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Thread: jtm30 bias mod problem

  1. #1
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    jtm30 bias mod problem

    I recently modded a jtm 30 with a bias pot (250k linear) in replace of R132. I first tried the 9.1v zenner and upped it to an 11v but it didnt seem to do much (dropped from 60mA to 55mA). I installed the bias pot and now my current meter shows 55mA when the pot is turned all the way to 250kohms and as I reduce the value the current sky rockets to 180mA! So I went back and started checking my voltages. I dont know what the hell happened but I lost my B+ voltage. I had it when I first started the modification. I checked the power transformer and im reading 420v across the supply. I get to the rectifier for the b+ and im reading about 140vac on the supply side and 2vdc/ 30vac on the rectified side. This doesn't seem right at all. It looks like they are using the neutral wire of the secondary b+ supply to setup the bias supply ckt. Maybe something I did down there is having an adverse effect on the b+. I have about 2vdc on all of my plates. Ive replaced every diode on the b+ rectifier (d106,d105,d110,d111). I also replaced d107, d108 & d109. Im lost at this point and trying to back track but everything seems good with my mod. Any suggestions would help me very much!
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  2. #2
    Old Timer Amp Kat's Avatar
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    Yes bias is derived from the plate voltage. D 107 converts it into negative voltage and looks like it's dropped along that 100k going to the 220k bias feed resistors. You may have blown the HV fuses. If you put you're red meter lead on pin 3 of the tube socket and black on chassis and you get nada you likely blew the fuse and you should get no neg bias voltage either. You should get oh I guess around 75 volts from that 100k to ground. I'd scrap all those zeners and pit a pot after the 100k and send the wiper to the 220k bias feeds and pin 1 to a resistor to ground. Thats just me though.
    KB

  3. #3
    Old Timer oc disorder's Avatar
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    Donegone when you put in the trim pot you may have shorted out the bias supply .
    This may have been too much for the 47n X class capacitor and it may now be faulty. In the past I have replaced this cap in similar ciruits.
    As Amp Kat said this could have blown the fuse or at least when it was drawing 180ma (no bias ?) the fuse could have blown and damaged the tubes.
    A fixed value resistor between the pot and ground will prevent rhis happening.

    Maybe up that R132 value to 220k then as Amp Kat said wire in a pot say approx 20 k with aprox 82k to ground.
    You may have to juggle these values to get the range you are after.
    By measuring voltage across c126 you can check if you are in the ballpark
    before you fit the o/p tubes bearing in mind the tubes may drag down the voltages slightly.
    By adding up the value of the zeners -39.1 volts... perhaps a range of -36 volts to -50 would be a value to aim for.
    Then refit your tubes and check the current.
    BTW how are you measuring the current ?
    Have you fitted tempoary 1 ohm cathode resistors or using another method?

  4. #4
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    Without looking at the schem,I can tell you that when replacing a fixed resistor with a pot I use a multi-turn cermet of about 2X's the value of the resistor.If you have a 10k resistor I would use a 25k or so pot.If you use a 250k pot you run the risk of going too high with your current draw,as you have seen.The 25k pot will keep you in a safer range and should be enough to cover what you will need.

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    I am using a tube bias tool (a plug that goes in between the tube and its socket) that puts my meter in series with the cathode. Its a pretty nice tool. Saves time so I don't have to install a resistor. The problem I guess I am having is where did my supply voltage go? And how can I be drawing current when I have no plate voltage?! I am unfamiliar with this method of setting up the bias and plate voltage using the same supply winding. Usually you will see individual supplies on the secondary of the transformer. One that provides b+ and one that provides the bias supply. I checked fuses and tubes and caps. Everything that everyone has mentioned has been double checked and none of those components are at fault. I just don't understand how the tube can be drawing current. Could it be the 2vdc on the plate? And how can I be getting 120vac on one side of the rectifier and 2vdc on the other. Why is my ac disappearing and not being rectified? I have undone my mod and changed it back to spec. and even with the 100k resistor the amp is still pull 55mA with no plate voltage. Where is my plate voltage?

  6. #6
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    Hi,

    I don't know if it can help but I did same biasing mod on my JTM30 amp, and I got same current problem (55mA) with a 200K trim pot. My power tubes just came out of the box (2x6L6GC from Electro Hx), so there was no reason it could come from tubes.

    I just bought a 500k trim pot and now everything is fine. With the trim pot set to max, I get a 37mA current and a voltage around 405 volts.

    This can seem very strange but I think this is normal as you can not really know what is the equivalent in terms of capacity of 3 zener diods.

  7. #7
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    Well the reason everything was goofy was because the PCB standoffs were actually floating grounds. BLAH. SO stupid. So yeah, the potentiometer method wasn't reliable because it seemed to have to dissipate too much power in order to be biased correctly. I didnt want the pot acting as a load resistor. It also seemed to be putting an unnecessary strain on the bias supply. You cant remove the zeners from the circuit becuase they take care of the AC ripple in the supply so that idea was a bust. SO the solution was to to add up the zeners to roughly -60 volts and I now pull an acceptable 25ma of current through my 5881's. I am happy. I tamed this amp and it felt good. Unfortunately everything I read online only suggested small voltage adjustments on the supply (I.E. replacing the 9.1v zener with a 12v). I needed way more negative bias to reduce the current draw to the appropriate values.

  8. #8
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    Considering the risk you mention if removing zeners, I have soldered back my zener diods, but was still convinced the pot method was OK. Specially if you do not want to open amp each time you change your tubes, and have to tune zener values, which is fastidious.
    So I have added a 220K pot between ZD103 and R132, so that I can now tune the bias easily, without having to open amp as pot is wired and set outside.

    I also built myself 2 bias probes by recycling old 5881 tubes (remark: you will need a 100W soldering iron, otherwise you will not be able to solder), with 1ohm resistors, so that each rebiasing is now really simple, you just have to read the values and trim the pot.

    Ps: I had announced 405V for plate voltage, this was a mistake, is in fact 465v.
    Attached Thumbnails Attached Thumbnails dsc00103.jpg   dsc00106.jpg  

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