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    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

help needed with transformer wiring

jimmy74

Member
2009-11-13 1:32 pm
currently building a 5F6A clone in which the seller of the kit has included a PT with colour code instructions that don't match up with the wires on the PT itself.
Here's a photo of the PT:

04042013503 | Flickr - Photo Sharing!

Here's a translation of what's written on the sticker:

220v = blue blue
340 = grey 0= brown 340 = grey
50v = white white
5v = red red
3.15 = yellow 0 = black 3.15 = yellow

As you can see there's only one red wire and in the lower left hand corner of the photo you can see the two black wires.

The red wire is infact 2 solid core copper soldered together whereas the white and yellow wires are each single solid core copper.... maybe the seller has gone and soldered the two 5V & 0V windings together?:whacko:
DC resistance between the 2 black wires = around 15 ohms, between the black wires and the red wire around 700K and between the black wires and the yellow wires up in the 10Mohm range but steadily and continuously goes down.
what do you think?

thanks
 
I think you are holding both leads of the multimeter in your hands when you are taking the resistance readings. That will give you a bad reading. What is happening is you're taking the resistance reading of your body, NOT of the transformer. Don't touch the leads when taking measurements. Especially when the transformer is live with electricity as that will KILL you. Be careful.
 
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jimmy74

Member
2009-11-13 1:32 pm
Ok I was holding the tester leads trying to keep the wires from not wobbling around. I repeated the test without touching anything with my hands, between the 2 black wires I'm getting 15 ohms and no readings between the black wires and the red wire and no readings between the black wires and the yellow wires not even in the 20Mohm setting. I have not tryed hooking up the PT to the mains, and I won't do so if I don't figure out what wire goes where beforehand.
 
When dealing with an unknown power transformer I like to screw all the leads into terminal connector block. Meter readings are more accurate when you can press the lead probes hard against the terminal screws.

Investigate the red lead further, I dont like the idea of a shorted coil.

By measuring the resistance of each lead against every other lead the separate coils should be determined including those with and those without a centre tap.

I assume the two blue wires will turn out to be the mains 220Vac input. You could use a small ac supply if you have one ( 12Vac or less ) instead of the mains and see what voltages are generated in the other coils. A little bit of maths should tell you what coil is what.


As DanFrank says - be careful
 

jimmy74

Member
2009-11-13 1:32 pm
Ok here goes I used one of those multi - connector plugs and screwed in very securely, the black wires, red, yellow and white wires. I heat-shrinked the grey and brown wires (B+) to get them out of the way and connected the blue wires directly to mains plug. Using the tester leads on the multi connector for each wire:

Black wires = 52.2vAC
White wires = 7.1vAC
Yellow wires = 5.6vAC

checking voltages between the white wires and the red wire gave me around 3.5vAC so the red wire is the CT for the 6.3v heaters!

What a mess the seller did when he winded this transformer up, the CT for the heaters being 2 solid core copper wires soldered together!! Never seen this before!
 
Well it looks like I need a variac for this, what sort would be sufficient for working mainly on guitar amplifiers and tube powered equipment?

Just keep in mind that variacs do not normally provide any isolation from the mains input. They are effectively variable auto transformers, where there is a single winding, and the variable output is a movable tap on the winding.

(You may already know this, of course, and please accept my apologies if that's the case. It's just that sometimes it is assumed that variacs do isolate.)
 

Rundmaus

Member
2005-08-21 10:46 pm
What a mess the seller did when he winded this transformer up, the CT for the heaters being 2 solid core copper wires soldered together!! Never seen this before!

That is exactly what I would expect from a center tap. The two halves of the 6.3V heater winding are made as two separate windings, the two wires are the ending of one and the beginning of the other half. What is so strange about that? I would be more worried if a center tap consisted of only one wire!

Rundmaus
 

jimmy74

Member
2009-11-13 1:32 pm
Well the mess was mainly in the way he messed up the colour codes even though I think he just messed up the coloured wires during winding as the sticker itself seems to be stock on these transformers. As for the CT being 2 ends, yes I know that, but I've really never seen it done this way before, not on mercury's, not on any of the PT's I bought from this guy in the past and nor on old fenders... it's would have been more logical if he had used big fat copper core wires for the OT secondaries or for the heater wires themselves.
 

jimmy74

Member
2009-11-13 1:32 pm
well I've finally finished it up, there was a faulty voltage splitter cap that I replaced (it was bleeding +200vDC from the PI plates to the bias). But now the amp seems to be very noisy, specially when cranking up both volumes and the presence, the amp starts to whistle other than just loud hum. I've tried passing the NFB wires away from any HV sections but that doesn't change anything, I've used shielded wire for the input jacks, that took a bit of the hum away but the amp is still very noisy. I'm also guessing that the speakers aren't powerful enough to handle the amp so I will definitely be getting rid of those (maybe a G12M65 +G12H75 would be right?).
Anyways there must be something wrong in there somewhere, if I turn up the volumes and presence (other controls up half way) and touch the output cable, the amp whistles instantly. I'm starting to think that maybe those +200vDC on the bias (before replacing the voltage splitter cap) may have seriously damaged one of the 6L6GC's? Would this make the amp unstable and noisy?

thanks
 

jimmy74

Member
2009-11-13 1:32 pm
I changed the NFB resistor to 56K and that got rid of the whistle but the hum is still there. Following the Ceriatone layout the only differences are basically the grounding points I made: one ground point for mains, one for the PT + filter cap board (which I also modded to fit the bias trimmer on) and one ground point for the main PTP + output jack. I did not do a main ground point for the pots and left them grounded to the chassis as single points. So I lifted the input jacks ground wire from the volume pot and soldered it to the 100k/820r resistor junction, but that did not change the quantity of hum I'm getting. Here is a clip of the hum I'm getting. The guitar is attached, both channels are connected together and I turn up both volumes to max, then presence to max, other controls are half way up. When you hear the hum going back and forth that's because I'm moving the input jack plate around. After a bit, I turn both volumes to halfway and strum on the guitar:

15052013007 | Flickr - Photo Sharing!
 

jimmy74

Member
2009-11-13 1:32 pm
I decided to do some tapping around with a screwdriver, very carefull not to hit the resistor and cap leads. I started around the 10uf 450v cap, as soon as I tapped it the hum got worse. I measured its capacitance with the amp off and drained and it measured around 10uf. I changed it out with a radial 10uf 450v cap I had but the hum is still there, just like it was after I tapped the previous cap. I've checked for continuity all through the board and everything is right. The only thing I noticed is that I had inverted the power switch wires.

Help!!