At Lee’s request, I am providing this post.
NOTE: I have been away from this forum for almost a year.
I wonder who the Phase Linear employee was that built the amp? I know a Gary C, Gary K and there were several Charles (Chuck’s) at Phase Linear. Most likely it was built up from parts that were stolen from Phase Linear. It also has no serial number sticker.
I know who most of the Phase Linear employees are and I have a list of over 200+ employees. A few employees were known to do a bunch of one offs. In most cases they were unauthorized with the involvement of a few other key employees who had access to machining, silk-screening, etc.
At the risk of repeating what others have already said....
The front panel plastic lens on this 400B has the words "Laboratory Standard Amplifier" on it which doesn't match the era an early 400 (“4 fin” heatsink) would have been made. Phase Linear went from the wording "Four Hundred Watts rms" on the 400's and "Seven Hundred Watts rms" on the 700B's in November 1974 to using "Laboratory Standard Amplifier". At the same time, the 400's added additional heat-sinking.
The XPL910’s have a 7636 date code which is the 36th week in 1976.
The top cover on the amp is from a 400 series 2 amplifier and they didn’t go into production until around 1978.
The transformer is a 400 series 2 transformer and has a suffix letter “B” which means it is “B” stock.
The power switch is a later cam operated switch that was used on 700B’s. Phase Linear started using these around Nov 1974 as well.
The relay in this amp is a Guardian relay used in 4000 series 1 pre-amps and 200 series amplifiers. These relays were added by some employees at Phase Linear as part of a DC speaker protection relay circuit. I also have approx (25) new old stock Guardian relays.
To my knowledge and to the knowledge of a few others I have spoken to over the years, there was never a production 400B. However, that is not to say that it couldn’t have happened as anything is possible. I have seen one other Phase Linear 400B. It had a blank front panel like the production Phase Linear series 1 amp 200B’s had.
Essentially, all one has to do to make a 400B is to use a 700B front panel and replace the plastic lens between the VU meters with a custom lens. Replacement lenses for the 700B have been made by someone on this forum in the past. Just have them make a 400B lens for you. I wouldn't pay much for the amp.
Just to show that replacement 700B plastic lenses are being made, below is a photo of (6) that someone (whose name escapes me at the moment) sent me a few years ago. NOTE: The lettering is reversed in the photo since the other side has some protective paper on it and would normally have a blue gel when installed in a amplifier. There are also some reflections on the upper rows which are not flaws.
The change in the wording and the additional heat-sinking (added to the 400 series 1 amplifier) was as an effect from the FTC "amplifier rule" released in May 1974 which took effect in November 1974. The FTC did not like manufacturer's using the model number to imply the power output of the amplifiers. So the 400 was no longer rated at 200 watts per channel and changed to 201 watts per channel and the 700B was 345 watts per channel. The sales literature also had to be updated. The heat-sinking change was in an effort to meet the 1/3rd power pre-conditioning requirement for 1 hour that was in the FTC standard at that time. In mid 1975, the 1 hour pre-conditioning spec was relaxed so that an amplifier could cycle on and off in order to "accumulate" 1 hour of pre-conditioning. The total amount of time for a Phase Linear to accumulate this 1 hour of pre-conditioning was about 1 hour and 20 minutes.
There is also the problem with buying other "factory original" rare pieces in that so many counterfeit/replica front panels are being made. As an example, there were at one time only (3) original black 700B's. Two were owned by a former Phase Linear employee and the other by a Motorola rep who sold transistors to Phase Linear. Now there is a plethora of black 700B front panels as well as black D-500's and Clair Brother 400 series 2 amps. Phase Linear never made a black front paneled D-500 or Clair Brothers 400 series 2 amp. I have some rare original purple Heil Sound amplifiers and wonder how long it will be before someone starts making copies of these front panels?
