Hiss... again, but no boos

[jfutral]

My latest build is an odd Vox/Fender blend. Not complete, but I have enough going that I can test various things as I go.

My current (no pun intended.. or not) problem isn't really that huge. My Fender pre-amp channel has hum and hiss. My Vox Top Boost channel is whisper quiet all the way to full open.

I did manage to reduce the hum to _almost_ nothing by re-routing and re-soldering some leads and changing out a capacitor. Oddly enough when I changed out the capacitor (a ceramic 250 pf to the treble) I reduced the hum but increased hiss.

To be honest, the hum and hiss isn't that bad and isn't really noticeable until I turn everything up past, like, 7 or 8. But I don't know why the Fender pre is noisy, but the TB pre is quiet as can be.

The hiss and hum ARE affected by the tone stack and volume, so if I am correct does this mean it is pretty much in the first half of the preamp? The tone stack/volume is standard Fender style with 250pf, .1uf, and .047 capacitors with a 10k mid pot.

Any thoughts? Ideas? Dreams?

Joe

Update: I reconnected the lower hiss capacitor. Now I'm not so sure the hum went down as much as the hiss went up with the replacement cap. I swapped out a plate resistor. That helped some with the hiss. Swapped out 1st stage tubes and nothing made a difference I could hear. Not sure what else to do. The difference between the two channels is still quite striking.

[jfutral]

New symptoms update. I've been reading txstrat's thread hoping to get some insight. Good thread, if you haven't read it yet.

1. I hear the hum with with amp powered up but still in standby. Very slight, but there. Same frequency as the Fender pre-amp hum, but the tone and volume controls can make it louder.

2. For the heck of it I put a meter to the chassis and various ground points. Pretty consistently 1.3-1.5 mVAC at almost all grd points.

I read RG's grounding tute and point 2 says this:

"Remnants of old “line reverse switch” must be totally removed; no cap from the AC line to chassis, either side."

This is an old Dean Markley chassis that was mostly gutted, but the power wiring remains the same. On the 120vAC in, the ground goes to a solo point that has a capacitor which then goes to a "Ground" rocker switch which chooses between the common and hot. Is this the kind of switch RG is talking about? Should I remove this?

Thanks,
Joe

[tubeswell]

Got a schematic which shows the accurate ground wiring?

I find that if I include the ground wiring on my schematics in such a way that it shows the actual schematic rendition of how each ground return is integrated with the relevant filter/decoupling cap node ground return point, it is easier to sort out a good layout which is hum free.

[jfutral]

Here is the schematic and a rough layout of the grounding. I kind of think the hum is really PT related. This is not the actual PT I plan on using, but it was one I had around to get things started while I wait for the Hammond I ordered to arrive. This PT can't handle the additional reverb circuit I plan on adding and I am pretty sure I am pushing it as it is since it was originally used in an old single channel 2-12ax7, 2-6v6 amp.

I think the Fender pre-amp is just amplifying the hum more than the Vox stages. I could be wrong about that. But the hum is present even in standby and with the tubes pulled. When the new PT arrives I guess I'll know for sure.

The preamp circuits go to a wire that runs along the edge of the circuit board (an eyelet board) so all the Fender grounds are behind the Vox grounds on their way to the grounding point.mI did run the filter caps' grounds to their respective stages' grounds. No hog tying this time!

The chassis is an old Dean Markley CD 30 or 40 or some such thing—a 2-6L6 amp. The guts were ripped out and the original iron was rusted and shot. The cabinet is still in good shape and the tube sockets still look clean, although I added two (one for the rectifier and a 9 pin) and will add more for the reverb. Tons of real estate.

Joe

[loudthud]

The low noise of the Vox channel is probably due to a couple if issues. 1) The Vox tone stack is pretty dark sounding, not much treble gets through and 2) the 2nd gain stage with the cathode follower can have practically zero headroom depending on B+. Unless you have a scope to dial it in, change the 1.5K to 820 ohms and the 56K to 100K. A 1 Meg volume pot might add a little brightness or increase the 100pF cap to 220pF or so.

