Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: 1blueheron on October 24, 2019, 09:05:21 pm

Title: Multiple center taps?
Post by: 1blueheron on October 24, 2019, 09:05:21 pm

Checking voltages on my Wurlitzer PT tonite.  It is unloaded and not connected to any tubes or caps, chassis is stripped.

Line voltage from wall is 121V.

CT1 to yellow (rectifier pin 2)=21V
CT1 to red (rectifier pin 4) 456V
CT1 to red (rectifier pin 6) 455V
CT1 to yellow (rectifier pin 8) 21V
CT2 to green 199V
CT2 to green 195V
CT1 to dark blue? 5V
CT1 to brown 165V

CT1 and CT2 both appear to be red/yellow

I assume this is a 450-0-450 transformer?  Measuring voltage to red wires from CT2, I got 226V on pin 4 and 560V on pin 6  assume this is wrong CT for reds?

What is the key to matching wires with CT's :w2:

Transformer is stamped with Stancor STC trademark and Part # 15123  306035

Title: Re: Multiple center taps?
Post by: sluckey on October 25, 2019, 02:00:37 am

Disconnect all CTs from chassis. Use your ohm meter. Connect one probe to a red wire. Now, one at a time, touch every other wire with the other probe. Tape all wires that show resistance together. This is one secondary. Now repeat for a yellow wire. Probably the other yellow is the only wire that shows resistance. Tape these wires together. This is another secondary. Now repeat for a green wire. Now repeat for the blue wire. Now repeat for the brown wire.

Title: Re: Multiple center taps?
Post by: 1blueheron on October 25, 2019, 05:39:23 pm

Thanks Sluckey,

I think I got it sorted using your tips.

I made this drawing and I believe it is correct now although the true wire colors are up for debate. 

Title: Re: Multiple center taps?
Post by: sluckey on October 25, 2019, 07:11:40 pm

Looks like a PT to me. You're gonna need that field coil or an equivalent resistor to get that B+ down to a usable level. Hope you made a schematic for the power supply.

Title: Re: Multiple center taps?
Post by: 1blueheron on October 25, 2019, 10:15:31 pm

No, Didn't draw schematic for power supply.  Stupid me. But I'll dig through my stack of resistors and see what I can come up with.

Would you discourage swapping out 5u4GB with a 5R4GYB to achieve taming it a bit? 

100 ohm 5 watt wirewounds on each leg of AC before the rectifier?

This one will likely end up being a marshall plexi 50 clone of some variant. 3 12AX7's  and a pair of 6L6GC, EL34's or KT77's.

I have some filter chokes to bring it down for the preamp but need to figure out what their values are.  Guess I could use the field coil speaker it looks to be in mint condition.  Stamped 68 on the back and P-232 which makes it Maganvox right?  Looks like Jensen A12 clone.