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Thread: Crate BV 6212 Problem?

  1. #1
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    Crate BV 6212 Problem?

    Hello Everyone,
    I am working on a BV 6212 with a hum balance control on it. First off, I can't find the correct schematic that shows the hum balance circuit. Only ones I could find were for the BV 60. This amp also doesn't have DC filament supply on the first preamp tube.

    When I got the amp in, it had an open screen grid resistor, a open 620 ohm power resistor, a blown 12 ohm resistor and a bad diode in the auto biasing circuit, and a bad power tube. I fixed all of those problems and put it back together with some new tubes and it works!! The problem is that it really hums, especially from channel 1. The hum balance control doesn't do much to alleviate the problem. I plugged a guitar into the effects return and the amp sounds much clearer. Still a little buzz, but hardly noticeable. So it seems the problem is in the preamp section. I have cleaned all the pots and jacks and they all seem to be fine. I have tried different preamp tubes to no avail. I also swapped in a 12AU7 in V3 per schematic and it helped a tiny bit with the hum. There was a 12AX7 in V3 when I got the amp. It really sounds like a ground issue. The amp hums even with nothing plugged into it. Does anyone have any ideas on what might be causing this? I have read some of the other posts on here and I know these amps are problematic and that some of you are familiar with them. Any help would be greatly appreciated!! Thank You!! (I included the schematic for the BV 60)
    Attached Files Attached Files

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    I should have posted this in the guitar amps maintenance part of the forum. Not sure how to move it? Sorry!!

  3. #3
    Senior Member Enzo's Avatar
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    You have a revision 1 board. The revision 2 boards fit the schematic you have. As far as I can asee, the only differences are the hum balance control was replaced by R120,121, and they added the small rectifier circuit for the DC heaters. And they added pairs of flyback diodes on the OT plate leads. The rest of the schematic should still work.

    SO a power tube shorted bad and took out the cathode resistor. COuld just as easily have shorted to heater as well. The hum balance does nothing? Did you check the control to see if it was burnt open? And from its wiper to ground is 100 ohm 1/2 watt R112, that is right next to the control. Did it burn open? That stuff alone could cause your hum.

    If restoring those parts makes no difference, then troubleshoot as you would anywhere else.

    Isolate the problem.

    You got hum even without something plugged into the input. FIne. What controls have ANY effect on the hum? Does turning the gain (or volume, depending on which channel you are in) down also turn the hum down? If so, the hum is in the first stage. If not, it is later. Either fchannel tone controls alter the tone of the hum? etc.
    Education is what you're left with after you have forgotten what you have learned.

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    Thanks for the help Enzo. I was hoping maybe you would have a look. I checked R112 and it reads right at 100 ohms. The pot is not shorted or open because when I turn it it does make some difference with the hum. It more or less changes the pitch of it. The volume control on channel 1 definitely effects the hum with it peaking about 3 o'clock. The tone controls don't effect it that much. On channel 2, the gain and volume controls effect it once again peaking about 3 o'clock. Something else I noticed is the bright switch doesn't do anything to the sound of the amp even when a guitar is plugged into it. It makes a slight clicking sound but does nothing to alter either the tone or the hum. Could there be something wrong with this part of the circuit? It sounds like I may have a problem there and somewhere in the first stage? Any suggestions on where to look for possible hum inducing areas in the first stage? As always, thanks for your help!! I've made it through quite a few repairs without having to post for help, thanks to the wisdom of people like you and all the other great forum members!!

  5. #5
    Senior Member Enzo's Avatar
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    You still want to check the pot. Just because it has some small effect doesnt mean it is not working half-assed. It may be fine, but check, don;t assume.

    Never think up reasons not to check something.

    Tell me something. YOu had the board out to service the blown up power tube circuit. Did you install the board back into the amp completely? Or is it just sitting there, and we'll put the rest of the screws in once it is fixed?

    There are screws mounting the circuit board. The one right by the input jack, and the opposite corner one right by the fuse holder, are points where the circuit ground is conected to the chassis. See the metal thing the screw goes through? On hewer boards there are a coulpe small bare wires making this contact. If these two screws are not installed and tight, the circuit is not properly grounded.

    Page 1, lower left, see R5? And R102 appears to have both ends grounded. It actually is there top insure various subsections of ground are conneted together. Note also how there are a couple different ground symbols in use. There are separate grounds. Now page 2 bottom center. See how J31 screws to chassis. (That symbol looks like a fan, but I think it is supposed to represent a philips screw), and that point is also where the two circuit grounds are connected together.

    Not only that, if the pots on the control panel are not tightened down secirely with nuts, THEY won;t be well grounded either, and they to can pick up hum.

    This may not be what is happening to you, but it is important when working on any amp.
    Education is what you're left with after you have forgotten what you have learned.

