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Thread: Trace Elliot GP7 AH150 Power Amp Question

  1. #1
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    Trace Elliot GP7 AH150 Power Amp Question

    Hi all, used to be a member several years ago but lost access to my account so have just been lurking since but have been having trouble with a Trace Elliot bass head I am fixing for a friend. I hope someone might be able to enlighten me!

    It is a Trace Elliot GP7 AH150 head from the (roughly) mid 90's and I have not been able to find a schematic, the internet is filled with examples of another similar GP7 head but it is not this one. In all my searches the only proof I ever found of this amp even existing was a single listing on a second-hand site. So I traced out the schematic myself (just power amp the preamp is fine) and realised it looked familiar. While searching for schematics I found one titled "Trace Elliot GP7 150-300watt SM" which seems to be fairly available and I assumed was for the newer looking version which is all over the place, however the final pages of this schematic depict an alternative power amp section using MOSFETS of similar (essentially the new equivalent) type to the amp I have. After tracing out the power amp I realised that it was essentially identical (have not confirmed all component values) to the suggested design in the schematics. This seemed strange to me as it seems they originally designed the amp using MOSFETS as in the one I have, they then changed to a different design known as the Bi-polar Bear using BJTs but then offered an apparently improved design based upon an older model!

    While this is all very strange and interesting (to me at least!) it's not particularly relevant to my question. Sorry if I have taken a while to get to the point but here it is...

    The design in the schematic is slightly different from the amp I have, the most notable difference being the introduction of a load of crap that if I'm totally honest have no idea what it's doing there. I have outlined this area in red on the image attached. If someone could please help me figure out:

    1. What this arrangement is known as and what its purpose is?
    2. Can it be omitted and if so what adjustments might need to be made?
    3. Do I need to re-evaluate my life for not knowing this?

    Also if anyone owns or has worked on this amp it would be interesting to know if the power section of your model uses K135s and J50s (like my friends), BJTs or BUZ901/6s.

    I'd happily spend months mucking about with this amp, it's what I like to do. However some people like to make music...

    Any help much is appreciated, and much respect to anyone selflessly sharing knowledge!

  2. #2
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    Quote Originally Posted by TonyZ View Post
    Hi all, used to be a member several years ago but lost access to my account so have just been lurking since but have been having trouble with a Trace Elliot bass head I am fixing for a friend. I hope someone might be able to enlighten me!

    It is a Trace Elliot GP7 AH150 head from the (roughly) mid 90's and I have not been able to find a schematic, the internet is filled with examples of another similar GP7 head but it is not this one. In all my searches the only proof I ever found of this amp even existing was a single listing on a second-hand site. So I traced out the schematic myself (just power amp the preamp is fine) and realised it looked familiar. While searching for schematics I found one titled "Trace Elliot GP7 150-300watt SM" which seems to be fairly available and I assumed was for the newer looking version which is all over the place, however the final pages of this schematic depict an alternative power amp section using MOSFETS of similar (essentially the new equivalent) type to the amp I have. After tracing out the power amp I realised that it was essentially identical (have not confirmed all component values) to the suggested design in the schematics. This seemed strange to me as it seems they originally designed the amp using MOSFETS as in the one I have, they then changed to a different design known as the Bi-polar Bear using BJTs but then offered an apparently improved design based upon an older model!

    While this is all very strange and interesting (to me at least!) it's not particularly relevant to my question. Sorry if I have taken a while to get to the point but here it is...

    The design in the schematic is slightly different from the amp I have, the most notable difference being the introduction of a load of crap that if I'm totally honest have no idea what it's doing there. I have outlined this area in red on the image attached. If someone could please help me figure out:

    1. What this arrangement is known as and what its purpose is?
    2. Can it be omitted and if so what adjustments might need to be made?
    3. Do I need to re-evaluate my life for not knowing this?

    Also if anyone owns or has worked on this amp it would be interesting to know if the power section of your model uses K135s and J50s (like my friends), BJTs or BUZ901/6s.

    I'd happily spend months mucking about with this amp, it's what I like to do. However some people like to make music...

    Any help much is appreciated, and much respect to anyone selflessly sharing knowledge!
    Your questions I cannot answer but this i ask of you. What are the symptoms? Can you link the schematic you found?
    As far as going from Mosfets to Bipolars it sounds similar to what was done on some Hartke 3500's.

    nosaj

  3. #3
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    Hi thanks for your response, I thought I attached the file but apparently it was far too technical a process for the likes of me. Lets try again...
    Attached Thumbnails Attached Thumbnails trace-elliot.png  
    Attached Files Attached Files

  4. #4
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    To be clear, I am not looking for diagnosis help. There were several damaged sections and I have opted to rebuild the power section so the original fault will no longer be present. I am trying to understand the differences in design between the unit I have and the published MOSFET design. Specifically whether or not I can build the power amp in the schematic without the portion highlited in red and use that as a direct replacement.

    If you are interested, the original issue was there was no signal output and around 19V DC across the output lugs. But as I said I have rebuilt this as there was physical damage as well so please don't waste time trying to diagnose. This outlined section is throwing my confidence for whether the new build will work though.

