Testing 14 differents 6JB6 Tubes ( Thanks to Al - W8UT )
The Drake T4XB was used as the "fixture" for testing 14 6JB6 tubes for power output. One reason for this test was to evaluate the EI brand tubes from Czechoslovakia, which, along with 2 others, were supplied by Dexter Francis. The T4XB had not been aligned or modified for the tests, and was used to feed a 50 ohm dummy load through a URM-120 wattmeter. Both cathode and screen resistors matched within + - 3%. The two original tubes, Sylvanias, were good, and the better one (more cathode current drawn), was retained as the "standard", and the tubes to be tested were mounted in the other socket. The baseline tubes could be loaded to over 300 ma total current, and would put out over 150 watts. For testing, the indicated current was held to 300 ma by maintaining plate loading at just under 50%. It was found that some tubes could be loaded "heavier", but that actual output power dropped when that was done. Little if any adjustment was required on the driver loading for the different tubes. Drake's tuning procedure calls for tuning for max. output power, and states that this will occur between 250 and 320 ma plate current. Specification says input of 200w.Tests were performed at 14.0 mc. Filaments were allowed to heat for approximately 2 minutes. Voltage across the individual cathode resistors was measured, and the current flow was calculated using the separate voltage and resistance values. Power per tube was calculated, using the measured plate voltage of 650. Output power was noted on the 0-500 watt scale with 10w/div. Only in a few cases was the reading interpolated to half a div. Current and power dropped slightly during measurement, and key-up was used between each tube's reading to equalize the amount of drop for each tube after key-down.
The test setup will not allow separate measurement of tube output power.
From the data, it can be seen that the 3 known "poor" tubes only took less than 1/3 of the total power, and efficiency, paired with a good tube, was lowest of all tests, at 53 - 60%, vs over 70% in all but 2 other cases. 2 of the new EI tubes also did not share the load well, at about 1/3 of total power, and the other 2 did take about 45%. Efficiency of the latter 2 was good, as best as can be estimated, since single tube output could not be measured.
I put the 2 "good/new" EI's in the T4XB, ID 3 & 7. Fiddled a lot, could get 100-110w out, at 300ma after neutralization, which was difficult on 14mc. Un-neutralized, I saw 340ma, 150w out, but don't know if it was all 14mc or where. They were matched, at 164 & 154ma, at abt 300 on the meter, 220v screen.
Changing bias voltage would increase plate current and output, but idling current went way up to do it, to 150ma or more total/pr, vs 80ma normal.
On to 21 & 28mc, they would not load up at all. Did not try to re-neutralize, didn't think it would make any difference. Did get 140w out at 300ma on 7mc.
I measured input grid-cathode, Cgk, and grid-screen, Cgs, capacitance, which seems to be a big factor, but is very consistent on/between most brands, at 10 - 11.5pf for each value, but significantly higher in the EI’s, at 13 - 15pf. Input capacitance is the sum of the 2, and if it's too high, it's about impossible to use a set of three in the TR4.
I could not neutralize the set of 3 EI’s in the TR-4. I think the high input cap. is the problem.
I believe the high Cgk & Cgs of the EI’s, the failure to load on 21 & 28mc, but the reasonable output on 14mc & lower, would limit their use to the lower frequencies.
The 3 "Good" ones were put in the TR4 and were tested there:
 | #10 ITT 156 ma 100w |
| 11 RCA 156 100 |
| 12 West 143 91 |
455ma 291w total
Output 170 watts, for 58% eff.
450ma on xmtr mtr, 638volts on plate, -59v bias
Originally, #12 seemed to be the better of the 3.
Note that these are 3 different brands, but did each match closely, within + - 3%, to the "standard" tube, and are + - 5% to each other in the TR-4.
In conclusion, finally, I think that tube matching may be necessary, but only if they are more than 10% apart in plate current.
Brand used seems to make little difference, except for the "foreigners". It is easy to measure actual current by voltage across the cathode resistor.
Cathode resistors and screen resistors should be within 5% (they were a mile off in my TR-3), so check them accurately, and replace as needed.
Tubes may be matched by juggling cathode R if the tubes are not too far off. (I don’t know how far)
You can’t tell how good a tube is by looking at it. Measuring transconductance in a tester may do the job, but doing it under actual operating conditions, as above, is probably the only reliable indication.
High Cgp is a definite no-no, but all domestic tubes measured were very close, across all brands.
Test Equipment
| - Drake T4XB transmitter |
| - Douglas Microwave URM-120 wattmeter |
| - Fluke Mod. 23 dmm |
| - Ballantine mod. 520 Direct-Capacitance Mtr |
| - Parker's uncalibrated eyeballs |
test tube |
ref. tube |
|||||||
ID nr |
Mfr |
Cgk |
Pwr in |
Pwr in |
Pwr in/pr |
Pwr out/pr |
Eff. |
Comments |
1 |
EI |
15 |
60 |
119 |
179 |
140 |
78% |
NEW |
2 |
EI |
14 |
61 |
125 |
187 |
100 |
54% |
NEW |
3 |
EI |
13 |
88 |
109 |
198 |
140 |
71% |
NEW |
7 |
EI |
15 |
92 |
103 |
195 |
140 |
72% |
NOS |
10 |
ITT |
11 |
96 |
95 |
191 |
140 |
73% |
good spare |
11 |
RCA |
11 |
96 |
99 |
195 |
145 |
74% |
good spare |
4 |
Syl |
11 |
46 |
116 |
162 |
90 |
55% |
known poor |
5 |
Syl |
11.5 |
55 |
112 |
167 |
100 |
60% |
known poor |
6 |
Syl |
11 |
38 |
112 |
150 |
80 |
53% |
known poor |
9 |
Tung |
10 |
98 |
100 |
198 |
140 |
71% |
NOS |
8 |
W |
11 |
101 |
95 |
196 |
130 |
66% |
NOS |
12 |
W |
10.5 |
97 |
98 |
195 |
150 |
77% |
good spare |
X |
SYL |
11 |
102 |
98 |
199 |
140 |
70% |
known good |
Y |
SYL |
11 |
97 |
102 |
199 |
140 |
70% |
used as std |
73,
Al, W8UT
New Bern, NC
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