R.L. Drake 6JB6 Tubes Compared
By Al Parker <W8UT@ARRL.net>

Posted 9/2/98

     There has been lots of speculation recently concerning 6JB6 sweep tubes, which are used in the Drake 4 - line transmitters. With the encouragement and editorial advice from Dexter Francis and Jim Lockwood, I have characterized a total of 19 6JB6 tubes for power, efficiency, and interelement capacitance. One reason for this test was to evaluate the EI brand tubes from Czechoslovakia, which, along with 2 others, were supplied by Dexter Francis.

∑- EI brand tubes will not work reliably in Drake equipment that requires 3 such tubes.

     ∑- All other tube brands work fine in 2X or 3X configurations, however.

     ∑- It is possible that brand ďEIĒ tubes would work satisfactorily in 2X configurations, at least on the lower frequencies.

     ∑- Tube matching may be necessary, but only if they are more than 10% apart in plate current.

     ∑- 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 donít know how far)

     ∑- Brand used seems to make little difference, except for the ďforeignersĒ.

     ∑- It is easy to measure actual current by voltage across the cathode resistor.

     ∑- You canít 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.

  • however,

     ∑- TV-7 test readings are indicative of tube good-ness.

     ∑- High Cgp is a definite no-no, but all domestic tubes measured were very close, and low, across all brands.

The details of how I performed my measurements and the raw test data follow. The Drake T4XB was used as the ďfixtureĒ for testing 19 6JB6 tubes for power output.

     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%, while tuning for maximum output.

     It was found that some tubes could be loaded to more plate current, 320 - 340, by increasing the load control beyond 5, however actual output power dropped when that was done. Drakeís 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. Little if any adjustment was required on the driver loading for the different tubes.

     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 tubeís reading to equalize the amount of droop 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, paired with a good tube, only took less than 1/3 of the total power, and efficiency was lowest of all tests, at 53 - 60%, vs over 70% in all but 2 other cases. Two 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% of the pwr. 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 best good/new EIís in the T4XB, ID 3 & 7. With a lot of fiddling, I could get 100-110w out, at 300ma after neutralization, which was difficult on 14mc. Un-neutralized, I saw 340ma, 150w out, but donít know if it was all 14mc or where. They were matched, at 164 & 154ma, at about 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 21 & 28mc, this pair would not load up at all. Re-neutralization was not attempted, I didnít think it would make any difference. This setup did produce 140w out at 300ma on 7mc, indicating that the EIs are frequency sensitive.

     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 EIís, at 13 - 15pf. Input capacitance is the sum of the 2, and if itís too high, itís about impossible to use a set of three in the TR4. I could not neutralize the set of 3 EIís in the TR-4 on 10 mtrs. I think the high input cap. is the problem.

     I believe the high Cgk & Cgs of the EIís, 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: 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, 638 volts on plate, -59v bias

     Originally, #12 seemed to be the better of the 3. Note that these are 3 different brands, but each tube did match closely, within + - 3%, to the standard tube, and are + - 5% to each other in the TR-4.

∑- Drake T4XB transmitter
∑- Douglas Microwave URM-120 wattmeter ∑- Fluke Mod. 23 dmm
∑- Ballantine mod. 520 Direct-Capacitance Mtr ∑- Parkerís uncalibrated eyeballs

     I tested 5 6JB6ís, labeled A thru E below, about 2 weeks after the initial group of 14. Very similar data was found. I did try increasing drive to get max. power out, & recorded pwr & T4XB mtr reading. Most would go to 155-160w out, at 330 ma., vs the ~300ma at test power. I didnít do that earlier, but wish I had, it was no trouble. TV-7 data was available on these 5 only.

                        test    ref.    
                        tube   tube   pair   pair
ID nr   Mfr     Cgk     Pwr     Pwr     Pwr     Pwr    Eff.  Comments
                     in     in     in    out
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
A     GE?AEC  5.8       99      92      191     135     71%   112 on TV-7
B    RCAsonar 5.7       98      99      197     140     71%   110 on TV-7
C       GE      5.7     98      99      197     140     71%   100 on TV-7
D    SYL?RCA  5.7       98      101     198     125     63%   94 on TV-7
E       RCA    5.6      97      104     201     130     65%   112 on TV-7
XvsA                    97      108     205     140     68%
XvsY                    96      94      189     140     74%

     D would only go to about 145w output for the pair, at 320ma, the lowest. So, TV-7 readings are indicative of goodness.

     Between the first and second test groups, the Cap. Meter changed calibration. I donít have a low cap standard, so could not recalibrate. However, the Cgk of the new 5 tubes are very close together, as were the ones before (except EI). The earlier one I re-checked did come in at the 5.5 - 6.0 area. Wish I knew which actual value is right, however the conclusion re: tube capacitance is still valid.

     This has been an interesting exercise, and at least I now have enough 6JB6ís to keep my Drakes going for the foreseeable future. (no, I donít want to sell any, now) Iíd gladly entertain any discussion or comments. I hope this does provide some light in the murky darkness of conjecture.
Sept. 2, 1998
A. W. Parker, P.E.
New Bern, NC

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