  5. #5
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    That section looks like a V+ and V- section with some regulation, though I am not super good with solid state circuitry yet. If that is what it is though, if you cut that out, the amp wouldn't work. I think there is some feedback circuitry in the highlighted section near where the signal comes in also.

    I have a Trace Elliot GP7 150 SM from 1995-1996 that I am currently rebuilding. It uses BJT's, and the bi-polar bear amp came before the MOSFET amps. They had 7 revisions of the BJT amp and then switched to a MOSFET based output section instead. The amp in the schematic was made after the one that I have, and I would guess yours was made after that, though I am not sure of exactly when. All of them (BJT and MOSFET) had some issues that had to be sorted out with mods or part changes apparently. This one came to me for free but in pieces so I don't know what is wrong with it and will be replacing a lot of parts just in case, including all electrolytics. I would guess that your preamp and EQ section are similar or same to mine but the power section is different. On mine, they have the power transformer plugging into the power amp board (bi polar bear board) and the other sections plug into the power amp board also. The diode bridge in your schematic, if the MOSFET amp is the same as the BJT one, is located on the power amp board, as are the main caps that filter the AC noise to ground.

    Just doing a quick search I saw some stuff on the Talkbass forum about the Trace Elliot AH amps, though the people there are mostly players and not techs like here. Still it may provide you some useful info.

    Greg

  6. #6
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    looks like protection or mute for the amp when its powered up.
    j112 is N-channel jfet will pull the signal to ground when the gate is at 0 volts.
    powered up the negative voltage opens the switch after a few seconds.
    Last edited by dstrat; 08-10-2017 at 08:04 AM.
    J M Fahey likes this.

  7. #7
    Senior Member Enzo's Avatar
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    You could eliminate that, but why bother? it is a common mute circuit. The AC from the main power supply is rectified to form a small negative supply. WHen that supply charges up C14, it turns off TR3, thus un-muting the input. This circuit eliminates loud noises at power up. Similar circuits are used far and wide.
    J M Fahey and dstrat like this.
    Education is what you're left with after you have forgotten what you have learned.

  8. #8
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    soundmasterg what colour is the front panel on the model you have. Green on black or green on silver?

    The model I have is green on silver and I only managed to see one example of this anywhere, it definitely looks older than the black faced ones.

    If my amp was made after the one in the schematic, then they found a way to make it work without any of the outlined section, as well as some other simplifications - none of which seem to have any advantage or real cost to be avoided.

    If my amp is older then the outlined section may well be a mod as you say to fix an issue, but that doesn't explain why this circuit would appear in an early BJT schematic and they then go to make an amp without these features. It certainly seems like the power amp in the schematic is supposed to be an improvement on the model I have but perhaps less is more?

    As for the layout, The GP7 preamp is self-contained and tests fine so I haven't had need to delve into in. The output board takes V+, V-, Gnd, and Signal Input from a 6 pin header and sends the output through a fuse not seen in the schematic. The bridge rectifier is a discrete unit bolted to the chassis between the power TX and the filter caps which are also chassis mounted.

    dstrat and Enzo thanks for clarifying that. Of course there would be no advantage in removing it where it already exists, but I am rebuilding a model that never included it and wondering what affect its addition/removal would have on the rest of the circuit and if anything needed to be changed to accomodate it. Parts are cheap but space is tight as I am building on tag board as opposed to PCB - strange I know but the original was badly damaged and I dont have etching capabilities and custom prints would blow the budget for this. It does seem a useful feature though so may have to look at other ways of mounting it.

    Anyway thanks for the help guys - I now know what a mute circuit looks like!

  9. #9
    Old Timer J M Fahey's Avatar
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    Itīs a momentary mute to avoid or diminish turn ON-OFF thump.

    When you turn amp ON, C14 charges to a negative voltage through 100k ; when amp is turned OFF it discharges 10 times as fast through 10k, just what the Doctor ordered.
    That negative voltage opens J112; when non present itīs practically an input short to ground, muting preamp signal.
    Circuit will work with or without it, but since itīs a very simple and inexpensive circuit, why not?

    ONLY case I would not add it is that IF you are making a new PCB without using PCB design software but plain copying the old one , and the old one did not have it, well, why complicate things?, just "photocopy" the old one .

    Donīt worry too much about exact MosFets; original Toshiba ones from the 80`s are unavailable (or horribly expensive if somebody has a couple); practically all modern ones (2SK/SJ, BUZ, etc.) are actually made by a single supplier, Profusion PLC or something, who grows the actual chips, which are bought by others and packaged in their own cases for resale, in fact Profusion has their own sub brand: Exicon, so google all options and get the ones more available depending on where you live.
    I guess you can buy them online from the UK factory if you donīt have a local source.
    Juan Manuel Fahey

  10. #10
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    Quote Originally Posted by TonyZ View Post
    soundmasterg what colour is the front panel on the model you have. Green on black or green on silver?

