Maggy 185 w/12AU7 1st stage...

[knockbill]

I'm revisiting this 185, since I had to replace a wonky fuse holder yesterday,,, Replaced the old 7025 phono stage with a 12AU7/12BH7 and Volume controls last winter, similar to Gordons mods, in this thread.... http://audiokarma.org/forums/index.php?threads/12au7-vs-12bh7-circuit-mods.747842/
but had some low level hum that I decided to chase while it was on the bench... Spent the most part of yesterday, til 2AM! and came to the conclusion it wasn't PS related, but ground loops in the input circuits/jacks... Here's what I found...

The hum can be eliminated by connecting the preamp jacks to line input (tape deck), or shorting jacks... but that eliminates the 1st (12AU7/VC) stage ... I suspect the problem is with the chassis grounds and the original jacks,,, there are two shorting jacks on the preamp stage, one the 1st(old 7025 phono, now 12AU7 stage), the other jack is open...

The goal is to use the old 7025 ceramic phono input w/VC , rewired for 12AU7 and retain the line input without the VC for use with the original passive tone stack, if I want to re-use the console... Other than the GL hum, the amp sounds very nice...
Not sure I can have it both ways,,, but I want to get rid of the ground loops and hum, regardless...
Figured its time to call in the cavalry!!
Thanks in advance...

 

[rudedogg]

I'll be watching this closely...thankyou! Now that i have acquired a 185 amp

 

[gadget73]

have you tried another 12au7? Just wondering if its heater-cathode leakage. That would make for a 60 cycle hum, same as a ground loop issue.

Failing that, are both sets of jacks grounded to the same point? Where is the grid to ground and cathode resistor for the 12au7 connected? Make that the same point if possible.

 

[knockbill]

Yep same results with any tube, or without a tube... I think the problem is the multiple ground connections around the 12AU7 stage jacks,,, I cut a couple of redundant ground wires from punched chassis lugs with no difference...
I'm considering removing the original phono jacks, as they are mechanically grounded to the chassis, and let them float with a separate ground, or buss bar to an insulated jack...

The cathodes are also grounded to their jack lugs... You can see the ground mess in the 2nd and 4th pic... maybe a buss is the way to go here...

This amp was dead quiet with the original, unused 7025 and that was a higher gain stage... and with a crappy wafer socket...
Maybe the volume pot is part of the problem, however the hum is the same with the VC in any position...

 

[CopperWizard]

Something simple? Try a different rca cable.

 

[GordonW]

I suspect the problem is WHERE the volume pots are located. That's very close to the power transformer and choke.

Is the power switch also in that general area?

I always put the volume pot, near dead center of the front panel (as close as possible to the gain stage that's feeding it). That reduces wiring length and gets the pot itself out of the "noisy zone" near the power supply.

Regards,
Gordon.

 

[knockbill]

Its more serious than that, only way to kill the hum is with a shorting plug in the pre amp stage, hums or without the 12AU7/12BH7 in the socket., but that eliminates the 12AU7 stage,,, shorting the 12AU7 jacks wont cancel the hum... I think its gonna need a new ground system...

 

[GordonW]

The pots are after the 12AU7, right? If so, any induced hum in them, will be there no matter if the 12AU7 is in the socket or not.

Shorting the preamp plug, shorts the output of the pot. That's why the hum goes away.

Regards,
Gordon.

 

[knockbill]

Thanks Gordon,,, I moved (from bottom L corner in pic) the pots and switch out of the chassis, bonded the VC cases and no joy... The switch was near the VC pots but it is in a metal box to isolate it... Unless all the tubes I tried have the same heater-cathode leak,,, its looking more like a buss or at least a star ground system is necessary on this one...

I could just back up and just change it to a 175 amp,,, but wheres the fun in that!!

Thanks for the input,, we'll get it...

