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RE: Dynaco A470 Output Transformers

Posted by strijw426 (A) on August 20, 2014 at 12:08:17

In Reply to: RE: Dynaco A470 Output Transformers posted by strijw426 on August 17, 2014 at 12:16:58:

I thought I would go back and provide some more detail.

Output Transformer Measurements
Test Conditions:
Center tap disconnected from power supply electrolytic
Allied KG-625 VTVM
Output tubes removed from sockets
Left Channel:
Center Tap to front plate lead = 92 ohms
Center Tap to rear plate lead = 103 ohms
Center Tap to front screen lead = 32 ohms
Center Tap to rear screen lead = 36 ohms
Plate to Plate = 192 ohms
Right channel:
Center Tap to front plate lead = 180 ohms
Center Tap to rear plate lead = 178 ohms
Center Tap to front screen lead = 58 ohms
Center Tap to rear screen lead = 36 ohms
Plate to Plate = 390 ohms

I have a Heathkit IB-2A Impedance Bridge and decided to take measurements of the two transformers being discussed. The test frequency used is the bridge internal 1kHz oscillator.

My measurements of the two transformers with their center taps disconnected from the power supply electrolytic were:

Following the Heathkit instructions for the bridge the setup of the bridge that gave the lowest left hand 100uA reading of the AC level when the range switch was set to the 10H range. Then the other dials are adjusted for best null reading as the generator level is gradually increased to maximum. The controls are basically adjusted for the best deflection back toward the left end of the scale on the meter.

DQ dial adjusted to read 1.0
Final readings of the CRL Dial were:
Left channel transformer 1.57 X Range dial reading of 10, so 15.7 Henries
Right channel transformer 1.42 X Range dial reading of 10, so 14.2 Henries.

Those measurements do not look all that different, but I am still troubled by the difference in the plate to plate DC resistance.

Thoughts?

Joe

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