Fender bassman 135 fix...

Discussion in 'The Workbench' started by gibson175, May 23, 2010.

  1. gibson175

    gibson175 New Member

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    Sorry to be asking about a fender amp, but you guys are a very knowledgeable crowd so i will ask anyway...
    A friend has given me his bassman 135 to fix (because i own a soldering iron!).
    q1:
    One of the 470 ohm 1w resistors on the power tubes cooked, so i am putting in 4 new ones at 5w each. I think this is better, but please let me know if its a bad idea.
    q2:
    One of the two 39ohm .5watt resistors wired between v2 and the hum balance pot also cooked. The dude i buy caps. etc from says he uses 2x 100 ohm for artificial center taps. Is this cool?
    q3:
    I am also replacing the filter caps because they have some brown rusty looking stain spots on their ends.
    Stock values are: 2x220uf285vdc - i am putting in 2x220uf300v. I think this is fine.
    3x16uf450vdc caps are being replaced by 3x16uf475vdc. I think this is fine too.
    There are two 70uf100v caps that need replacing, but he only has 100uf100v caps - is this okay?

    Okay thats everything. Cheers!
    http://www.ampwares.com/schematics/bassman_135.pdf
     
  2. MM54

    MM54 Well-Known Member

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    A1: Good idea.

    A2: I've no idea.

    A3: The filter caps are fine, and it depends where in the circuit the other ones are located.
     
  3. gibson175

    gibson175 New Member

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  4. gibson175

    gibson175 New Member

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    Thanks for your help matt. At least question 1 is sorted!
    The serial is 841824. I dont think this amp is identical to the schematic.
    http://www.ampwares.com/schematics/bassman_135.pdf
    I believe the 16uf 450 caps in this amp are 20uf500v on the schematic.
    There is one 70uf100v cap down the bottom of the schematic just above the diamond shaped circuit. in the amp they are located under the doghouse with all the other big caps from question 3.
     
  5. MajorNut1967

    MajorNut1967 New Member

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    hmmmmmmmmm!

    A1: Not a good Idea, we would rather take out a resistor then an OT or worse! Resistors are cheap.

    A2: You can use two 100ohms to create a virtual zero, thats fine.

    A3: All is ok there!

    And don't be afraid to ask Fender questions, what do you think the first Marshall was a copy of?
     
  6. MajorNut1967

    MajorNut1967 New Member

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    Ok the 16mfd will have the amp a bit loser on the bass response and if its been repair before with 16mfd that's not unusual. 20mfd just a hair tighter not a lot to hear really.

    The 70mfd cap is from the bias circuit, if its ok leave it alone.
     
  7. Joey Voltage

    Joey Voltage Well-Known Member

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    If it looks bad replace it, but just make sure that whatever caused it to destroy itself is fixed too, or else you gain nothing... look for signs of arcing on the socket.

    thats a humdinger, and depending on the value of the pot, it should be fine to go with 100r's

    replace all the filters, and choosing a cap with a higher voltage rating will never hurt, Also 100uf is fine for the bias supply.... wont add to performance, but it certainly wont detract.
     
  8. gibson175

    gibson175 New Member

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    Thanks joey. The resistors that cooked cooked when the owner put in a new set of all jj tubes in both the pre and power sections. He replaced the old tubes because he was running it on two power tubes because two noname chinese ones were not working. Anyway, i reckon he might have not inserted one correctly because jjs are generally good reliable tubes. The sockets all look fine. the only signs of problems are one burned out 470ohm and the one burned out 39ohm hum balance resistor.
    I will be careful to put a set of all workingtubes in case the problem was caused by a bad (new) tube.
     
  9. Wilder Amplification

    Wilder Amplification New Member

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    I second the Major's assessments. 470 ohm 2 watt resistors are typically what most Fenders used stock. You don't want to overrate them too much. The fact that one got taken out when the valve shorted is a good thing as once the screen resistor goes it basically pushes that valve into cutoff and turns it off. As such, the screen resistor acted as a fuse. Overrating it will make it harder to blow in the event of a catastrophic failure where it can cause much more damage, such as taking out an OT or possibly even a PT.

    Two 100 ohm resistors off the heater winding to create a false center tap is also common practice as well.

    And 100uFs are fine for the bias filters as well.
     
  10. Iron Mang

    Iron Mang New Member

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    Didn't Major say all that in the previous post, but yet you thank the other guy? Am I the only one seeing that? You better figure out what took out the 470 before you do anything and since you said the 39ohm res in the heater circ. is burnt most likely you or he plugged in a bad tube (with heater short) or a wrong tube, like a 7027?
     
  11. gibson175

    gibson175 New Member

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    My aplogies. Many thanks to Major and John of course.
    I am pretty sure he put the right tubes in - but one of them may have been not working from the start causing the failure. I wasn't there when it happened, but unless he changed them, all the right type of tubes were in the correct spots. They were all brand new, so it may be a possibility that one was not right at the get go, causing the failure. I believe the amp was working fine, then he installed a new set of tubes, turned it on and smelled smoke..... I will be sure to use tubes that i know work for sure after replacing the burned resistors.
    Point taken on the 5w resistors tho, i will get some 1 or 2watt resistors from another supplier.
     
