[davemc]

I need to replace the output transformer in a 37-660 I am repairing. I measured the voice coil resistance as 1.2 ohms, so I assume the impedance is about 2 ohms. The tube manual says I need a load impedance of 10K ohms.  Maximum plate current 80ma (from tube manual) and output power 10 watts (from Riders).

I have a NOS Utah 5999 universal output transformer in stock. It is about the same physical size as the original. Searches online say it is rated at 12 watts (PTOP says 8 watts).  But I have not found any sources for the maximum DC plate current or impedance ratios available (but those can be measured at 400Hz). I suppose that a Hammond 125E would be a better choice since it is rated at 15 watts and 80ma maximum plate current.  But it is HUGE and also pricey. Any advice? Other alternatives? Thanks.

[Mondial]

I would certainly give the Utah transformer a try. If it is rated at 12 Watts and the same physical size as the original it should be fine. Plate current rating should not be of too much concern as it is a push pull output stage and there is not a problem with core saturation since the two plate currents cancel the static magnetic field in the core.

You can also measure the winding DC resistance of the Utah and see how it compares with the original if you are worried about the DC drop due to the winding resistance. You can calculate the impedance transformation by measuring the turns ratio from primary to secondary.

[davemc]

Thanks. According to Riders, the 37-660 transformer measures 700 ohms total. My original is open, unfortunately. The Utah 5999 measures 419 ohms total. I do not know what that implies. Perhaps the original has higher inductance, and thus perhaps more bass or less distortion. The owner just wants it to work, so I guess I should not worry.  I plan on using 10000 ohms for the plate load (6F6 push-pull pentode) and 2 ohms for the secondary (measures 1.2 ohms DC). So I am looking for a ratio of about 70:1 I suppose. I use my old HP200A audio generator to drive the primary at about 10 volts at 500Hz and look at the voltages on the various taps.