Star City Antique Radios and Test Equipment

The Universal Output Transformer Instruction Sheet

A universal output transformer comes with an instruction sheet showing which taps to use for any combination of plate load and voice coil impedances.  Below is a portion of the instruction sheet for the Stancor A-3830 transformer that I use.  Examples of how to use the sheet follow the tables.

A-3830-20W Primary Impedance - Push-Pull Tubes
PRI SEC 4000 5000 6000 7000 8000 9000 10000
Brown
and
Blue
CT-Red 		1-2 .25 .32 .36 .43 .50 .57 .64
1-3 .54 .67 .80 .94 1.08 1.21 1.35
1-4 .97 1.22 1.44 1.69 1.94 2.19 2.44
1-5 2.80 3.47 4.16 4.88 5.60 6.27 6.95
1-6 12.00 15.00 18.00 21.00 24.00 27.00 30.00
Brown
and
Blue
CT-Red 		2-3 .05 .06 .08 .09 .10 .12 .13
2-4 .23 .30 .36 .41 .46 .53 .59
2-5 1.35 1.69 2.00 2.35 2.70 3.04 3.38
2-6 8.80 11.00 13.20 15.40 17.60 19.80 22.00
PRI SEC 4000 5000 6000 7000 8000 9000 10000
Brown
and
Blue
CT-Red 		3-4 .06 .08 .08 .10 .12 .14 .16
3-5 .86 1.08 1.28 1.50 1.72 1.94 2.17
3-6 7.40 9.40 11.20 13.00 14.80 16.80 18.70
Brown
and Blue
CT-Red 		4-5 .45 .56 .68 .79 .90 1.01 1.13
4-6 6.20 7.70 9.20 10.80 12.40 13.90 15.50
Brown
and Blue
CT-Red 		5-6 3.30 4.12 4.96 5.78 6.60 7.42 8.25

Single Tubes Only
PRI SEC 4000 5000 6000 7000 8000 9000 10000
Brown
and
Red 		1-2 1.02 1.28 1.52 1.78 2.04 2.30 2.56
1-3 2.16 2.70 3.24 3.78 4.32 4.86 5.40
1-4 3.98 4.95 6.00 6.98 7.96 8.93 9.90
1-5 11.00 14.00 16.80 19.40 22.00 25.00 27.80
1-6 48.80 61.00 73.20 85.40 97.60 109.00 122.00
Brown
and
Red 		2-3 .20 .27 .32 .38 .40 .47 .54
2-4 .94 1.18 1.40 1.64 1.88 2.12 2.36
2-5 5.20 6.70 8.00 9.20 10.40 11.90 13.40
2-6 36.00 45.00 54.00 63.00 72.00 81.00 90.00
PRI SEC 4000 5000 6000 7000 8000 9000 10000
Brown
and
Red 		3-4 .24 .32 .36 .42 .48 .56 .64
3-5 3.50 4.35 5.20 6.10 7.00 7.85 8.70
3-6 28.00 34.70 42.00 49.00 56.00 62.00 69.50
Brown
and
Red 		4-5 1.82 2.27 2.72 3.28 3.64 4.09 4.55
4-6 24.00 30.00 36.00 42.00 48.00 54.00 60.00
Brown
and
Red 		5-6 13.00 16.50 20.00 23.00 26.00 29.50 32.70
PRI SEC 4000 5000 6000 7000 8000 9000 10000

To find the correct taps first choose the push-pull or single section of the tables.  Next find the column headed by the value that is closest to the desired plate impedance.  In that column select the row with the value that is closest to the desired voice coil impedance.  To the left of that row are two columns indicating the correct primary and secondary taps.

For example, if you need to match push-pull tubes needing a 10,000 ohm plate-to-plate impedance to an 8 ohm voice coil, select the 10,000 ohm column in the upper table.  Move down the column to the row with 8.25 in it - this is the row closest to 8 ohms.  The columns to the left show that the primary taps are brown and blue with a red center tap, and the secondary taps are 5 and 6.

As another example, if you need to match a single tube needing a 7,500 ohm plate impedance to a 16 ohm voice coil, look at both the 7,000 and 8,000 columns in the lower table.  Move down the columns to find the closest match for 16 ohms.  That would be 19.40 in the 7,000 ohm column.  The columns to the left show that the primary taps are brown and red center, and the secondary taps are 1 and 5.

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