sound city 50+ MK4 tone stack noise remedy?

20 Studio, 50+, 50PA, 50R, L100, B100, L120, L120PA, L120R, 120 SLAVE, B120, B150, L150, L200, B200, Concord, and SMF

Re: sound city 50+ MK4 tone stack noise remedy?

Postby ThisLifeILead on Wed Dec 17, 2014 2:03 am

Well i finally got the sound city opened up.
Someone has been a busy beaver in here.
Couple new coupling caps. Full retube and recap.
There is a residual hum/hiss in the background that is not affected by the volume controls but the hum seems to change with the mid tone control. With everythjng turned down it had a fair bit of background noise. The mid control seems to introduce a diff tone and gradually take over the background hum.
I really hope its not related to all this work done in here.
First two pre tubes, if pulled, still hums. BUT removing 3rd or 4th pre tube and the hum goes away. V3a is treble driver v3b is gain stage beforre p.i. v4 is p.i.
I found a leaky coup cap from v3b plate to v4a grid. Made no noticeable difference. There is a cracked mustard cap on cathode of v4b. Goona change that next but judging by pulling tubes would it make sense that the problem is tje circuitry between v3 and v4 ?
"Great spirits often encounter violent opposition from mediocre minds" - A.Einstein
ThisLifeILead
Squire
 
Posts: 582
Joined: Fri Feb 12, 2010 6:15 pm
Location: frozen prairies

Re: sound city 50+ MK4 tone stack noise remedy?

Postby Papa Dog on Wed Dec 17, 2014 7:12 am

ThisLifeILead wrote:First two pre tubes, if pulled, still hums. BUT removing 3rd or 4th pre tube and the hum goes away. V3a is treble driver v3b is gain stage beforre p.i. v4 is p.i.
I found a leaky coup cap from v3b plate to v4a grid. Made no noticeable difference. There is a cracked mustard cap on cathode of v4b. Goona change that next but judging by pulling tubes would it make sense that the problem is tje circuitry between v3 and v4 ?


check v3 and v4. bad V3 could make a big diff. V4 should be a 12AT7 and not a 12ax7.
Most likely the noise is coming aboard between 3 and 4.
pics would help.
DA 50+ schematic
Image
rev 1973 schematic with updates
the 50 plus is different to the 120 and 200+. Those two have another gain stage (v4) and 4b is a cathode follower. Your PI being plate driven right off 3b might be the source of many headaches. I would pay attention to c12 and c23 (1000p mustard) and the resistors in that area.
There Ain't No Such Thing As A Free Lunch.

"The time has come," the Walrus said,
"To talk of many things:
Of shoes--and ships--and sealing-wax--
Of cabbages--and kings--"
Papa Dog
Knight
 
Posts: 1337
Joined: Mon Mar 15, 2010 6:48 pm
Location: Narly Ampwerks

Re: sound city 50+ MK4 tone stack noise remedy?

Postby ThisLifeILead on Wed Dec 17, 2014 2:25 pm

Thnx for the reply Pdawg.
C23 i replaced to no avail. C25 is also a mustard and has a crack which looks due to drying.The tubes are new tung-sols.
I will try diff tubes and resolder the connections between v3 and v4 and swap c25. :)
"Great spirits often encounter violent opposition from mediocre minds" - A.Einstein
ThisLifeILead
Squire
 
Posts: 582
Joined: Fri Feb 12, 2010 6:15 pm
Location: frozen prairies

Re: sound city 50+ MK4 tone stack noise remedy?

Postby geoff_bell on Wed Feb 04, 2015 9:20 pm

the hiss gets quieter when you replace pre-amps tubes with ones with lower gain. i stuck ecc81 and ecc82 vales in mine
geoff_bell
Knight
 
Posts: 1335
Joined: Tue Jan 23, 2007 4:23 pm
Location: N. Ireland

Re: sound city 50+ MK4 tone stack noise remedy?

Postby ThisLifeILead on Thu Mar 05, 2015 10:04 pm

allrighty, i swapped all out of spec resistors, p.i plates were both over 25% out of spec.
all preamps were replaced with new tung sol 12ax7's. will try a 12at7 in p.i.
i found that the monkey who made this mess grounded both sides of the coax cables. all the pots were grounded THROUGH the coax shielding.
well.after fixing this the tone stack is injecting no more hum when turning them up. one step closer.
but there is still a residual background hum.
all caps and grounds measure out ok.
i wouldnt be suprised if a 12at7 did the trick.
lastly when i pull v2 the hum is still there unchanged. if i pull v3 or 4 it dissapears. so the noise is being injected between plate of v2b and grid of v3a.
i resoldered the grid of v3a to no avail. the search continues
"Great spirits often encounter violent opposition from mediocre minds" - A.Einstein
ThisLifeILead
Squire
 
Posts: 582
Joined: Fri Feb 12, 2010 6:15 pm
Location: frozen prairies

Re: sound city 50+ MK4 tone stack noise remedy?

Postby pdf64 on Thu Mar 05, 2015 10:32 pm

One one, I found that the cambric wire insulation along the long wiring runs became leaky, putting buzzy B+ into the grid circuit (terminal 7 V3).
Try easing wires apart where they're gathered together and see if the noise improves.
The leak was pulling the grid Vdc up a few volts too.
My band:-http://www.youtube.com/user/RedwingBand
pdf64
Knight
 
Posts: 1243
Joined: Thu Dec 14, 2006 9:55 am
Location: Staffordshire UK

Re: sound city 50+ MK4 tone stack noise remedy?

Postby ThisLifeILead on Thu Mar 05, 2015 11:43 pm

will look into that! thnx man :highfive:
"Great spirits often encounter violent opposition from mediocre minds" - A.Einstein
ThisLifeILead
Squire
 
Posts: 582
Joined: Fri Feb 12, 2010 6:15 pm
Location: frozen prairies

Re: sound city 50+ MK4 tone stack noise remedy?

Postby ThisLifeILead on Thu Mar 05, 2015 11:59 pm

even with a 12au7 in the p.i the background hum is quite pronounced.
With the treble, mid and bass off. (grounded) treble grid measures:11ohms between the shield and cable mid grid: 33ohms between shield and cable and bass grid: 11ohms between shield and cable. Volume pots when grounded dont show a reading....
"Great spirits often encounter violent opposition from mediocre minds" - A.Einstein
ThisLifeILead
Squire
 
Posts: 582
Joined: Fri Feb 12, 2010 6:15 pm
Location: frozen prairies

Re: sound city 50+ MK4 tone stack noise remedy?

Postby ThisLifeILead on Mon Mar 23, 2015 7:42 pm

i gave up. I returned it to my friend. its quieter. the tone drivers inject little noise now but there is still a noticeable 60hz hum. It was modified by some monkey and they added a choke and the wiring is really messy. instead of completely redoing the amp i just said thats the best i can do for now. :Zap:
Thnx for all the input fellas!
"Great spirits often encounter violent opposition from mediocre minds" - A.Einstein
ThisLifeILead
Squire
 
Posts: 582
Joined: Fri Feb 12, 2010 6:15 pm
Location: frozen prairies

Previous

Return to Sound City

Who is online

Users browsing this forum: No registered users and 1 guest