[jfutral]

Sounds like I have something to do this weekend! But does this mean I'll lose the peace and quiet of the Vox channel?

Joe

[jfutral]

Definite boost on the high end. Altered the distortion somewhat, too. Nothing bad, just different. Clean seems a bit meatier, over-drive has more bite. Not as much of a difference as I was thinking. Probably be more of a difference if I played a humbucker guitar. My strat sounds good, though. I was thinking about the 820s already and your suggestion sealed the deal.

There is more hiss, but still considerably less than the Fender side. Still, as tube amps go, it isn't bad at all, but I feel like there is something I'm missing that could make it quieter, something is not necessarily wrong, but maybe something is not adequate or working at cross purposes or something. The treble control on the Vox side now adds to the hiss on the Fender side even when the Vox volume is down.

Joe

[jfutral]

After hearing the Vox treble affect the hiss on the fender side I went back and re-reexamined the pre-amp wiring. I decided to de-couple the Vox and Fender pre-amps and run them independently to the ground point. I found one point where the Vox crossed over the Fender ground leads and eliminated that. That fixed the bleed or cross-over, which ever that is. Also quieted the Vox side back down. So that re-inforces my feeling (love those "feelings", huh?) that something is wrong, minor or not.

One thing I noticed and don't remember noticing before in previous builds. When I got close to the Fender tone stack ground I would get an increase in buzz/hum. I've had that kind of proximity type noise happen with the signal path before, but never a ground lead. Am I just not that observant and this is normal, or is something else going on?

Joe

[tubeswell]

The way you have your grounding looks suspiciously like it would create ground loop hum (but its difficult to be sure from that block diagram). Have you read Merlin Blencowe's http://www.freewebs.com/valvewizard2/grounding.html

[jfutral]

Yeah, that was a pretty pathetic drawing. I've been trying to squeeze in troubleshooting while loading in a show and drafting two light plots. I forgot that I don't have any ground leads running to the PI tube. I did at one time, but eliminated that a few revisions ago. Didn't need it. I was running ground leads to the center post on the 12ax7 sockets. I also had the cathode circuits grounded there, but that's gone, too.

I have read the article and try to incorporate what I retain, which is never as much as I should retain. But your post and this article made me think of something that might be skirting my attention, but it will be tomorrow before I get to look at it next.

Joe

[jfutral]

I THINK I found one source of the hum. The AX7 tube after the Fender tone stack had a weird off balanced heater feed. One side was showing over 7 VAC and the other was 5.x (can't remember exactly). All other tubes were within .2 or so (6.5 to 6.7ish). Swapped that tube out and my hum is greatly reduced. Still there but not nearly as bad. I may have hit the limit of what can be done without rewiring the whole thing. I've double checked all grounds and no one is doubled up anywhere, that I can see. I did a few other little things last week (before I got swamped with work) and I remember that did a little bit to help the hiss.

I will get another chance to rework some things, like the wiring, this week when the final PT shows up, I hope.

Joe

[jfutral]

Here is the full schematic with grounds labeled. I've been doing some major reworking since my Hammond PT arrived and I took the advice of a few others in another thread to decouple the Vox and Fendish pres' B+.

But over all I am still at three chassis points—Power (A+B on the schem), PI (C), and Preamps (D+E), topologically, A+B is closest to the power-in corner, C is about middle of the chassis and a bit toward the front, and D+E are in the farthest corner from the power-in corner, close to the input jacks.

I would say the biggest improvements on the hum were 1) to ADD 100R resistors on the filament supply. I had them with the previous PT because it didn't have a CT. But this PT does. I wasn't sure what to do with the center tap so I left it. Everything still measures properly and the hum is less than before. And 2) to reverse a lead shield ground on the lead from the Fender volume pot to the ax7 it feeds).

The hiss on the Fendish side is a little louder now, with just a hint of a buzz, after all the dust has settled. Volumes have to be cranked past 7 to hear it, but it is there. The Vox side is still pretty darn quiet so I am happy with that.

If I get some time tomorrow I'll post some clips. I'm pretty happy overall. Now I am just looking at adding a reverb (the tank is just sitting there asking to be used) and an FX loop.

Joe