Home Made Pre amps...

Tried em with 2 different mono and the Maggy.... I understand what you are saying,,, I have read small AC between chassis, and clipping them together removes the reading to ground, but the hum stays the same...
Trouble is I don't know if these were ever hum free, as they are home made... I assume the original Fishers were tho...
I'll rebuild the PS and isolate the grounds tomorrow and see what happens...
 
Is there hum in both units, and is it when they are working together, or when they are working separately, could it be a ground loop between them?
 
Hi Scott,,
They hum separately or together regardless of what amp they are connected to... Hums with preamp volume control zeroed, and gets a little louder as its turned up... I tried everything I could think of to eliminate ground loops....
 
Just put covers on one pre, hum stays the same with or WO,,, I think its time for a redesigned PS and umbilical set up...

The PS for the WA-P2s is tube rectified with SS heaters... it is quiet, except for a little hum thru the phono stage on a couple amps with higher gain...
 
Pulled the four 6V6s in the Maggy and its quiet... Think its time to lift the heaters and isolate the grounds... if that doesn't work, I'll remove the AC from the preamp chassis...
 
guessing he means output tube on the preamp, whatever is last in line before the signal leaves it.
 
Not sure what he means then! I pulled amp output tubes... Can't pull a tube from the preamp, its series wired.....
Just took a break, I hit a snag, figured it needs less than 47uF 1st cap off the 6X4, so I wanted to put an 5-10uF oil cap right after the rectifier, all I have are oval ones which would mean hacking up the PS chassis,,,
 
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Ohhh... forgot about the series filaments, I had worked last night, and went to bed after the last post I made.
I was thinking that with the tube removed it would rule out a hum generated within the preamp, and prove a ground loop or not. Can you send me one of the schematics that are close to the build, I can possibly help more. I cannot read the previous schematic, it is too small
 
Rebuilt the HV supply, new caps, resistors, and added the 10uF stage after the 6X4... Removed all the AC from the preamp and umbilical, except for the 6V pilot tapped off the rectifier filament... Added a dedicated ground wire for the heaters, and lifted them off the chassis... Heater supply is 17.9VDC for each tube and a half...

I didn't replace the 1000/25V caps in the heater supply, but they seem fine...

Scott just saw your message,,, the Fisher 50C manual on HiFi engine has a clear schematic.., but the difference is in the separate PS mine has, and I'm pretty sure that's where the problems are,,,
Here's the updated PS, and schem...
 

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On the filament supply the center pin is grounded, and 18 volts are fed from each end,with a dropping resistor you could duplicate it, You could also try connecting a larger cap, between the plate, or cathode of the last tube to see if a ground loop is the problem, as the cap should shunt the hum to ground.connect it with the amp off, and discharge between reconnecting.
The volume control looks like it grounds the grid of the last tube, so no signal should be present
 
Good grief,when I saw the pictures my first thought was Pultec.The colour,the layout.Geez,that is some beautiful workmanship.The faceplates were certainly made by someone with good skills and equipment. While the slots (with their rounded ends) for the lever switches were likely done on a milling machine,the square-cornered hole for the loudness switch was punched.That,or a really steady guy with a file and lots of spare time;)
 
Thanks for the input,Scott....That's how the filaments are connected here, except pin 9 was originally soldered to the chassis to complete the ckt.. Filament winding puts out 17.9VDC after the filters, and is fed to each end (tube and a half) from the CT... Outside tubes use 12V, middle one is tapped... Pin 9 was soldered to the chassis to complete the ckt, so I lifted it, and ran a second dedicated wire from the PS and isolated the whole heater string from the chassis in both the PS and preamps, figured that where the noise was coming form, but it didn't change,,, I'm thinking I need to reference the filament string to ground now, thru a cap... Also, a 3wire line cord to the PS chassis may help...

By last tube you mean V1B? If so, the cap would go from cathode, bypass the two resistors(R21 ,R22) then to ground...
I don't follow the VC statement?
 
Yep, he didn't have a CNC machine!!! To bad the wiring doesn't match the metal work... this thing is a mess!
 
The volume control, when it is at minimum, should ground the input of V1B, so there should be no signal at the output, unless a ground loop,or hum on the B+
Grounding the cathode, bypassing the resistors,should remove any signal to the RCA jack without shorting it,
so if there is still hum, it should be some kind of grounding problem.Connecting a large cap to the plate should eliminate hum from the B+ line
 
OK,,, I get it now,,, the VC at minimum doesn't kill the hum... so, try a bypass cap around the resistors... if it still hums big cap from plate to ground? how big are the caps? like .1 films?
Since I put in a small 1st node cap after the rectifier, would bigger PS caps help? I have some 100 to 220uF lytics,,, or, if the hum is in the B+, think the HV should be SS as Fisher originally used? I just tacked the PS together til it gets sorted out, so changes won't be too hard ...
Thanks again...
 
If the cap from the cathode to ground doesn't silence it, likely a ground loop or grounding problem, something like 1mfd should do it, not too critical, if it silences it, remove it, and try a 20mfd or so from the plate to ground.
 
Thanks Scott,,, I'll try it tomorrow, and take notes,,, had enough messing with it today!! At least the PS is in better shape...
 
22uF cap on the plate(s) of V1B, kills the hum substantially,,, still has enough hum to make the phono stage noisy, I think... Since that node feeds both plates on V1, I put a big cap (220uF) in the PS which quiets it a little more...

So we proved the noise is on the B+,,, which may be easier to deal with than finding a ground loop!!!! I can still experiment tying the heater, HV, chassis and line ground together with caps, and try to eliminate the ripple... Variac is cranked down, as only one pre is connected, B+ gets too high,,, so heater volts is down...

Also,,, seems a choke or 2 may be helpful in the PS, rather than huge caps? I think modifying the PS to be as noise free as possible is the way to go forward,,, rebuilding the preamps any further at this point isn't going to affect hum...

Thanks again for the info and support...
 

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