presume), does this create a big ol ground loop? I'm guessing yes...

[martin manning]

I don't think you want the jacks grounded to the chassis (that's what is meant by "isolated"). You are probably right that the shield is continuously connected through the tank, so you should only need to ground one RCA jack's shield. Isolate the jacks from the chassis, and run a wire from the place that the filter cap for the reverb power supply node is grounded to one reverb jack's shield lug.

[jaret]

What I meant was, the RCA jacks are themselves isolated but I had run ground wires from both rings to ground, defeating the purpose of isolated jacks! Thank you for your suggestion. However, I'm using a 4x20uf multican filter cap, which has a common ground for all stages. I know this isn't ideal for the kind of grounding scheme you describe, but I'm guessing it'll be an improvement over my 'ground it wherever you feel like it' approach

[martin manning]

My wording on the ground location was carefully considered, meaning however you have that filter grounded, run this reverb loop ground to that same place. I hope it works!

[M Fowler]

Run shielded wire to the reverb pot and may need to shield other reverb wires.

I have built several amps with the RCA jacks bolted to the chassis and no low level hum after I used shielded wire.

Mark

[tubeswell]

Hi jaret

Can't see your amp pics (I can't remember my gmail password to access the google site, and can't be bothered filling out the gazillion questions needed in order to get a new password sent). Can you post pics here?

[jaret]

Sorry again! I've attached it this time.

Mark- I'm already using shielded wire for the return line from the RCA to the grid of V2B. The 220k resistor between V2B and V1B makes an unshielded run of about 2 inches from the reverb pot to the 3.3M junction, where shielded wire is used to the V1B grid. Neither side of the reverb pot is shielded, but the 220k and 4.7n are soldered directly onto it and make short runs to the turret board.

[M Fowler]

Jaret, Between what you reported and Martin's comments I don't have any answers for you, sorry.

Grounding out the grid of V1B or either reverb pot terminal kills the hum completely

In your situation would grounding the pot with the rest of the reverb circuit ground path work? Isolate the pot from the chassis?

It would be hard to wire it any differently since you already have short runs.



Mark

[tubeswell]

Grounding out the grid of V1B or either reverb pot terminal kills the hum completely.

Try a new pot there. Could be the pot you have has been badly soldered in and is a leaking liability. I had this sort of issue once with a presence pot in a bassman - chased my tail in circles until I replaced the pot and the problem went away.

[martin manning]

Where is the ground for V1B's cathode? What if you ground the reverb jack shield there?

[jaret]

Ok, I've isolated the rca shields and grounded one of them with the RT ground at the common cathode ground for all preamp cathode and grid leak grounds, which is then earthed to the chassis. This ground point also collects the pot grounds (Trem depth>Trem freq>Reverb>Volume>Input Jack>Ground). I used this grounding scheme because the layout on the turret board was constrained for space, not because I thought it was optimal.

This common earth, unfortunately, is on the other side of the chassis from the common earth connection for the four filter stages. Would it be better to run a wire from this common preamp ground across to the filter earth connection?

I'm still getting the hum although the reverb wet signal sounds much stronger. I guess the other option would be to try replacing the reverb pot, as tubeswell suggests.

Thanks to all of you for your continuing help. I really appreciate it!