Q: What in the heck is an XPL 909?

A: The XPL 909 was a selected Delco DTS 411 (see data sheet). While I did not work in the Engineering Dept. I know that the XPL 909s were selected for low gain. Lower gain transistors current share better in a parallel configuration. They were probably given an SOA test too. Over the years Delco became less agreeable to the selecting process and the reliability declined somewhat in the late 70s. Delco discontinued the XPL 909 in 1980. The four digit number under the XPL 909 is the date code (last two digits of the year + week).

Q: What in the heck is an XPL 910?

A: The XPL 910 was a modified Delco DTS 310 power Darlington (see spec sheet).

Over time, Delco offered other power transistors to Phase Linear. The XPL 910 was a selected Delco DTS 310 (see data sheet). Internally it looks like a transistor with a 100 ohm resistor across the emitter to base junction (see internal schematic). The advantage of the larger Darlington die was higher heat dissipation. They tend to test "leaky" on a transistor tester but are perfectly OK to use. The XPL 910 was used from 1975-1976.

Q: What in the heck is a Green XPL 909?

A: Green XPL 909s (logo and date code in green instead of the normal blue) were dubbed "the green death" by the production techs and were inferior parts. Delco offered them as an alternative to the blue XPL 909 but Phase Linear only purchased a couple batches due to high failures on the production line. All are stamped with a 77xx or 78xx date code. Some surfaced via mail order years later. They were inferior and should automatically be replaced.

Q: What in the heck is an FPL 909?

A: The FPL 909 was made by Fairchild and used as an alternate vendor to Delco starting in 1976. I never saw an FPL 909 spec sheet during my 7+ years at Phase Linear but I'm certain it was just a selected FT 411. The FPL 909 quality did not decline like the XPL 909 and remained reliable right up until Fairchild discontinued it in 1980. FPL 909s stamped Hong Kong are just as good as American made ones. FPL 909s have a 3 digit date code (last digit of the year + week).

Q: What in the heck is an RCA 66546 and 66430?

A: Both are selected RCA 410's (see data sheet). The selecting was done to improve reliability of the Model 200/200B amp. The 66546 had a slightly larger die and the 66430 had a slightly higher Ft. Both can be used interchangeably with the 410 as drivers in quasi-comp 400s and 700s.

Q: What in the heck is a TP 9054?

A: The TP 9054 is a Motorola MJ 15024 (see data sheet). TP was one of the engineer's initials. This transistor was used extensively in Series 2 amps.

Q: What in the heck is a DB 15025?

A: The DB 15025 is a Motorola MJ 15025. DB was one of the engineer's initials. This transistor was used as the compliment to the TP 9054 in the fully-comp Series II 400s and 700s. Both Motorola parts are currently available from a number of vendors.

Q: Can I mix different output transistors in the same channel?

A: NO! Never mix different ouputs in the same channel for current sharing, stability, and reliability reasons.

Q: Can I use ECG, SK, or NTE transistors?

A: NO! General purpose substitute transistors are NEVER recommended, even if you don't mix them. Even if you use the same cross reference part number, they may come from more than one manufacturer or batch.

Q: Can I update XPL/FPL 909 or XPL 910 output transistors with MJ 15024s if I don't mix them?

A: It depends on whether the amp is Series 1 or Series 2. The XPL/FPL transistors had a Ft of 2 MHz. The MJ 15024 has a Ft of 5 MHz. In other words, the Motorola is more likely to oscillate. This is a problem with the earlier Series I drive boards (PL 0171, 400C, 14, 14A, 14B and 20) which were designed for the XPL909. Updating output stages requires careful testing at different frequencies and levels with a distortion analyzer. It is possible to stop oscillations but there is no set "fix" for this update. Each one must be evaluated individually. It should only be done by someone experienced in Phase Linear amp repairs with the proper test equipment. A significant percentage of Series 1 amps won't respond to these fixes and will not meet specs with these parts. Series 2 drive boards are more compatible with the MJ 15024 but, again, each one must be evaluated indivdually.

Q: Should older output transistors be automatically replaced with MJ15024s?

A: No. Not unless the original output transistors are impossible to find. PL909s were good parts and are better suited to Series 1 amps. Email me or Ed for availability. See above caution.

Q: Is it ok to use the newer ON Semiconductor MJ 21193/94 or MJ 21195/96 as substitutes for the TP 9054/DB 15025 (MJ 15024/25)?

A: Yes, if the amp was originally built with theTP 9054 and MJ 15025. If the amp was built with PL 909's or 910's, see the above caution. ON Semiconductor introduced these parts several years ago. The specs are nearly identical to the MJ 15024/25 except for the Safe Operating Area (SOA) spec. The MJ 15024/25 are rated at 2 amps @ 80 volts, the 29113/14 at 2.5 amps @ 80 volts and the 21195/96 at 3 amps @ 80 volts. According to ON, they have the largest die of any TO-3 power transistor on the market which means they can dissipate more heat and operate more reliably. Cost of all three is roughly the same so the MJ 21195/96 is the obvious choice if available.