5f1 Champ clone

[xtian]

I'm gutting and rebuilding a 5f1 from a botched job found on CL. Guy had so many errors and poor lead dress (with resultant bad hum) I decided to start over.

There's no brand or ID on the PT, so I'm having to guess at the last lead. See photo. Here's what I'm sure of:

red/red/red+yel = HT 325/325 vac with center tap
green/green/green_yel = 6.3 vac with center tap
yellow/yellow = 5vac
black and white are currently connected to 120vac mains

That leaves the orange wire. This could either be an alternate mains lead (for 115 or 125 vac, say), or it could be an earth, and should be grounded with the mains earth (green). Probably the first choice, right?

[M Fowler]

The orange wire is the shield wire ground that one.

[xtian]

Next question: The 5f1 schematics I have show PTs with no 6.3vac center tap. It's common to create the artificial center tap with a pair of 100 ohm resistors. What's the best thing to do with a REAL heater center tap if you have one in order to best eliminate hum?

[xtian]

The orange wire is the shield wire ground that one.

You sure, Mark? Seems like that would be a quick way to burn the tranny if incorrect...

The orange wire was not connected at all in this nominally working Champ.

[xtian]

The orange wire is the shield wire ground that one.

Measured from orange wire to ground, when power is on, is 50vac. Make sense for shield wire?

[M Fowler]

I tried to upload a schematic from Triode Stores to support my reply but my system is weird tonight.

The orange wire is the shield wire it is usally connected to the transformer bolt.

[M Fowler]

Are you going to have cathode bias if yes you can lift the 6.3v CT from ground and connect it to the + side of the bias cap to elevate DC and eliminate hum.

Some people will put in a humdinger pot to eliminate hum.

[M Fowler]

I have come across some PT's that had a 5v center tap wire which I didn't know not to connect to ground and I found out the hard way.

Mark

[xtian]

Third question: The OT is a PP model. He had blue connected to the power tube and brown connected to B+ (left red unconnected).

Is this an acceptable hack, or should I get a true SE OT?

[xtian]

So, if I'm measuring 50vac from orange to ground, doesn't that mean, DON'T CONNECT IT TO GROUND?

[M Fowler]

Yeah I believe your correct.

PT are all wired differently most reserve orange as shield.

I found a PT on Marsh wedsite that has orange as 100v wire and black as the 120v so not sure what is going on with your PT.

I checked Mojo and Weber but no orange wire used in their PTs.

What brand is it?

[M Fowler]

Found it on Triode and that is most definitely a shield.

[xtian]

So, I don't want to forget my earlier questions:

1. What's the smart thing to do with the heater center tap

2. What do you think about the PP OT in this SE amp?

[tubeswell]

Next question: The 5f1 schematics I have show PTs with no 6.3vac center tap. It's common to create the artificial center tap with a pair of 100 ohm resistors. What's the best thing to do with a REAL heater center tap if you have one in order to best eliminate hum?

Same as you would with 2 x 100R resistors (but don't do 100R as well - do either/or) i.e.: run the CT to the 6V6 cathode to elevate the heaters (or make a higher elevation (say ~40V) with a voltage divider from the B+)

Re: the PP OT - with that Pt you've got just enough juice for a 5E3ish amp, or because of the bias tap - a tweed PP harvard 5F10, or even a 5F11 vibrolux, or a 6G2 princeton

[Deric]

If that orange wire is just a shield there should be no voltage on it...if there's 50vac on it don't ground it.

50vac sounds like maybe a bias tap???