New test data comparing different 7591's

[slafferty]

Hi,

Just wanted to let you know that an addendum with new test data has been posted on the Tronola article comparing various 7591 tube types, including JJ's and EH's. You can find it at:

7591A tube comparisons

Please understand that these tests results are just a humble attempt to present data we measured on a small number of tubes. They are hardly a rigorous scientific evaluation. Yet they are better than nothing. Comments are welcome.

All the best,
Steve L.
Tronola DIY Electronic Projects and More

[pentode7591]

Steve,

I enjoyed your analysis on these tubes. I am a Ham Radio fellow, and I use a pair of 7591's in RF service. I have tried old "Fishers", the EH's, and the JJ's. There is no contest; the old 40 year old tubes I have from an old Fisher AMP are by far more linear, and produce more power.

These are good RF tubes since they have low inter-electrode capacity (0.25pf G1 to P), and dual G2 terminals at the base. I use them from 160m to 20m (1.8 Mhz to 14Mhz).

I use these in class AB2, grid driven (from a 1/2 12BH7 cathode follower), with a plate voltage of 550V (@ 150ma), G2 voltage is +375 from a 6W6 series pass regulator. I use an adjustable TL431 to set the G1 bias at about -19v to set the per tube idle current to ~ 25ma.

The "Fisher branded" RF power on 80m (3.8Mhz) is about 70 watts carrier, or 80 watts PEP single sideband. They also do 20 watts AM where at 100% modulation the PEP power is also 80 watts. The EH's show about 5% less power, but have to be driven harder (well into class Ab2) for max power. The problem with the EH's is poor linearity as seen on the AM trapezoid pattern. The Fishers are near perfect whereas the EH's show a kink as we go into grid current. There is also a bit of a slight 'S' in the trapezoid walls. I haven't looked at the JJ's in a while, but they are down at least 10% from the EH's.

If anyone is interested I could compile some data and post some pictures.

Regards,
Jim
WD5JKO