By: Ronald Baker / WB4HFN

Question:  Is using the 6JU6 a good substitute for the 6JB6?

I’ve seen the ads saying the 6JU6 tube is a substitute for the 6BJ6 tube used in the Drake transmitter equipment.  Have you priced a matched pair of 6JB6’s for your Drake?  If you have you probably understand my interest in a lower cost substitute replacement. I purchased a pair of RCA 6JU6 tubes and tried them in a Drake T-4XC transmitter, and did a comparison between those and a pair of Sylvania 6JB6’s.

For comparison, the RCA 6JU6’s were new and the Sylvania 6JB6’s had a few hours of use on them but still deliver full power on all bands.

First the Sylvania 6JB6’s were tested, the transmitter was properly neutralized per the Drake instructions. Tune-up on each band went accordingly to the instruction manual and obtaining full rated power output on all bands.

Next I installed the RCA 6JU6’s, the first thing I found was neutralization on 10 meters was very erratic and unstable. The transmitter even went into oscillation at first, I eventually found a neutralization setting where things seemed to settle down.

I did find I needed to readjust the neutralization periodically as I changed bands because it seemed unstable and even periodically going into oscillation.   Power output from these tubes was substantially improved on 80, 40 and 20 meters reaching almost 200 watts output on the lower bands. On 15 and 10 meters I found a substantial decrease in power with the plate tuning being somewhat erratic.

With some investigation I found that by readjusting the R.F. Trimmers on the side of the transmitter I could get additional power out on 20 meters and full power output on 15 and 10 meters.   Even after retuning the trimmers I found the plate tuning and loading on 10 meters not vary stable, it seems to drift and required periodic readjustment the longer it was in use. I also noticed that when the plate tuning was adjusted out or resonance it had a tendency to go into self oscillation.   Readjusting the neutralization seemed to help but even this seems to change or drift around with use.

Here is the measured power output on each band:

The 6JB6’s results:             The 6JU6’s results:

80 meters = 130 watts       80 meters = 180 watts

40 meters = 130 watts       40 meters = 180 watts

20 meters = 120 watts       20 meters = 140 watts

15 meters = 110 watts       15 meters = 135 watts

10 meters = 110 watts       10 meters = 130 watts (with noticeable instability)

Based on my tests I’ve concluded on 80, 40 and 20 meters there is a noticeable increase in power output.   On 15 and 10 meters an increase in power output was obtained as well, but only after some circuit retuning.  10 meters the tuning never appears stable, requiring periodic final tank circuit readjustment.   Neutralization on 10 meters never seemed to stabilize and seemed to drift with use.

For my experience you can draw your own conclusions.   Personally I’m still using the 6JB6 tubes and will happily sell a pair of 6JU6’s very cheap.

Ron Baker / WB4HFN