Star City Antique Radios - Determining Plate Loads

Determining Plate Loads for Output Tubes

In order to use a universal output transformer you must know what load impedance the plates of your output tubes should see. This, unfortunately, is not something you are likely to find in the service information for the set you are working on. Fortunately that impedance depends on the tubes used and their operating conditions and can be determined by other means. I created the table below to use in my shop. It contains data for the most common pre-WWII output tubes drawn from tube data books and other sources. Note that the loads given for push-pull operation are plate-to-plate impedances as given in universal transformer instruction sheets. You can download this table in Adobe Acrobat PDF format.

Tube Single Tubes Push-Pull Tubes
Amplifier
Class Plate
Voltage Load Amplifier
Class Plate
Voltage Load
18, 42, 6AD7, 6F6 A
Pentode 250 7,000 A
Pentode 315 10,000
285 7,000
A
Triode 285 4,000 AB₂
Pentode 375 10,000
285 7,000

AB₂
Triode 350 6,000

19

B 135 10,000
38, 38A A 100 15,000

135 13,500

180 11,600

250 10,000

41, 6K6, 7B5 A 100 12,000 A 285 12,000
125 11,000

167.5 9,500

180 9,000

250 7,600

315 9,000

42 see 18

43, 25A6 A 95 4,500

135 4,000

160 5,000

45 A 180 2,700 AB₂ 275 3,200
250 3,900

275 4,600

46 A 250 6,400 B 300 5,200
400 5,800
47 A 250 7,000

50 A 300 4,600

350 4,100

400 3,670

450 4,350

53, 6N7

AB₂ 300 8,000
59 A
Pentode 250 6,000 B
Triode 300 4,600
400 6,000
A
Triode 250 5,000

71, 71A, 71B A 90 3,000

135 3,000

180 4,800

76 A 100 12,000

250 9,500

89 A
Pentode 180 8,000 B
Triode 180 9,400
A
Triode 160 7,000

2A5 A
Pentode 250 7,000

285 7,000

A
Triode 285 7,000

3LF4, 3Q5 A 90 8,000 Series filament
110 8,000
A 85 9,000 Parallel filament
90 8,000
110 8,000
3Q5 see 3LF4

6AC5 A 250 7,000 B 250 10,000
6AD7 see 18

6F6 see 18

6G6 A
Pentode 135 12,000

180 10,000

A
Triode 180 12,000

6K6 see 41

6L6 A
Pentode 250 2,500 A
Pentode 250 5,000
300 4,500 270 5,000
350 4,200 AB₁
Pentode 360 3,800
A
Triode 250 5,000 360 6,600
300 5,000 360 9,000

AB₂
Pentode 360 3,800

360 6,000
6N7 see 53

6V6, 7C5 A 180 5,500 AB₁ 250 10,000
250 5,000 285 8,000
315 8,500

6Y6 A 135 2,000

200 2,600

7B5 see 41

7C5 see 6V6

25A6 see 43

25A7 A 100 4,500

25AC5 A 110 2,000 B 180 4,800
180 5,000

25B6 A 105 1,700

200 2,500

25L6, 50L6, 50A5 A 110 2,000

200 4,000

25N6 see 25B5

35A5, 35L6 A 110 2,500

200 5,000

35L6 see 35A5

50A5 see 25L6

50L6 see 25L6

70L7 A 110 2,000

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Tube	Single Tubes			Push-Pull Tubes
Tube	Amplifier Class	Plate Voltage	Load	Amplifier Class	Plate Voltage	Load
18, 42, 6AD7, 6F6	A Pentode	250	7,000	A Pentode	315	10,000
	A Pentode	285	7,000	A Pentode	315	10,000
	A Triode	285	4,000	AB₂ Pentode	375	10,000
	A Triode	285	7,000	AB₂ Pentode	375	10,000
				AB₂ Triode	350	6,000
				AB₂ Triode	350	6,000
19				B	135	10,000
38, 38A	A	100	15,000
		135	13,500
		180	11,600
		250	10,000
41, 6K6, 7B5	A	100	12,000	A	285	12,000
		125	11,000
		167.5	9,500
		180	9,000
		250	7,600
		315	9,000
42 see 18
43, 25A6	A	95	4,500
		135	4,000
		160	5,000
45	A	180	2,700	AB₂	275	3,200
		250	3,900
		275	4,600
46	A	250	6,400	B	300	5,200
46	A	250	6,400	B	400	5,800
47	A	250	7,000
50	A	300	4,600
		350	4,100
		400	3,670
		450	4,350
53, 6N7				AB₂	300	8,000
59	A Pentode	250	6,000	B Triode	300	4,600
	A Pentode	250	6,000	B Triode	400	6,000
	A Triode	250	5,000
	A Triode	250	5,000
71, 71A, 71B	A	90	3,000
		135	3,000
		180	4,800
76	A	100	12,000
76	A	250	9,500
89	A Pentode	180	8,000	B Triode	180	9,400
89	A Triode	160	7,000
2A5	A Pentode	250	7,000
	A Pentode	285	7,000
	A Triode	285	7,000
3LF4, 3Q5	A	90	8,000	Series filament
	A	110	8,000	Series filament
	A	85	9,000	Parallel filament
		90	8,000
		110	8,000
3Q5 see 3LF4
6AC5	A	250	7,000	B	250	10,000
6AD7 see 18
6F6 see 18
6G6	A Pentode	135	12,000
	A Pentode	180	10,000
	A Triode	180	12,000
6K6 see 41
6L6	A Pentode	250	2,500	A Pentode	250	5,000
		300	4,500	A Pentode	270	5,000
		350	4,200	AB₁ Pentode	360	3,800
	A Triode	250	5,000		360	6,600
	A Triode	300	5,000		360	9,000
				AB₂ Pentode	360	3,800
				AB₂ Pentode	360	6,000
6N7 see 53
6V6, 7C5	A	180	5,500	AB₁	250	10,000
		250	5,000	AB₁	285	8,000
		315	8,500
6Y6	A	135	2,000
6Y6	A	200	2,600
7B5 see 41
7C5 see 6V6
25A6 see 43
25A7	A	100	4,500
25AC5	A	110	2,000	B	180	4,800
25AC5	A	180	5,000
25B6	A	105	1,700
25B6	A	200	2,500
25L6, 50L6, 50A5	A	110	2,000
25L6, 50L6, 50A5	A	200	4,000
25N6 see 25B5
35A5, 35L6	A	110	2,500
35A5, 35L6	A	200	5,000
35L6 see 35A5
50A5 see 25L6
50L6 see 25L6
70L7	A	110	2,000