Any Phase Linear Experts out there?

Discussion in 'Solid State' started by fdejong, Apr 12, 2010.

  1. fdejong

    fdejong AK Subscriber Subscriber

    Picked up some Phase Linear gear and am wondering about this 400 in particular.

    On the back, I believe, are the output transistors, but I'm wondering how the amp is able to work with (what I believe) are two missing per channel?

    The amp pictured turns on and produces great sound despite the missing? outputs - anyone have and idea how that's possible?

  2. just dave

    just dave vintage rules!

    What is that goo on the right vu meter?
  3. fdejong

    fdejong AK Subscriber Subscriber

    Some very old masking tape that said 'good'.
  4. Sir.Byrd

    Sir.Byrd Lunatic Member

    I'd sell it while it still works.
    Looks like it's been modified, those might be replacement transistors. The power switch has been replaced, numerous parts are missing, and there are two knobs on the front which were definitely added!
  5. fdejong

    fdejong AK Subscriber Subscriber

    Yes, definitely mods. The power switch and channel volume knobs both have been added, afaik the original never had them. I actually took off the top cover so I don't think anything is missing.

    Going to spend some time cleaning it up - just was curious about the outputs.
  6. cademan

    cademan Addicted Member

    It will play if a transistor or two are missing. The two that are missing per channel, will diminish the output. They were probably shorted and the person just did a type of jury rig. The output transistors are wired in a parallel set-up so its not going to put out its full rated power.

    Plus, the remaining transistors may heat up a bit faster as the bias is going to be off for 6 transistors, instead of 8.
  7. fdejong

    fdejong AK Subscriber Subscriber

    Thanks Cademan - that's the info I was looking for. I have some 'spare' 400s - is putting in a new transistor something that I can do myself?
  8. cademan

    cademan Addicted Member

    Well, you are gonna want to find a service manual, and make tests and adjustments to make sure all the output transistors, are sharing the current equally.

    They also tend to oscillate unless you have the exact parts, and the right test equipment.

    I own a 400, series II and they use different outputs, but from what I have seen on my friends 400, they are wired in parallel, same as my series II.
  9. d3imlay

    d3imlay AK Subscriber Subscriber

    The outputs are wired in parallel so that they share the heat dissipation. The amp will still work, and still put out the same power, but will fail at high volume levels. From the pic, it looks like the outputs have all been replaced with a sub. The outputs are fairly inexpensive. The service manual, which I have on PDF, is very good. You really should have an oscilloscope and signal generator to check out this amp.

    The 400, unlike the 700B, did not come with a power switch or level controls. I've got some info on my web site. At this time I use MJ21196's for outputs.

    Phase Linear info

    See the service bulletins.
  10. bharper

    bharper tube junkie...

    I have used mj15024 transistors for the top three transistors (those are the outputs, the bottom one is the driver) and had very, very good luck with them, and they are quite available, and will not break the bank. make sure you have the mica insulator under each one, and use thermal compound.

    the 15024 is a much more robust part, in mine I removed the 2n1304 and 2n1305 I believe they are, from the driver board, removing the fold back current limiting. That circuit in its own is known to blow up 400's when driven hard, becasue of its loop response time, it may turn on both sides at once.

    with those transistors, and no current limit, my amp has been driving a 2 ohm load for over 5 years without a single problem. with a huge amount of power.

    downside of no current limit however, you short the speakers leads, and nice things are not going to happen, so do this at your own risk. ymmv....

    there is a newer series of mj transistors that are arguably even better than the 15024's, they are mj211xx, forgot what the two last digits are, but for a 400, you want the npn varity.

    I have been very happy with all my phase linear amps over the years, don't let anyone convince you otherwise... the newer output transistors really wake them up, and make them much more bullet proof. on ebay,you can also find matched output transistor kits, cap kits, driver board transistor kits, led meter light kits, about anything you might need.

    Good Luck!

  11. BinaryMike

    BinaryMike Pelagic EE

    I've experimented with disabling the factory safe-area-limiting circuits on a number of amps over the years. This mod often improves performance, but I cannot recommend it in good conscience to most folks. You risk blowing the output stage transistors with highly reactive speakers, which is to say most speakers.