  12. gibson175

    gibson175 New Member

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    is just normal carbon film okay for the 470ohm 2w resistors?
     
  13. Lane Sparber

    Lane Sparber Well-Known Member VIP Member

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    Yes, that type of resistor is fine. I usually use the flameproof metal film for safety, but that's just me. Here's a link to them on Mouser if you are interested (470 ohm 2 watt resistors):

    CPF2470R00FKE14 Vishay/Dale Power Resistors - with Leads

    Also, I'd suspect a bad JJ power tube caused the fried screen resistor if there's no evidence of arcing. Given the set of symptoms you describe, maybe the heater even shorted to plate. This could fry the screen resistor AND take out the resistors on the filament circuit. I worked on a '74 Fender Twin Reverb two months ago and it caused that EXACT same thing to happen (fried screen resistor and cooked filament resistors) inside the amp. What I'm getting at is that you should be VERY careful as you power up after doing your work if you're using the same power tubes. As Joey sagely advised above, if the power tube was bad, and you put it right back in there at start-up, you'll be right back at square one. As we all know, there are no guarantees on tubes, and they CAN be faulty right out of the box...it's happened to me, at least.

    Good luck!

    -Lane
     
  14. gibson175

    gibson175 New Member

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    Thanks Lane - i will definitely use known tubes when i power it up. I have to wait a couple of days for the parts so i will chime in again when its all done.
     
  15. Joey Voltage

    Joey Voltage Well-Known Member

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    and didn't I already say this a above ;)
     
  16. Joey Voltage

    Joey Voltage Well-Known Member

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    That is if you are absolutely lucky! In my experience most the damage has been done by the time that screen resistor pops. and is the last place I would look for protection or insurance



    what he has is a humdinger he wants to measure the pot to see if 100r will work fine. remember Jon you don't want to go crazy with the value here. I suppose I could just look at the scheme and tell him though, but memory tells me it is 100r or so, so the two 100rs are just within the limit. plus I already said this crap already.
     
  17. MajorNut1967

    MajorNut1967 New Member

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    In your experience! Well why don't you explain the vastness of your experience with this particular amp and why sticking with the stock values is not necessarily a good thing?

    But what you didn't think about is if the was an instance of parasitic oscillation or a lead dress problem! And that if there was parasitic oscillation going on guess what one of the first things to get roasted would be? Does 470 mean anything to you? The thing about parasitic oscillation is that it is usually in an upper frequency range that you would never be able hear or detect, much less diagnose. And I know you're going to come up with that, "the likely-hood of that happening in a production amp is zero to none!" Well how do you know?

    So Joey, the engineers at Fender used those values for a reason and I trust that they know or knew a lot more then you do!
     
  18. MajorNut1967

    MajorNut1967 New Member

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    Hey why don't you refrain from the unnecessary comments!
     
  19. Joey Voltage

    Joey Voltage Well-Known Member

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    ohh... you again (shakes fist), It's alright though, I like you anyway.


    You can keep with stock values all you want, but there is also nothing wrong with increasing the value of the screen resistor/stopper than the minimum required to do the job. regarding wattage, There seems to be two schools of thought, the ones that will use the bare minimum wattage, and pray this resistor acts as a fuse in some cases (which in the experience I have often fails to do so, or does after it is already too late), and the ones who advocate using the highest practical wattage you can find to protect against overload as well, where an undersized screen resistor is likely to burn and Arc, not pop.

    If you are hoping this thing is going to act like a fuse, you have to pick a type of resistor that does indeed pop open, and not prone to arc. Not all types of them will pop! 2W is the bare minimum I would use.

    The primary function of the screen resistor is to limit screen current, in order to bring it/them to a happy working condition throughout the swing of the valve under signal conditions. 470 does mean something to me, it most likely was the minimum value chosen to provide what I said above, with a bit of safety built in for those operating conditions, for that valve type, and retain output power at the same time. If you really want me too, I will post the math on what the minimum value for 6l6 types under those voltages are. There is no wrong or right value depending on what you are designing around, and you can certainly go higher if you want to (just not lower than the minimum considering some built in safety), and like the sound. In other words just because it is a 5881/6L6 valve doesn't mean it needs a 470r screen resistor. 1K is perfectly fine.
    The grid stoppers typically are more responsible for damping that, and yes those should be there!, most older marshall type amps are pretty unstable with out them. There are other causes of oscillation too that most people don't consider... like the use of -fb. Most amps come dangerously close to oscillating due to an unstable feedback loop when using more feedback then they can, and I would say that most amps do ring ultrsonically.

    I would say that the engineers at ATT/Western electric knew more.
     
  20. MajorNut1967

    MajorNut1967 New Member

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    I'm not going to bother quoting any more of this, but!

    Ya me again and I don’t give a crap whether you like me or not.

    You never did answer my first question about, “Well why don't you explain the vastness of your experience with this particular amp” And I personally don’t need a lesson on screen resistors, but thanks.

    So is it your belief that grid stopper resistors stop all sources parasitic oscillation that affects the power tubes? What about a lead dress issue that starts the oscillation early in the circuit, Will the grid stopper cure that? What if the oscillation actually starts in the power supply?

    And lastly Joey I wasn’t speaking about the engineers at ATT/Western electric, I was speaking about you and the engineers at Fender and that they know or knew a lot more then you do!
     

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