Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Colas LeGrippa on March 22, 2012, 06:47:11 pm

Title: Output transformer primary gnd lead ?????????
Post by: Colas LeGrippa on March 22, 2012, 06:47:11 pm

Why would an output transformer have a primary lead tied to gnd ? The amp is an Ampeg Jet J12, no such wire appears on the schem, strange or am I missing something ?

Title: Re: Output transformer primary gnd lead ?????????
Post by: Dave on March 22, 2012, 08:07:08 pm

Maybe its just a ground wire for the bell like you find in some power transformers?

Dave

Title: Re: Output transformer primary gnd lead ?????????
Post by: HotBluePlates on March 22, 2012, 08:30:35 pm

Can you post a picture of it? As you say, it's not on the schematic.

Title: Re: Output transformer primary gnd lead ?????????
Post by: Colas LeGrippa on March 22, 2012, 09:11:17 pm

my camera is out for the moment, but let me explain : it is a push pull tranny : as usual : red HV, blue plate no1 , brown plate no 2.......and unusual: grey to ground. It is not the gnd from the secondary that has it's own.

Title: Re: Output transformer primary gnd lead ?????????
Post by: jojokeo on March 22, 2012, 10:10:38 pm

With power off, disconnect it and do a continuity check. Then I'd power up the amp with this being taped off. I've only seen OT's (Mercury) where they would ground the voice coil side. Doing that on the primary is very odd indeed.

Title: Re: Output transformer primary gnd lead ?????????
Post by: Ritchie200 on March 22, 2012, 11:36:06 pm

Is that a UL amp?

Jim

Title: Re: Output transformer primary gnd lead ?????????
Post by: HotBluePlates on March 23, 2012, 08:09:43 pm

It might be an electrostatic shield. Does the OT have a copper band around the core?

Title: Re: Output transformer primary gnd lead ?????????
Post by: Colas LeGrippa on March 23, 2012, 10:44:36 pm

no, it is an output transformer ''naked'' as my girl friend is, just beside me ( she's passing the broom, but don't think I am a macho, I just spent three days on doi'n the ceramic floor in the basement, while she were on her facebook,  now it's her turn to work a bit hey hey ).

Colas

Title: Re: Output transformer primary gnd lead ?????????
Post by: tubeswell on March 23, 2012, 11:54:15 pm

As HPB says, it might be an electrostatic (faraday) shield (between the primary and secondary windings)

Title: Re: Output transformer primary gnd lead ?????????
Post by: Colas LeGrippa on March 24, 2012, 09:03:32 am

It is very unsusual, never seen that b4

Title: Re: Output transformer primary gnd lead ?????????
Post by: jim on March 24, 2012, 09:20:38 am

Some are manufactured with that lead soldered to the cover that is screwed to the chassis.  The lead would end up on chassis ground anyway because of your output jack.

Title: Re: Output transformer primary gnd lead ?????????
Post by: sluckey on March 24, 2012, 10:54:14 am

I bet there's no continuity between that grey wire and the primary. Just because the grey wire exits the OT through the same hole as the primary wires doesn't mean it's actually connected to the primary winding. Have you done a continuity check yet?

Title: Re: Output transformer primary gnd lead ?????????
Post by: Colas LeGrippa on March 24, 2012, 11:34:26 am

not yet , i'm working on it, I have checked all grounds, solders, tubes , rectifier..........could it be the multiple cap can ? though all voltages read fine. All coupling caps leak .200VDC , is that too much ?

the power tranny has been replaced but the guy who did it taped off the heaters ct lead......the ac cord has been replaced as well for a three lead one but the grnd was not connected to the chassis.....to make a long story short, I am correcting lots of errors, but the HUM' s laughing at me : :l2: ''FIND ME IF YOU CAN, I'M WELL HIDDEN''  :cussing:

I will swap the OT temporarily for a new one and see what happen............ :w2:

Colas The Gripper