Magnatone Ultralinear OT tap identification

[twilightofthedogs]

Hi folks, I have a magnatone starlite bass 442 amp that I am troubleshooting. One thing that I wanted to double check is if the OT primaries are correctly attached. It looks like they have been unsoldered. My question is: Is there any way to tell which of the taps go to the grid and which go to the plates? Which pair goes together? Thanks in advance.

[52 Bill]

My question is: Is there any way to tell which of the taps go to the grid and which go to the plates? Which pair goes together? Thanks in advance.[ATTACH=CONFIG]21871[/ATTACH]

I'd start by reading the resistances across the primary wires. There should be 5 wires, find the center tap and read the resistance from the center tap to the other 4 wires. The two that have the highest readings will probably be the outer ends of the primary winding. The two with the lower readings will be the taps. You can read the resistance from the taps to the ends to find the matching ends. Of course, the outer ends connect to the plates and the taps connect to the grids.

If the resistance readings are inconclusive you will have to apply a signal to the secondary and scope the primaries to see which wire is which.

[twilightofthedogs]

Thanks Bill. I checked 153 on the plate and 52 on the grid of one, 94 on the plate and 21 on the grid of the other. Seems about right, resistance-wise. I might have expected the two tubes to match up a little closer, but who knows.

Another issue, the tubes are redplating when a signal is applied. Fine when at idle. Negative voltage into the grid is what it should be (-15vdc), but when a signal is applied the negative voltage drops down to about -30vdc.