    In my own amp designs, I just go with massive overkill in the type and quantity of output devices. You can sometimes do this when you're modding an existing product, by adding an extra set of outputs in parallel. Dynaco actually sanctioned this concept: Remember the "Double Dyna" 416? Another benefit of paralleled outputs, mentioned in Douglas Self's book, is a remarkable reduction in crossover distortion.
  12. Nikko75

    Nikko75 Listener and Music Lover

    My Phase Linear caught fire and if it wasn't for the protection circuit it would have damaged the speakers too.
  13. d3imlay

    d3imlay AK Subscriber Subscriber

    Although this is possible, the usual failure mode is shorted outputs. Once an output fails the protection circuit doesn't work. I don't permanently disable the protection circuit. The designer has a patent on it. I don't feel I should second guess his decisions.
  14. BinaryMike

    BinaryMike Pelagic EE

    There are two kinds of protection circuit in most transistor power amps. One is to provide safe-area limiting for the output devices, because they cannot withstand maximum rated current and voltage simultaneously for very long (the secondary breakdown phenomenon). The other is to protect the speakers in case of amplifier circuit failure. I don't believe anyone here is advocating defeat of the second type!
  15. bharper

    bharper tube junkie...

    If you do a little research on phase linear 400's on the web, you will find this is a quite common modification, and is generally credited wtih increasing the reliability of the 400. Some claim as much power as 900 watts peak per channel into 2 ohms!

    My infinity RS4.5's in my avatar have dual 12" infinity beta woofers, 4 ohms each, in parallel. Have driven those now for over 5 years with the 400 driving the bottom ends, at some pretty high levels, without a single problem.

    The fold back current limiting in the 400 to the best of my knowledge is more toward protecting the amp from a short circuit on the output, it is not a speaker protection circuit. Biggest problem is that circuit tends to allow both sides of the amp to conduct at the same time when driven at high levels, destroying the amp, and creating the flame linear syndrome...

    I do know mine has performed well into 2 ohms with the heavier rated output transistors and the current limit removed, I doubt a 400 would drive 2 ohms without both of these mods being done.

    One gentleman claimed he had removed the current limit on over 30 phase 400's used in COMMERICAL (band) duty use, and had never had a single failure!

    From what research I had done previous to doing the mods, it seemed to be clear that the current limit circut in THESE amps created more problems than the fixed.

    Here is a link to the straight text of some of the more popular 400 modifications that I have referenced here.... I use the 2sd555 nec's as drivers to the mj15024's...

    click here
  16. Nikko75

    Nikko75 Listener and Music Lover

    In mine it was a voltage driver that failed, took out half the board on the left channel.
  17. d3imlay

    d3imlay AK Subscriber Subscriber

    The P/L amps didn't come with an output relay. All they have is a V/i limit.

    Yeah, I've seen this site before. When somebody presents test data proving their point, I'll change my view. Oscilloscopes don't have opinions. I've changed out at least a 1000 silpads and never had a problem. I think any mechanical problems with silpads are from people who tried to reuse them. As for temperature differences, show me the test data and the conditions it was obtained.
  18. releone71

    releone71 Active Member

    I was watching the mods list posted by bharper and asking myself whether the upgrade of the rectifier could be useful in case of replacement of the main capacitors with higher value devices. Is there any relation?
    I've just seen also there is a guy on eBay who is selling a large stock of NECs 2SB555/2SD600 still packed ($19 per pair!!!). Possible? Since they were among the alternative devices suggested by the factory, can you tell me more about them? I've heard clashing opinions. Some say they are the worst (weakest) to put in a PL, some other they are excellent and indestructible. Can you make a comparison with the Motorolas/ON? Thanks.
  19. bharper

    bharper tube junkie...

    I think the article was mentioning a 40 amp bridge, would go in that range, or even a 50 amp, 400piv or over should be fine. With larger capacitors, the starting surge current will be higher, and the old bridge is just that, old....

    The 400 uses all npn transistors, the ones you mentioned are a complimentary pair(npn/pnp)... I am using some 2sd555 nec's in the lower driver position (very bottom position of each row) because I had some, still would probably go one of the mj series for the outputs.

    That sure is a very nice phase linear 700B that you are using for a stand for your 400, I believe that is one of the best looking amps they ever made!
  20. releone71

    releone71 Active Member

    Yeah, a 40 amp bridge. Think I will go with that or anything better. I was going to replace the old main caps with a new pair of 22,000mF from Vishay when I saw the suggestion regarding the rectifier.

    Sure...I was asking because I have a 700 Series II. It works nice but, for safety reasons, I would prefer to replace the mix of transistors in the output stage (complementary). Most of the output devices are the original Motorolas TP9054/MJ15025 but some Necs and Toshibas are running together with them.

    I agree with you but...unfortunately that is not mine!!:D Hard to find a 700B in Europe with 240VAC power supply!!:tears: Therefore I will go on taking care of the only 700 I was able to find locally: a '79 Series II.:banana:

Share This Page