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Thread: Biasing Blakstar HT5R

  1. #1
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    Biasing Blakstar HT5R

    Hi.
    Want to install some new valves into this amp.
    It has two trim pots inside labeled 'bias' and 'bal'.
    There are also a couple of Test points on PCB.
    Anyone have a setup procedure for this amp??

  2. #2
    Supporting Member Jazz P Bass's Avatar
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    I cannot find any hard data on setting those pots.

    I did find this link that has a few voltage measurements.
    JVM Forum ? View topic - Blackstar HT5 Head

  3. #3
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    As it states: The other pot sets the overall bias voltage, the other one bias voltage balance between the push-pull pair. Note: when you adjust balance, setting that increases bias voltage at one half also decreases it at the other half.

    You can read bias current by measuring voltage drop across the shared cathode resistor of the output stage (R33 at least in plain HT-5) but do note that the cathode resistor is 4.7 ohms (at least in plain HT-5) instead of 1-ohm or 10 ohms so you actually need to employ some very, very basic math to calculate the current from the voltage. Note that since both push-pull halves share the cathode resistor you are reading the current flow through both halves. Current flow through one half is naturally about half of that, given we assume that balance between the tubes isn't way off. Cathode voltage is also hard-limited to maximum value of about 650mV by a parallel diode (though cathode voltage should be well below that under normal operating conditions).

    So, nothing new here: You measure plate voltage (which should be around 430V - 440V, and which will vary in interaction with the bias setting) and then set bias current according to what's considered safe dissipation for 12BH7. Max plate dissipation for the tube is 3.5W (check datasheet of the EXACT 12BH7 tube type your amp uses to verify the rating) and isn't it common rule of thumb to bias to about 60% - 70% of max plate dissipation for class AB....? I guess the exact setting borderlines subjective issues of preference (hotter or colder bias) and objective issues of safe area of operation (which in the amp is already quite close to limits to begin with). Given that, there is no exact setting and even everything you measure will fall to certain margin of tolerances. Anyway, this is just as ordinary fixed bias tube amp as any other one. If you have a scope you can simply try biasing that removes the crossover notch, assuming the idle dissipation falls to ballpark of safe figures that way.

    No, Blackstar won't give any guidelines or setup procedures. According to them the amplifier has an automatic bias system and therefore you don't have to worry about biasing replacement tubes.

    ...And NO: There is no proper "bias voltage" setting. If there was, there would be no point to make that voltage user adjustable to begin with. Where you set bias voltage depends on individual tube characteristics and plate voltage, which you probably should know already. With certain grid bias voltage on a given tube you end up to certain current draw and dissipation figure at given plate voltage. I don't see the point of the threads quoting "stock" / measured grid or bias voltage figures as they are completely meaningless variables. You rebias, they change. That's the point of biasing.

    Given that, there's much more sense in this thread:
    http://www.ax84.com/bbs/dm.php?thread=473836
    Reading between the lines, the stock bias setting seems to be insanely hot 7.5mA * 430V = 3.2W (!!!), which is well above the 70% figure of class-AB, in fact, close to cooking the tube at maximum plate dissipation even when idling. Add to that the plate voltage is also very, very close to maximum ratings of the 12BH7 (450V). If there ever was deeper rocket science in proper and safe biasing then rest assured that Blackstar at least didn't seem to give a damn about it.

    As said, this is a plain fixed bias amp. The only difference is that there's also a bias balance adjustment and that the plate dissipation of a medium power triode such as 12BH7 is about decade lower than that of generic power pentodes or beam tetrodes. Biasing routine, however, is just the same.
    Last edited by teemuk; 04-23-2014 at 10:42 PM.

  4. #4
    Supporting Member Jazz P Bass's Avatar
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    Blackstar HT5 Schematic

    Here is the schematic of the HT5.
    Attached Files Attached Files

  5. #5
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    Ok. Got some help from Blackstar. Measure voltage across 4,7ohm resistor on cathode. Adjust to mV reading of 46mV. Adjust balance pot for least amount of hum from speaker.
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  6. #6
    Supporting Member Jazz P Bass's Avatar
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    Blackstar HT5 Bias Procedure

    Amazing that they responded.

    This may be the only place on the Internet with that information.
    Attached Files Attached Files

  7. #7
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    So that's bit less than 5 mA per half, resulting into idle plate dissipation of about 2 watts.

    Sounds way more reasonable than that 7.5 mA reading.

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    Just joined to say thanks for the voltages. Just replaced my power tube and needing re-biasing. Thanks aging for sharing...couldn't find anything definitive elsewhere (just a whole bunch of people saying it didn't need biasing...)

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    I have a HT-5 with an issue in the power-tube circuit. R39 was burned, so I replaced R39 and R38, and tried to repair the damaged PCB (no traces lifted, but some scorching). When energized, R39 is dropping 230vDC! R38 is not dropping any DC, which seems correct, since no power tube is installed. So, what other components might be drawing current thru R39?

