Talk:Heathkit SB-221 HF Amplifier

Using 3cx1200a7 ceramic tubes

http://www.redwaveradio.com/8_cd6d4ebd835b1158_1.htm

• Is this direct replacement, which would require no modifications?

Just

• plug and play?

It is a good replacement but not quite plug and play....

The basing of both tubes in the same, so the same socket could be used.

I originally thought the same filament transformer could be used, but it's voltage would have to be increased by 2.5 volts AC.

The filament current required by the 3CX1200A7 is 21 amps; but the original filament transformer for the pair of 3-500Z's was 30 amps., so there is enough current available.

I have seen people use the same filament transformer and it may have decreased their output power as compared to what would be possible if the filaments were run at full voltage; but if you start out at 5.0 volts and then unload the transformer winding by reducing the current drain by 1/3, the voltage may go up to 6.0 volts or more.....

The plate voltages are compatible at 3000 to 3500 volts.

The bottom of the chassis would have to be pressurized to force air through the cooling fins of the ceramic triode. that was not necessary with the 3-500Z tubes.

And the PI-TYPE output tuning network may have to be changed to match the new value of plate capacitance and the new operating point (plate impedance) of the ceramic tube.

• if the drive into amp was not change is it a significant power

increase?

• 1% 5%
•  ??

According to the Eimac Catalog, the 3CX1200A7 should produce about 1500 watts output with only 85 watts of drive (with 3600Volts on the plate).

Eimac would rate a PAIR of 3-500Z tubes at about the same output if we run everything within the Eimac specifications.

We all know both tubes can be 'pushed' to much more than that.

My experience is that a single 3CX1200A7 can as well or better than (3) of the 3-500Z's....

Henry 'conservatively' rates a pair of 3CX1200A7's at 5000 watts output in one of their 'export' linear amplifiers.

And I have seen over 15KW PEAK from one of those 'export' henry amps..

Dennis

MECHANICAL DIFFERENCES IN 3-500Z and ZG TRIODES

Recently I bought a "house brand" 3-500ZG triode, and an Amperex 3-500Z, from a reputable national RF tube dealer. The 500ZG was stamped with their name, and had PRC on it (Peoples Republic of China). I noted that the envelope was slightly different in length and diameter, but not by much. But the annoying thing was that the anode with grid/filament basket inside, were not concentric with each other. In other words, the spacing from one side of the anode cylinder was closer to the grid than on the opposing side! This is not an old Chinese tube, but a 2004 date code product I believe. With such a misalignment, the electron optics would appear to be suboptimal (with anode to grid space too close on one side and too far on the other side). The net (average) performance of such a tube may pass test parameters, as the increased capacitance on one side would balance against the decrease on the other. Likewise for the mu factor. However, the sloppy construction did not give me much confidence in that product or the makers Q/A

The Amperex tube, on the other hand, was likely made in their Covimag plant in France, and was aligned as a new tube would be expected. It clearly had a better 'look' to it. I plan to run both tubes in a particular socket soon, althought its not an RF socket but a pulsed HV modulator. The tube on the right is the obsolete Eimac 3-400Z/8163 triode that is being replaced by the best 3-500Z or ZG. The same distributor has two other variants, both priced even less than their brand-name tube. Differences include the name stamped on the tube and the warranty terms. As for the tubes above, the mechanically inferior tube was about half the (list) cost of the Amperex triode. The second photo shows three chimneys, the SK416, the SK406 and the “house brand” chimney with no marking. The exhaust hole on all three is different in diameter and location. The first one goes with the short, fat 3-400Z. Our distributor had at least one more variant, the SK406B, said to shorter than the unmarked chimney on the right (SK406A). The two sizes of 3-500Z should be carefully used with the appropiate chimney, depending on airflow requirements and design. The use of a radiating anode cap would mandate that air be forced to flow around the cap. It is good practice to direct a steady stream of cool air to the anode seal.