    The model I have is green on silver and I only managed to see one example of this anywhere, it definitely looks older than the black faced ones.

    If my amp was made after the one in the schematic, then they found a way to make it work without any of the outlined section, as well as some other simplifications - none of which seem to have any advantage or real cost to be avoided.

    If my amp is older then the outlined section may well be a mod as you say to fix an issue, but that doesn't explain why this circuit would appear in an early BJT schematic and they then go to make an amp without these features. It certainly seems like the power amp in the schematic is supposed to be an improvement on the model I have but perhaps less is more?

    As for the layout, The GP7 preamp is self-contained and tests fine so I haven't had need to delve into in. The output board takes V+, V-, Gnd, and Signal Input from a 6 pin header and sends the output through a fuse not seen in the schematic. The bridge rectifier is a discrete unit bolted to the chassis between the power TX and the filter caps which are also chassis mounted.

    dstrat and Enzo thanks for clarifying that. Of course there would be no advantage in removing it where it already exists, but I am rebuilding a model that never included it and wondering what affect its addition/removal would have on the rest of the circuit and if anything needed to be changed to accomodate it. Parts are cheap but space is tight as I am building on tag board as opposed to PCB - strange I know but the original was badly damaged and I dont have etching capabilities and custom prints would blow the budget for this. It does seem a useful feature though so may have to look at other ways of mounting it.

    Anyway thanks for the help guys - I now know what a mute circuit looks like!
    I did some research tonight and it looks like the GP7 AH amps were made during the same time as the GP7 SM like I have. Mine is green on black. The Trace Elliot GP7 SM150-300 file that is available with the schematic of the 300w bi polar bear and MOSFET amp show that the BJT model was earlier and the MOSFET one was later. I saw on DIYAudio forum that there were something like 7 revisions of the BJT version before they switched to the MOSFET design, but Trace Elliot made their name before the sale to Kaman in 1992 with MOSFET designs, so who knows what was going on there. You could email the designer of the amps, Clive Button. He has his own website.

    I also found the italics below from somewhere (don't recall where)

    The reason the AH150 cooks its output transistors is instability and this is caused by a really cheap and nasty earth connection via one of the PCB mounting posts. If a shakeproof washer isn't used on both top and bottom of the board to ensure a solid connection, it can easily work loose, cause the board to go unstable and fry the output transistors and the drivers. The 'fix' is to solder a solder tag with the right diameter to the connection pad that the mounting post sits on and run a seperate bonding wire to the earth chassis connecting point just above the mains switch. Then bolt the whole lot down tight onto the mounting posts by shakeproof washers and solid screws. If it has blown its self, it's recommended that you replace all the TIP drivers and make sure they all come from the same supplier. Also replace the quiescent current transistor that sits on the heatsink beneath heatsink compound. Make sure that it is |COMPLETELY covered in heatsink compound, otherwise the fan may cool it down and cause the amp to run very hot. Finally, replace the TIP35/36s with 2SC4468 and 2SA1695. These are the power transistors used in the 300 watt version and are much more reliable. Note that all these fixes/upgrades applies only to the Bipolar Bear bipolar power amp. The later MOSFET version has its own issues.

    Greg

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    Hey all, not been having much luck with this amp. Hoping someone can educate me.

    I've rebuilt the power amp and now when I apply power R13 cooks within a second or two, virtually the entire V+ - V- is being dropped across it. I disconnected the phase inverter from the pre / power amp at the 2 inputs and 2 outputs, replaced TRs 4/5/6 (if i remember they actually tested fine) and tried again with the same result.

    I admit i'm not familiar enough with this kind of circuitry to be confident i'm not missing something simple, and there are a few areas which seem strange to me so either I dont understand them or the less likey option is there may be a mistake in the schematic. Either way its beyond me. I understand this is a long tailed pair etc but am unsure of the function of TR4 and D1 (is it a current mirror??) and the fact that both the outputs are connected through 220 ohm (trim pot) doesn't smell right, but im assuming theres an explanation. I still have no idea why R13 is blowing...

    Id love to have noticed something like stupid like a backwards wired-component or other suck muckup that would explain this but unfortunately I think I did a decent job of putting it together. I know its a lot to ask for insight into something I've built that you've never seen so im really hoping something in the schematic jumps out at someone more knowledgable than me!

  12. #12
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    When you replaced TR4,5,&6 did you use exact replacements or subs? I'm wondering if your replacements might have a different basing diagram.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

  13. #13
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    TonyZ, just a random thought. Since you said that R13 cooks quickly when you apply power, I wonder if an auto transformer or a dim bulb tester with very low wattage bulb would help you bring up the power part-way to look for a problem.

  14. #14
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    Hey guys thanks for the input, turns out I didnt do as great a job of putting it together as I thought I did oops. There was a short due to partly melted insulation which I had failed to spot. It was so slight I could barely see it even after removing the cable! Replaced it and its all working now, just some noise issues to iron out (hopefully thats not an understatement) but I think swapping a couple of runs of cable for sheilded should do the trick.
    g1 and The Dude like this.

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