 

[knockbill]

The pots are between the input jacks and 12AU7 grids,,, Schem in post #1

 

[knockbill]

Not quite,,,, The 185 has two sets of inputs, one pair on line jack, and one pair on the 12AU7 input with the VC pots...There are two shorting jacks on the line input,,, and one shorting jack/one open jack on the 12AU7 stage... The hum remains with both pair of jacks open.. Also hums with an input device(CDP/TD) in the 12AU7/VC jacks.... Doesn't hum using the line jacks, but there is no volume control...

It is a low grade hum, not controlled by the volume pot(s),,, and I lowered it somewhat, by removing redundant grounds around the input jacks... Looking to getting it as quiet as it is using the line input...

 

[knockbill]

Don't think so, the line jacks have shorting contacts,, I cleaned them well... I'll try it with an input device (CDP) plugged in each set of jacks... 185s were originally designed to leave the line jacks unused(shorted) as they only came with phono consoles...

 

[knockbill]

They aren't open to hum,,, no grounding switch, just shorting contacts on the line jacks,,, CDP is connected to the 12AU7/VC jacks... 12AU7 input jack has 47K from jack to grid, 470K grid to ground and 500K VC pot...

 

[knockbill]

OK,,, by "line in jacks" that is the jacks connected to the line stage (12AX7s)... With an insulated rod holding the line jacks open there is no hum...

 

[gadget73]

so that says the hum is in the 12AU7 stage.

Same hum with no tube too? Maybe heater windings laying too close to an audio signal path.

 

[knockbill]

With no 12AU7 and line jacks insulated open there is no hum... with no 12AU7 and line jacks closed hum is as before...
Heater leads are as original, never hummed before,,, There was originally a 7025 in that socket, which has more gain, and it was quiet...

 

[knockbill]

How is the grounding lug in the jack connected? Right there at the jack to the chassis or is it a long wire to some chassis tie? Can you connect a clip lead to it and see if grounding to some other point like star or B+ filter ground kills the hum?

Line jacks,,,, Riveted to chassis, thru grd tab, and has a lead from grd tab of each jack to chassis lug,,, clip lead to main PS grd doesn't eliminate hum...

12Au7 jacks... connected the same way, I removed the redundant grd leads to the chassis lug, with no hum difference,,,

This is why I think I'll need to build a star or buss grd,,, I had a RCA console amp I couldn't get totally quiet, and I lifted all the chassis ground wires, used a buss tied only to the main CT to ground kill the hum...
Maybe just lifting these input grounds and using insulated jacks, connected to the PS lug will help...

 

[knockbill]

so that says the hum is in the 12AU7 stage. Same hum with no tube too?

Including the VC pots,,, as it never hummed with the original phono 7025 stage, With the pots bonded to a chassis that carries all the amp grounds,, seems its possible the problem could be adding pots?

 

[knockbill]

Since this is no longer being used the same as the original, why not just disable the self grounding contact and call it a day? Use standard phono input grounding plugs in them if you want to. If it hums with those then there is some serious AC in the chassis around the jacks, maybe even corroded rivets to chassis. But if the jacks are quiet without them...hurray!

The line stage self grounding contacts work as designed,,, the problem seems to be with the 12AU7 stage, I can remove the original phono jacks and replace them with insulated jacks connected to a lifted ground system for the 1st stage...
Corroded rivets wouldn't matter as the grds are soldered to the punched chassis lugs...

I still have the console cabinets, and want to keep the all the options....

 

[knockbill]

I lifted the all the grounds on the 12AU7 stage, including the jacks... All grounds terminate on the same isolated lug, and that goes to the CT at the power supply... Hhum didn't change at all...
This amp has a two wire polarized cord/plug, and grounding the chassis to the wall outlet ground doesn't change the hum...
Unless I did something terribly wrong with the 12AU7/12BH7 stage, or the VC pots, I'm running out of ideas, other than a complete floating buss tied only to the CT...

Here are the pin readings for the numbers guys,,,
12AU7 pin 1...103V,,, pin 3...3.17V,,, pin 6... 101V,,, pin 8... 3.22V

12BH7 pin 1...81V,,, pin 3... 3.60V,,, pin 6... 84V,,, pin 8... 3.45V