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    Thanks Enzo. I put the amp all back together after fixing the power tube circuit because I saw those ground points and just figured it would be easier to test it all back together. Therefore, I can't test the hum balance pot. I did check resistors R5 & R102 and they are grounded on both sides with respect to the chassis ground. Also, all pots are tightened down to chassis. The bright switch not working has me concerned. The switching circuit is a little over my head in the schematic. Pushing the switch does nothing to alter the tone of the amp. I am wondering if a problem with this part of the circuit could be inducing hum? I sure would like to do a few more tests before I take it all apart again to try and narrow this down. I thought about re-flowing the solder joints when I take it apart again just to be on the safe side. Any other ideas would be greatly appreciated!! Thanks Again!!

  7. #7
    Senior Member Enzo's Avatar
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    First, yes you can test the pot. One way is to unsolder it from the board. The other way is to pull the 6v wires from the trannsy off J34 and J35, and pull the tubes from the sockets. Now that pot is wired to nothing but the wiper resistor and the now unused traces. SO connect an ohm meter to J34, J35. It should measure roughly the 200 ohms of the pot. Then you can also meassure resistance from ground to either end while turning the pot. The resistance should look like 100 ohms through 300 ohms, since the pot is 200 itself and R12 is 100.

    Bright switch cintrils a JFET in teh cathode leg of stage 2. Is voltage getting to one side of the switch, and does the switch turn that voltage on and off on the other side of that switch. This control voltage in turn switches the JFET Q1 off and on, which shunts across R21 when on. SO measure resistance across R21 while the amp is running and see if the switch changes the resistance there.
    Education is what you're left with after you have forgotten what you have learned.

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    Enzo. You are a genius!! I pulled all the tubes and checked the pot the way you suggested and sure enough, it was bad. I clipped the pot off so I didn't have to take the amp apart. What do you think about putting in two 220 ohm resistors in place of the pot? R12 is wired to ground on one end so I was thinking of just attaching the resistors to the ground side of R12 and the other side of each resistor to the outer two terminals where the pot was. This would seem to be the same as the updated schematic. Let me know what you think. I haven't had a chance to mess with the bright circuit yet. If I can get the hum issue resolved first, then I will tackle the bright circuit. Thanks again for your help. I've got to learn to never assume things work!!

  9. #9
    Senior Member Enzo's Avatar
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    A bazillion Fenders with just two resistors seem to work OK, and the Crate folks seemed happy enough with it to replace the pot with them. Stick them in pplace of the pot and see how it goes.
    Education is what you're left with after you have forgotten what you have learned.

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    I finally got around to replacing the pot with the two 220 ohm resistors and low and behold, the amp is hum free!! Thanks Enzo!! Now to the bright circuit. On the schematic, point "F" is wired to one side of the switch. Point "F" is also connected to one side of R44, R46, R53, & R55. There is 19.4vDC present at this "point". There is also 19.4vDC at the junction of R22 & R23 with the switch in the out position. Also, the voltage on the other side of R22 & leg "G" of Q1 is .189vDC. Resistance across R21 is 93.3 ohms. With the switch in the in (on) position, the voltage at the junction of R22 & R23 drops to .004vDC, R22 & leg "G" of Q1 drops to 0vDC, and the resistance across R21 drops to 86.6 ohms. Does all this sound correct? Is Q1 just switching in C60 (.1uf) to bypass R18, the 1.5k cathode resistor? I am still kinda new at the solid state stuff, so I guess I need to learn more about these switching circuits. If the voltages seem right, should I suspect that C60 is bad? I really can hear no difference between the bright switch being on or off. Thanks again for your help!!
    Last edited by tonecat; 09-28-2011 at 10:39 AM.

  11. #11
    Supporting Member Jazz P Bass's Avatar
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    Q1 FET is not switching.
    R21 should read 100K & 100 ohms when Q1 is switched in & out.
    Q1 may be bad.

  12. #12
    Senior Member Enzo's Avatar
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    F is just a positive supply. It is derived from supply B. Look slightly above and left of center, just above the channel 2 LED. F is thus across C24. F is used for various control circuits.

    JFETs are ON until turned off by gate voltage. As Jazz said, Q1 is not working. It sounds shorted.

    Remove Q1 and then measure the voltage at the G end of R22 as you exercise the bright switch. See the voltage now? SOunds to me like Q1 is shorted. With the bright switch open, you get zero volts on Q1 gate (G = gate). With the switch closed, that 19v flows through R22, but won;t pull G up because it is shorted to ground. Thus your .18 volt.

    Look at the gate of Q2 while channel switching for an example of how it ought to look. Assuming Q2 is OK.
    Education is what you're left with after you have forgotten what you have learned.

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    Hey Everyone,
    Just thought I would follow up and thank you for the help!! Sure enough Q1 was bad. A lot of things wrong with this amp all at the same time. I think its back to about as good as it can be. Another learning experience for sure. Thanks again!!!!

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    Hi guys,

    I am having the exact hum issue as what was in this thread and know that the hum balance is the issue. Someone has fixed it once, now I have the same issue again. I would like to get rid of that hum balance pot if it is that touchy and unreliable, I read that you replaced the pot with 2 220 ohm resistors to simulate the design chanmge that Crate made, wired one side of the resistor to the outer terminals of the original pot, and want to confirm where you put the other end. Did you put it to R12 or R112. Just asking because R12 is on the other end of the board.