    I cannot find shorts in TR2 or TR3. Supporting resistors R40/43, R41/42 read OK.

    TR4 is an NPN transistor: I read only 4.2K resistance between base and emitter. Is that fault condition?

    screen-shot-2016-10-27-2.24.48-pm.png
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  10. #10
    Supporting Member The Dude's Avatar
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    Does your meter have a diode check function? If so, check the transistor using the diode check.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

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    Hmm...NPN reads OK on diode test...but D20 is shorted. Will replace.
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    D20 read OK out of circuit. Besides, it's just protection for the cathode path. Still chasing the short.
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  13. #13
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    Diode test checks the diode, but resistance check tells resistance. Your resistance test will reveal the 4.7 ohm resistor in parallel with the diode, as spelled out in the schematic. That 4.7 ohm resistor is your short, ie nothing is wrong there. The diode is there to limit voltage drop across R33.
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    True, Enzo, no confusion.

    I have two different HT5 schematics that both show the junction of R32 and R34 connected to ground. This does NOT match my amp. On my amp R32/34 connect the bias balance pot to the bias pot. If the markings on the SMD resistors indicates what I think, they're 12K, not 33K as on the schematic...and they DO NOT connect to ground.

    Anyone know where to find a different schematic? Guess I'll call them in the morning.

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  15. #15
    Senior Member Enzo's Avatar
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    http://bmamps.com/Schematics/Blackst...-schematic.pdf


    Please note on this drawing that "0V" means ground.
    Education is what you're left with after you have forgotten what you have learned.

  16. #16
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    The problem is this:
    The board around TR2 and TR3 is becoming conductive. The fix: Remove TR2 and TR3. Drill out the centre pin hole of each mosfet. Re-insert the fets and solder the outer pins back. The centre pins of the mosfets go through the enlarged holes and should not be touching the pcb.
    Then take some wire and solder the centre terminals to their appropriate points on the two resistors.
    Thank me later.
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  17. #17
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    Thx, Enzo. That's the schematic I already have--and I'm saying it does not match my board.
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  18. #18
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    Quote Originally Posted by diydidi View Post
    Drill out the centre pin hole of each mosfet.
    Took your advice. But, since I have new 3W resistors standing tall above the PCB, I just lifted the middle leg of the mosfet out of the PCB and connected it to the leg of the resistor. Didn't solve the issue, though--that resistor (R39) is still dropping 248vDC.
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  19. #19
    g1
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    Can you post your DC voltages on the Fet connected to R39 ?
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  20. #20
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    Measurements are taken with a 60w bulb limiter in line. R39 gets hot to touch, but doesn't smoke with the bulb limiter in place.

    TR2
    D: 303vDC
    G: 0
    S: 0.6

    TR3
    D: 62
    G: 0
    S: 3
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  21. #21
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    You have to drill the pcb. Its conductive!!

  22. #22
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    The MOSFET's drain is no longer touching the PCB at all. How will drilling out the hole help?
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  23. #23
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    @diydidi, now I will thank you for your advice! Drilling the PCB would not have helped in my case, because the damage was too widespread. I had to pull C46 out, invert it with leads up, and rebuild the circuit above the PCB. That worked!

    So, thank you!

    The amp is running smoothly and sounds good. Assuming the amp is now operating as designed, R38/39 are dropping 100v each, meaning almost half a watt each. This makes them too hot to touch within the first minute or two of operation. It's no surprise this slowly cooked the PCB, with the original 1-watt resistors right against the board!
    Jazz P Bass, The Dude and diydidi like this.
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  24. #24
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    Have one of these in where TR2 Rd was burnt. Replaced it AND TR2 with IRF830. This amp has STK830 in it. Also replaced C45 because if that was compromised it could have caused the problem too. TR2 Rd is still getting too hot and voltages are incorrect.

    TR2
    D: 42v
    G: 0v
    S: 3.3v

    There is 10k from gate to ground. All components around the transistor test good and are indeed all connected. The Source transistor and diodes are good too. I'm posting now because I've checked everything I see fit to check. Came into the shop because it was crackling and new tubes did not help. So diydid... you seem quite convinced the board is conductive. Has this been something you've fixed more than once and is a common issue with em?
    Last edited by lowell; 03-22-2017 at 02:57 AM.

  25. #25
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    @lowell, is your PCB discolored from heat?
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  26. #26
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    No it isn't

  27. #27
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    I wouldn't expect it of conducting until it turns darker.
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  28. #28
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    Guess what...drilled holes between the pins. Fixed it. Havent listened for the crackle yet...but the overcurrent is fixed. Both drains have 280v now.
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  29. #29
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    Completely fixed, crackle gone. I drilled the holes between pins on both PI FETs so it wouldn't come back with the other FET doing it. Never would've thought to do that. So it begs the question, what is wrong with the Blackstar pcb? Or, is it a case of bad pin-layout. IE the pins making a triangle shape on the board would be best.
    diydidi likes this.

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