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Help for novice racking Collins 356A Tube Preamp
Old 2 weeks ago
  #1
Here for the gear
 

Help for novice racking Collins 356A Tube Preamp

Hi All.

So I bought a Collins 356 module some time ago and am finally trying to get it running. Right now just trying to do a basic test that its working before I start to build a case and add 48v, pad etc... This is my first time doing a project like this.

the schematic is here: http://collinsradio.org/archives/manuals/356A.pdf

I have two linear power supplies for is 250v from International Power (https://www.mouser.com/productdetail...iylMDitA%3D%3D) And +/- 6v is a second hand one made by Lamda.

I wired it up last night and nothing blew up or caught fire. But its not working yet. Maybe a grounding issue?

With a mic plugged in I get a huge amount of 60hz hum, but it does pass some audio. So maybe I am wiring inputs/outputs wrong?

I have the mic wired to pins 1 and 2 of the plug (3rd pin of the XLR is connected to nothing), the schematic also shows pin 7 as an input but on my module that pin is not connected to anything inside....
How should I go about making a balanced connection to this thing?

Also when i disconnect the mic the preamp starts self oscillating really loudly. around 1.5k and up....
I dont know why this might be...

I do see a wire that used to connect pin 1 of T202 to pin 7 of V202, its hanging loose as if the solder joint broke off. When I try connecting it tho I get a different oscilating sound, this time coming out of the microphone!
The preamp has been recapped so there are some resistors and other things in there that look like they are not in use anymore, but this one looks more accidental....




Another thing I am looking at that seems strange, is on T201 pins 6 and 7 are jumpered together. meaning they are both going to ground? They also connect to pin 1 of v201, so that is going to ground too? pin 2 of v201 is also connected to the same ground lug...


Any advice on how to proceed would be greatly appreciated!!
I am very novice with this stuff and haven't been able to find any general guides online that answer questions like "how to connect a power supply to a tube amp" or "how to wire a balanced mic input"

thanks
Asher
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Old 2 weeks ago
  #2
Lives for gear
 
Radardoug's Avatar
 

Pins 1 and 2 should wire to pins 2 and 3 on the XLR. Pin1 on the XLR should wire to ground (pin 3 on preamp). Pin3 and pin 4 on the preamp should be connected together.
You should be using only one side of your 6 volt supply for the filaments, 6 volts not 12 volts for the filaments. Connect one side of this supply to ground, (pin3 and 4 on the preamp.)
Old 2 weeks ago
  #3
Gear Head
 

Quote:
Originally Posted by SquidRecs View Post

I do see a wire that used to connect pin 1 of T202 to pin 7 of V202, its hanging loose as if the solder joint broke off. When I try connecting it tho I get a different oscilating sound, this time coming out of the microphone!
The preamp has been recapped so there are some resistors and other things in there that look like they are not in use anymore, but this one looks more accidental....




Another thing I am looking at that seems strange, is on T201 pins 6 and 7 are jumpered together. meaning they are both going to ground? They also connect to pin 1 of v201, so that is going to ground too? pin 2 of v201 is also connected to the same ground lug...


Any advice on how to proceed would be greatly appreciated!!
I am very novice with this stuff and haven't been able to find any general guides online that answer questions like "how to connect a power supply to a tube amp" or "how to wire a balanced mic input"

thanks
Asher
It seems you don't have output of preamp connected to output transformer. Your explanation about connection around V201 and T201 most likely don't reflect actual connection to pins.
This nice preamps tend to come properly connected, although many need some minor components replacements. Maybe get in touch with technician to get him replaced what's needed and don't risk shorting output transformer or something similar. It doesn't take much time to do it.
Old 2 weeks ago
  #4
Here for the gear
 

Hi thanks for the reply!!
I got it working yesterday by connecting B- to ground and reconnecting that bad solder joint.

I still have not changed the 6v connections though. I may have misdescribed before when i called it +/-6v the powersupply does not actually say that. (someone warned me that would be a 12v supply) But nonetheless, I should tie the negative out of the 6v supply to ground? The schematic has 2 pins labeled for 6.3v. should i connect them together? leave one empty?

It was working yesterday but I only had it on for a sec, I hope im not overpowering the heaters!

thanks for all the help!!
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