5f1 Champ clone

[xtian]

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

Yes, but I really am after a 5f1. OTs are pretty inexpensive. You think I should replace it?

[Dingleberry]

If you look at the picture you will certainly notice that the power transformer is not original. If you measure 50V from orange to ground that rings me bells that it certainly is a bias tap. Don't connect it to ground.
That power transformer is probably taken from some other project, not from champ. Just leave it unconnected and use some shrink tube to completely cover the wire end avoiding shorts.

-T

[martin manning]

Here's an idea... ground the orange wire from the PT through say a 10k resistor, and see if the voltage drops. This should be completely safe; only 1/4W dissipated if the potential stays at 50V. What I'm wondering is, if it is a shield, if it could accumulate significant potential without a path to ground.

[M Fowler]

I would contact the seller and ask some important questions regarding the maker of that PT and if he/she has a schematic for that PT.

[Phil_S]

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?

If the OT is large enough (wattage rating) you might get away with using it. The problem is that a SE application tends to saturate the OT rather quickly. To compensate for this, an air gap is present in a SE OT. A PP OT does not have the air gap.

You also have the problem of figuring out whether you want to use all of the primary winding or just the CT to one outer leg. This, along with your choice of speaker impedance sets the primary impedance. In a 5F1-like thing, your primary impedance should be about 5K. You solve the problem by determining the turns ratio: http://www.radioremembered.org/outimp.htm or http://sarris.info/main/calculating-out ... -impedance

For very little money you can get a very good SE OT here: http://www.musicalpowersupplies.com/2.html I wouldn't try to make the PP OT work. I don't think it will give you the best tone.

[xtian]

I wouldn't try to make the PP OT work. I don't think it will give you the best tone.

I agree. Going to order a new one.

I would contact the seller and ask some important questions regarding the maker of that PT and if he/she has a schematic for that PT

Already did. It was a mutt lot of parts from David Allen. I spoke with David, and although he remembered the order, he was not able to find specs on the PT.

No matter. The orange wire is, without a doubt, not a shield. Otherwise, the PT is fine for this application. The voltages were all looking good when the circuit was still together.

Thanks, everyone! I'll post more once I get my parts.

[xtian]

Received my new OT from musicalpowersupplies.com. Quick question: I have two wires to choose from. 5K or 7K ohm primary side OT wire to pin 3 of output tube?

[ChrisM]

You probably want the 7K primary for a Champ. I would add a SPDT though and be able to switch between 5K and 7K.

[M Fowler]

The orange wire is, without a doubt, not a shield.

I did find another PT like that one that had orange as the bias tap 50v, so you were smart to DMM the wires.

Champs use 7k primary OT

Mark

[xtian]

My PT has a true 6.3vac (heater) center tap. I want to figure out what to do with it, following KISS (the acronym, not the band). Currently it is not connected to anything.

1. I understand the purpose of the heater center tap is to reduce hum induced in the tubes, right? So if there's no hum when the circuit is finished, is my simplest option to leave the center tap unconnected.

2. Next, if there is hum, I can connect the center tap to the cathode of the 6V6.

3. And if there is still hum, as suggested, I can:

Make a higher elevation (say ~40V) with a voltage divider from the B+)

Please say if any of these three options is incorrect (especially #1, leaving the center tap unconnected.)

[Structo]

Yes, but don't use the heater center tap AND the 100 ohm resistors together.

Only one or the other.