The AM-30469 was custom made for Gates by Elecro-Engineering. The tertiary winding is actually a cost-cutting measure, allowing the finals to run with less grid drive, since the full class-C plate modulated level of grid drive is needed only during the positive half of the modulation cycle. Instead of using a larger driver tube such as an 813 to drive the 833A's, they get away with using a pair of 807's, which are also plate modulated about 20%, giving the grid drive a boost during the positive peaks when it is needed. The idea is not a new one; it was first described in RADIO magazine sometime before WWII.

According to the schematic, in the BC1-T, taps 7 and 9 are used. Make sure the polarity is correct so that both the driver and final are modulated in phase, otherwise you have reduced grid drive during the positive peaks. This would result in downward modulation and severe distortion.

I have one of these transformers with a namaplate attached, with the following info.

 Taps Voltage Rating (V)  Power Rating (VA)  Function
1-2-3  2450 (2 is CT)    Primary
4-5 1800 750 Secondary
6-7 80 2 Tertiary Taps 
6-8 110 4 Tertiary Taps
6-9 155 8 Tertiary Taps


For all practical purposes, treat the V-A rating (volt-amperes) as watts.

Note they don't rate this transformer in terms of primary or secondary impedance. Given the DC plate voltage the BC-1 series uses, the voltage rating must be in RMS, not peak. The turns ratio is in direct proportion to the voltage ratio.

The secondary voltage, 1800 v X 1.414 = 2545 volts peak, which is just about what the BC-1 runs as DC plate voltage.

The primary-to-secondary turns ratio, 2450:1800 calculates to a turns ratio of 1.36:1. Squaring that figure gives the impedance ratio, which my calculator says is 1.852:1 . At full power the BC-1 runs about 2600v @ 600 mills. That makes 4333 ohms. So the the 833A's would see 8025 ohms plate-to-plate load.

Given the fixed ratio of the transformer, you can calculate the figures for your transmitter in a similar fashion. I suspect the transformer could handle a Class-C load impedance anywhere between 2000 and 8000 ohms, which would correspond to roughly 4000 to 16000 ohms plate-to-plate load for the modulators.

Experiment with the tertiary taps to get the best modulation linearity. Note the low power rating of this winding. Since you are only modulating about 20%, you only need audio power (0.2)squared = 0.04 times the DC input to the driver. If the DC input to the driver is 100 watts, that calculates to 4 watts, which is about what at the tertiary winding rating.


TNX to Don - K4KYV for the info.


30 November 2003