View Full Version : Phase Linear PL400 series one output transistor?

05-01-2009, 05:54 PM
I just picked up a non-working Phase Linear amp, and I probably won't get into it until next week. I did however pop the covers off the outputs. They are all NEC 2sd555's.
Does that sound right? Were these ever an original part for this amp? I ask because one channels insulators are clear, while the other channels are orangish. Someone's been in here already? The guy who sold it to me said both channels were very "garbled". He plugged in the amp to show me that part of the lights were out, and I noticed the meters both slowly climb. Hmmm. Any advice not having to do with fire extiguishers?:D

05-01-2009, 09:06 PM
Bump: for the Fridaqy night crowd.:music: Any Phase Phreaks out there?

05-01-2009, 09:13 PM
Orange insulators sound like Kapton. Someone's been in there.

05-01-2009, 09:20 PM
I don't know a lot about PL stuff, but I sold two units recently to someone who is an expert.

This guy said that the desired caps are the PL909. He sent me a play by play of his rebuild, very interesting.

Best of luck with your PL. Cool stuff, and great-sounding when up to spec.

05-01-2009, 09:37 PM
Schematic is available at Sign up for a free account and you can download.

If you scroll through the manual, the output transistor in this quasi-complementary is the PL909 which was a selected version of the Delco DTS-411. When the transistor was discontinued in 1981 by Delco, the service bulletin dated 10-81 from Phase Linear indicated their preferred and recommended sub was the NEC 2SD555 and needed to sub all transistors in each channel (no mixing of old and new).

So yes, someone has already been in there, and the replacement transistors are correct replacements. However, that still doesn't mean that there isn't trouble since its a non-working amp.

Best to query or search the AK forums for more current ON-Semi transistors. There may be 2SD555 fakes out there. I'm pretty sure there are PL400 rebuild thread here or at


05-01-2009, 09:48 PM
Many thanks for all the input, guys. I'll start a seperate thread, with pics for this amp. It has potential...
Thanks to mech for your advice. I won't fire this thing up without a checkup.:thmbsp:

05-01-2009, 10:14 PM
MJ15024 has been a recommended replacement here in AK, do a search on this number with Phase Linear. Important to match betas (gain). Even suggested to use them for the drivers too.

Also, after replacement, strongly recommended to check for oscillation via scope and add caps as needed per service manual addentdum.

Besides AK resources, here's maybe a nice overview of the overhaul:

05-01-2009, 10:42 PM
Holy crap! Thanks for all the info, mech.:tresbon:

05-01-2009, 11:19 PM
2sd555's were not the originals. They were Fairchild semiconductors, FPL909's.

05-02-2009, 07:22 AM
Best replacement is OnSemi 21996's. It can handle more current than all other replacements. 15024/21994 also work fine.

05-02-2009, 09:40 AM
The Phase Linear equipment has also been supported by Dean Nissen (the last factory service manger at the Lynnwood Washinton plant) since the factory shut down in December 1982.

Dean and I shared a web site on AOL that went down on October 31,2008 when AOL shut down all AOL web sites. An archived copy of the web site can be found at

This is what Dean had to say about replacing the original output transistors:

Q: Can I update PL909, XPL909, FPL909 or XPL910 output transistors with MJ15024's 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 MJ15024 has a Ft of 5 MHz. In other words, the Motorola (now OnSemiconductor) 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 MJ15024 but, again, each one must be evaluated indivdually.

Q: Should older output transistors be automatically replaced with MJ15024's?

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.

The same can be said of using the NEC 2SD555's and other MJ series (MJ15024, MJ21194 and MJ21196) output transistors.

Beware also of the many counterfeit 2SD555's and MJ series transistors currently being sold on the internet and ebay.

I have an excellent supply of original used/tested output and driver transistors available for Phase Linear amplifiers. (600) PL909's and XPL909's, (200) FPL909's as well as (100) RCA66546's (selected RCA410's).

Dean has registered a new site on which I hope he will have up and running sometime this year. Stay tuned.