JMP50 build - bias circuit resistor?

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So I managed to pick up a Marshall ST1 PCB from ebay a while ago. Its setup as a 1987x which I've decided to convert to 1986 spec. Most of the parts have now been changed but I'm in two minds about how to do the bias circuit. You see its currently setup with a 220k resistor to drop the voltage to the bias circuit as its fed from one of the legs of the HT. 
The transformer set that I've bought (inMADout) has a 60v bias tap which I'd like to use. What resistor value would I have to replace the 220k with? for some reason in my mind I have 22k but I've not tried to figure it out as I wouldn'tknow where to start.
Can anyone help? or even better, guide me through the calculation
cheers
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Comments

  • ICBMICBM Frets: 71960
    What's the HT on your amp? There are two different voltages for JMP 50W Marshalls which require different bias voltage  - around -32V for the lower HT (sub-400V) and around -39V for the higher (450V+), give or take a bit. But they vary even then, hence why Marshall fitted the trimmer even though back in those days they specified a bias *voltage*, rather than valve current draw.

    If you assume that with the bias trimmer centred the resistance to ground is 60K, the resistor between the two caps is 15K so you need about 5/4 times the bias voltage at the diode, and the same basic calculation to work out the voltage at the transformer, given the feed resistor, so you can reverse the calculation. If the 60V is AC the rectified DC will be around 90V (ie double the bias voltage), so I think you may need "about" 75K for the feed resistor.

    But since the resistor is upstream of the diode it doesn't quite work as neatly as that and sometimes it's easier just to bung in a value and see what happens!

    Also make sure the two bias caps are new if you've bought a complete board second hand. Originals very often go leaky and will pull down the bias voltage - many techs incorrectly bodge the feed resistor value to get enough bias voltage, but this is always the wrong solution.

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • gjonesygjonesy Frets: 145
    thanks for the comment @ICBM, but youre still moving a little too fast for me (maybe I'm a bit thick).
    The transformer I've bought is 345-60-0-345; 3.15-0-3.15; 0-5-6.3
    I admit I'm probably wrong and bow to you guidance on this but I thought as the 60v would be half wave rectified I wouldnt need a resistor as high as 75k

    the pcb was "new" so I've not actually got any of it assembled into the chassis yet, I've got the transformers and sockets mounted and heater wiring done so far. I was hoping to get this resistor right before installing as I'll have to lift the pcb to change it if its wrong.


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  • ICBMICBM Frets: 71960
    edited May 2014
    gjonesy said:
    thanks for the comment @ICBM, but youre still moving a little too fast for me (maybe I'm a bit thick).
    The transformer I've bought is 345-60-0-345; 3.15-0-3.15; 0-5-6.3
    I admit I'm probably wrong and bow to you guidance on this but I thought as the 60v would be half wave rectified I wouldnt need a resistor as high as 75k

    the pcb was "new" so I've not actually got any of it assembled into the chassis yet, I've got the transformers and sockets mounted and heater wiring done so far. I was hoping to get this resistor right before installing as I'll have to lift the pcb to change it if its wrong.
    You'll need to just try it - I didn't calculate it really, I'm just wildly guessing :). The difference between full and half wave rectified will make a difference, but how much depends on how much current is being drawn, and I'm really not sure about that.

    I would try the 22K first, that way you'll get an idea if the voltage is miles too high or not. Best method is probably to solder a plain wire (a cut-off component lead) into the hole which connects to the diode, and tack the resistor between the top of it and the wire from the transformer until you get it right, then you only have to lift the board once to finally assemble everything, including any other changes you might want to make… I would almost guarantee there will be something anyway!

    Lifting the board on a PCB Marshall is much easier than most people think - you don't have to disconnect anything, just undo all the pots and jacks, and tilt the whole thing up with the loom still attached. The only problem is if Marshall cut the wires to the filter cap too short to allow that - they sometimes did. In that case you can either unsolder them, or loosen the clamp and push the cap inside the amp, it will usually go through the hole.

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • gjonesygjonesy Frets: 145
    Thanks for the tips, I might have a look for some pcb pins and see if can use them in a more permanent fashion (if its looks ok)
    Fortunately as I'm putting this together myself from parts I'm already planning on leaving the filter cap leads a little longer. I'd previously learnt this lesson with a '72 (IIRC) purple slp and they were really short - not fun.
    I'm probably not going to get a chance to look at this this week, but I'll update once I've had a chance to do it.
    Many thanks

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  • gjonesygjonesy Frets: 145
    So after chatting with @thisisguitar I finally generated enough motivation to get this finished (sort of). Its built to organ specs (100pf on the bright channel pot) but with 100K feedback resistor on the 8ohm output and a LarMar PPIMV. Presence is wired backwards so I need to correct that and theres a touch too much hum for my liking so I'll need to look at the lead dress.
    I might change to 47K on the FB resistor or experiment with different spk output but so far I'm quite happy with it. So glad I put the PPIMV on it, its great to be able to whack the volumes up and turn the PPIMV down. 
    Next stop deciding on what colour tolex for the headshell! any suggestions?


    image
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  • ICBMICBM Frets: 71960
    gjonesy said:
    theres a touch too much hum for my liking so I'll need to look at the lead dress.

    any suggestions?
    That could be the choke - you've got it lined up with the power transformer. The coils are aligned: PT vertical front to back, OT vertical side to side, choke vertical front to back in the pic. Are the mounting holes a square, and can you turn it through 90º?

    Could also be lead dress though.


    Colour… with the black control panel, something like white or red would look really smart.

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • gjonesygjonesy Frets: 145
    edited January 2015
    @ICBM the PT is a laydown type, I thought this would mean the coils were horizontal so it wouldnt couple with the choke as much. Im no expert so please educate me if Ive got it wrong.
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  • ICBMICBM Frets: 71960
    edited January 2015
    In a lay-down, the coil is vertical with its axis horizontal. You can easily see it inside the amp, but you can also tell from the shape and alignment of the raised rectangular part of the bell cover. The coil is aligned with the coil axis on the long side of the rectangle, not the top face - ie in your pic, the coil axis is front to back, ie 'in' and 'out' of the pic as we look at it. It's the direction of the axis that's important, not the direction of the wrap.

    The OT also has the coil vertical but with its axis along the length of the amp chassis.

    The choke also has the coil vertical and with the axis front to back. Ideally, you would want the choke to have its axis vertical, like one of these:


    You can't achieve that with the choke you have, but the next best thing would be to have it at right angles to the PT, even if it's then lined up with the OT, if you're trying to avoid hum. Although there is possibly a chance that it could couple with the OT instead and cause instability! (But I doubt it.)

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • gjonesygjonesy Frets: 145
    Cheers @ICBM, I'm not sure that I've got my head round all of it but I'm sure it'll come together when I sit down again with the amp. Looks like I'll be lifting the board then - hopefully I've left enough slack to just take the pots out and angle the board up.
    Is there a quick n dirty way to check before taking it apart? like moving a sheet of aluminium between the transformers to see if theres any effect on the hum? 
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  • ICBMICBM Frets: 71960
    I doubt it - and it would need to be steel to have any effect. But the coupling isn't directly line-of-sight, so it may not work anyway.

    The easiest way of testing it is to temporarily take the choke out of the circuit (leaving it in place physically) and substitute a large resistor, eg 100-ohm 5W.

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • gjonesygjonesy Frets: 145
    Understood, I'll let you know when I get round to it
    many thanks for the help
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