Issue with an X88R build

[MGW]

I recently built an X88R preamp and I'm having some interesting noise issues. While the guys over at the forum are wonderfully helpful, I figure the more input the better. The PT in the build is a Hammond 270CX. I removed the bells for whatever reason I don't really know if that makes a difference or not. The preamp circuit is ground to the chassis at the ground lug by the IEC jack and nowhere else. The switching circuit is ground to the chassis by the ftsw jack on the back panel. The buzz I'm getting is a combination of 180hz and 300hz and other harmonics of this. While trying to eliminate this buzzing noise, I discovered something, the buzz goes away with the trafo isolated from the chassis. However, when I do this, the 60hz grows much louder. If I touch the iron (I know...not the best practice) the 60hz dies down to a tolerable level. I've probed pin 6 on the input stage and the buzz (with trafo bolted to chassis) is there.

There is also something interesting happening with the gain pots, if they are turned all the way down the buzz gets louder (with trafo bolted to chassis). I would think that if the gain pot was turned down nothing would pass to the next stage and thus the noise introduced at the front end would be gone. I'm at a loss here.

Any thoughts on where to go next.

Thanks
Scott

[LeftyStrat]

I'd put the bells back on. They shield the PT. Once that is done I would try changing the orientation of the PT, while listening to the buzz.

[Tillydog]

... The preamp circuit is ground to the chassis at the ground lug by the IEC jack and nowhere else....

Any thoughts on where to go next.

Ground the preamp circuit at the input jack and nowhere else, then see what you're left with.

[MGW]

no change at all.

[LeftyStrat]

Did moving the PT change anything?

[Gaz]

Sounds like a ground loop perhaps, which was suggested by grounding everything at the input jack. Since it's a tested design it has to be a wiring error. Keep searching, you'll find it eventually.

[roberto]

I removed the bells for whatever reason I don't really know if that makes a difference or not.

Not a good idea in a small chassis. Replace them and be shure the PT is correctly grounded to the chassis, and the chassis to the ground of the plug.

The preamp circuit is ground to the chassis at the ground lug by the IEC jack and nowhere else.

Ground the preamp input at the chassis and then the chassis to the IEC.

The buzz I'm getting is a combination of 180hz and 300hz and other harmonics of this.

They are all harmonics of 60Hz.

There is also something interesting happening with the gain pots, if they are turned all the way down the buzz gets louder (with trafo bolted to chassis).

Well, the gain pot works like a "mixer" between the guitar signal and the ground loop noise. All the way down, the noise is maxxed. Do you have some pics of the preamp?

[MGW]

I don't have any pics on hand, but I can definitely take some and upload some. Is there anything in particular that you want multiple angles and up close shots of?

[gingertube]

Um... lets go back to basics.
What have you done with the tube heaters?
A real or artifical centre tap (CT) to 0V is essential, sometimes works better to chassis rather than 0V.

I've had good results tying the CT to a positive voltage of around +50V rather than tying it to 0V. Given that the X88R has cathode followers in it I would definitely recommend tying the heater to a positive voltage of +50 to +70 volts, that will minimize the heater cathode potential.

To generate the +70V rail use a resistor divider off the B+ with a cap across the bottom resistor, something like 220K + 56K for the divider with say a 10uF across the 56K (adjust as required).

[MGW]

It's an artificial CT. The PT doesn't have a CT on the filament winding. I've had it grounded at a lug on the side of the chassis coming off of the lamp on the front panel, I've wired them directly to V6 and tied it to the chassis lug by the IEC jack, tried by the B+ CT (where it is currently). I've tried elevating them. Nothing has changed anything.

If I ground the grid of the second stage the pre is dead quiet. So it has to be something getting into the first two stages. Interestingly enough, I did discover that the closer the input wire is to the chassis wall the louder the 60hz gets...much louder. I wouldn't think a shielded cable on the input would have done that as significantly as it did.

[Tillydog]

I did discover that the closer the input wire is to the chassis wall the louder the 60hz gets...much louder.

That shouldn't happen - something is 'floating'....

I don't think you have a ground on your preamp board.

You need to connect the ground connection on the PCB near the input to the chassis at the input jack.

You also need to connect the ground on the power supply board to something - try it where it is on the chassis for now, but you may get better results connecting it to the same place at the input jack, or by connecting it directly to a fat ground trace on the preamp board somewhere away from the input.

[roberto]

Stupid question, but....
what about shielded wires grounding and where have you used shielded wire?

[MGW]

The preamp is now grounded at the input jack on the chassis. The power supply is grounded to the preamp board (just like a real Soldano). The preamp ground is then connected to the input jack. I have a big fat star washer on the input jack. I have tried grounding the preamp board and power supply separately...one on the input jack..the other at the rear ground lug. I have tried moving the B+ CT to different locations. I have tried three or four different heater artificial CT grounding options...including elevated heaters. I have swapped every tube out of the pre with known good tubes. It makes sense that something is getting into the ground somewhere given what the gain pot is doing. I'm just at my wits end trying to figure out what. I can probe the grid of V1 and can hear the noise but barely. Probing the plate of the input stage...same noise but louder.

There is shielded cable from the rear input connected to the front input and from the front input to V1 input. The shields on both are terminated at the ground tab on the input jack. I also have shielded cable from the gain pots to their respective valve grids. All terminated at the common ground for the gain pots. I have used shielded cable to the stages with the cathode followers. There is shielded cable from the output of the masters to the grid of the output buffer as well as shielded cable from the board to the output jack. The all of the jacks are isolated from the chassis except the switching and the input jack.

I ordered new bell ends from weber so I could orient the transformer a different way and to put bells back on it. Don't know when they are arriving though.

[Tillydog]

The all of the jacks are isolated from the chassis except the switching and the input jack.

Just to be clear: Is the grounded input jack something that you've just added, or was it there all along? (Your notes on the layout don't mention it.)

What are the voltages at anode(plate) and cathode of each stage?

Any pictures?

[jelle]

Pictures would help!