    Thanks

  15. #15
    Senior Member Enzo's Avatar
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    It isn't "that touchy and unreliable", this problem was a year and a half ago, and yours is the first add-on so far.

    The hum balance pots can burn out whan a power tube fails. SO can the 100 or 200 ohm resistors that take its place.

    But it is a simple change, so go ahead. But if the pot is bad, remove it. Run a resistor from each side ot the 6vAC, and then the other ends just run to ground, forget the extra resistor.
    Education is what you're left with after you have forgotten what you have learned.

  16. #16
    Supporting Member Jazz P Bass's Avatar
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    Just to clarify the purpose of the balance pot.
    If there is any imbalance inside the transformer itself, winding to winding, a balance pot may help with ac heater hum.
    If there is an imbalance, using two resistors of the exact same value will not help (the imbalance).
    Thus was born the hum balance circuit.
    The potentiometer sets the two resistor values (whatever value that may end up being).

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    Thanks for the info guys. In checking the pot using the manner Enzo described above by pulling the power tubes and checking the pot using J34 and J35, the reading I get is 140 ohm across the terminals, and then on each side to ground I get roughly 100-140-100 ohms turning the pot, which is not anywhere near the numbers above (and seems messed up anyway since it should go Low-Hi through turning it not Low-Hi-Low). So this, and the fact that this is a repeat issue, points me to the pot. Right after this was fixed before, the amp had another issue on the power tube side, so maybe it fried the pot too. I am OK with replacing the pot itself it there are other benefits as Jazz P Bass points out. I imagine I can get the pot through Crate.

  18. #18
    Supporting Member Jazz P Bass's Avatar
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    I may be mistaken but the pot may be fine.
    The way to test the pot is from the middle leg to either end.
    In the exact middle position both legs, with respect to the middle, will read the same.
    As you turn the pot, one leg will go high as the other leg goes low.
    So in the end, instead of two 100 ohm resistors you end up with something like a 120 & an 80.
    That is how the imbalance gets balanced out.
    To actually set the balance pot, with the amp warmed up, you turn the pot until the output (ie: at the speaker) hum is at a minimum.
    That is correct. At a minimum. It is not a cure all.
    You can simply use your ears or you can hook up a meter at the speaker terminal to read Volts ac.
    This is assuming no input at the signal jack.
    Last edited by Jazz P Bass; 01-25-2013 at 03:39 AM.

  19. #19
    Senior Member Enzo's Avatar
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    Did you remove all the tubes AND disconnect the 6v wires from the transformer?

    If you have a 100 ohm resistor from the pot wiper to ground, then at either end of the pot, the wiper will create a 100 ohm reading to ground. If the pot is centered, and let us say the pot is a 200 ohm pot, then from the center there should be about 100 ohms to either end. BUT, if the transformer is still connected - it has a VERY low resistance - then it is like the two ends of the pot art wired together. SO with centered wiper, we see essentially two 100 ohm resistances in parallel - about 50 ohms. Now when you turn the control to the center, either end - and remember the ends are tied together through the transformer winding - you get that 50 ohms plus the 100 ohm resistor, 150 ohms. Pots are notoriously not accurate in value, so your 40 ohms is close enough to 50.

    And if you did disconnect the transformer, you mentioned pulling the power tubes. But you also have to pull the small tubes. Tube heaters have very low resistance as well, especially if several are in parallel, so they can do the same thing the transformer winding does.

    SO for a 200 ohm balance pot with a 100 ohm tail resistor from wiper to ground, I'd say your 100-140-100 sounds pretty good.
    Education is what you're left with after you have forgotten what you have learned.

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    Enzo, thanks for the clarification for pulling ALL the tubes. I did pull the 6V wires before, but only the power tubes. After I pulled all the pre-amp tubes and re-checked, the pots tests as it should the way you originally mapped it out. About 200 across the terminals and then from 100-300 on either leg to ground. So maybe I am back to the drawing board, even though this has the exact same symptoms that this did when it was "professionally" fixed before (that was when I was told the problem at that time was in the hum balance control). I have a VERY noticeable hum on channel 1, not really noticeable so much on channel two, but the hum is there whether I have a guitar plugged in or nothing plugged in, no change. The controls don't seem to have much effect on the hum (vol at 1 or 10, tone, etc.), other than as i mentioned switching channels seems to mostly get rid of it, but that was also one of the original symptoms. The hum balance has effect, but even adjusted at it's quietest it still has a real noticeable hum indicating an issue somewhere.

  21. #21
    Old Timer soundguruman's Avatar
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    hahahahahahahahahahahhhhh!!!!
    hehehehehehhohohohohhehehehehehehehhoho.

  22. #22
    Noodle of Reality Steve Conner's Avatar
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    What?! :-O

    JPB: The purpose of the hum balance pot is to correct imbalance in the tube heaters, not the transformer. You're supposed to adjust it for minimum hum whenever you replace a preamp tube.
    Jazz P Bass likes this.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

  23. #23
    Supporting Member Jazz P Bass's Avatar
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    Oops.
    I could have sworn it was for the tranny.
    Thanks for the clarification.
    JG.

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