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Thread: Bugera 333 212 combo, very low output

  1. #36
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    Quote Originally Posted by bwheat View Post
    Is there a good way to actually test that the tube is doing what it is supposed to when it is in the amp. I have thought about buying some cheap Russian reflector brand tubes just to test this thing out and keep them as spares.
    DMM Checking the voltages on the pins will tell you how the tube is performing.
    Do you have one? Even the cheap Harbor freight ones will work. My problems has always been when the 9v battery gets low readings go wonky.

    nosaj
    nosaj

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    Senior Member Enzo's Avatar
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    Sure, by the performance. A tube is noisy or not, it hums or not, it amplifies or not, etc.
    Education is what you're left with after you have forgotten what you have learned.

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    Quote Originally Posted by nosaj View Post
    DMM Checking the voltages on the pins will tell you how the tube is performing.
    Do you have one? Even the cheap Harbor freight ones will work. My problems has always been when the 9v battery gets low readings go wonky.

    nosaj
    nosaj
    I have a nice one, I will get the tubes back in and take some measurements. I have a few of the harbor freight ones as well, they work fine, but the leads always break on them. I am usually upside down under the dash of a car with them so I am kind of hard on them.

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    Finally got a little extra time and cash to work on this thing. I replaced the preamp tubes, and it is a tad bit louder, but still about the same as before. Still has a little bit of a hum also. I will make a video here in a bit as to what it sounds like.

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    Would is work to test the pre amp and power amp to run a cable from the line out or head phone jack on a separate amp to the effects return to test the power and then run the cable from the effects loop out unto the front of the other amp to test the pre amp? Also when the effects loop is kicked on should there still be signal going thru to the speaker without anything hooked to the effects loop. I get a very low sound that still goes thru. Also a loud popping noise thru the speaker and then more hum when it is switched on. I recorded a clip of this, but my iPad video wouldn't load on here for some reason.

  6. #41
    Senior Member Enzo's Avatar
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    Yes, you can apply a signal to the FX return from elsewhere. Hell you can even just plug the guitar in there. And yes, you can send the preamp out from the FX send to some other amp.
    Pdavis68 likes this.
    Education is what you're left with after you have forgotten what you have learned.

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    Quote Originally Posted by Enzo View Post
    Yes, you can apply a signal to the FX return from elsewhere. Hell you can even just plug the guitar in there. And yes, you can send the preamp out from the FX send to some other amp.
    I have tried the guitar plugged into it, and I just get some light popping noise when you strum it. Trying to find my spare cable to hook up to another amp. Is it normal to get sound when the effects loop is on and the guitar plugged in the front? Seems as though nothing should make it thru as the preamp is not hooked to the power amp when it is switched on, is this correct?

  8. #43
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    It depends if it is a series loop, or parallel.
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    I finally located my extra guitar cord today and tried hooking the other amp up. When I do effects loop out to the input on the other amp I get the same thing, a humming sound and very low volume on both amps at the same time. When I hooked the line out from the other amp into the effects return I get full power, very loud volume, but it is very distorted even on the clean channel. It seems that the power side and output transformer are working good. I am going to pull it apart and do some investigating in the effects loop, preamp side of it here in a bit. The loud noise and hum when it is activated leads me to believe that there is something going on in the area where the pre amp and power amp are connected.

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    Didn't find anything obvious, I measured the resistors in the board that the effects loop jacks, are on, and also in the section on the front board where the harness goes to. Everything I tested seemed to be in spec. Any ideas on where to look next?

  11. #46
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    Quote Originally Posted by bwheat View Post
    ......When I hooked the line out from the other amp into the effects return I get full power, very loud volume, but it is very distorted even on the clean channel.....
    Can you explain what you mean by that? If you are feeding the effects return directly, the preamp shouldn't have anything to do with the test, yet you mention "the clean channel".

    At any rate, if I correctly understand your post, we have determined the power amp works and the preamp has problems? If that's the case, I would next do a quick check of preamp plate voltages. You may have an open or out of tolerance plate resistor.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

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    Quote Originally Posted by The Dude View Post
    Can you explain what you mean by that? If you are feeding the effects return directly, the preamp shouldn't have anything to do with the test, yet you mention "the clean channel".

    At any rate, if I correctly understand your post, we have determined the power amp works and the preamp has problems? If that's the case, I would next do a quick check of preamp plate voltages. You may have an open or out of tolerance plate resistor.
    I was referring to the amp that I had hooked up to this amp being on its clean channel, not the actual amp that I am working on. I have done all the voltage measurements with tubes in and out as well as plate resistors. The first page of this has all the results from those tests.

    Actually just looked again, those measurements were from the power tubes and phase inverter. I will get some numbers from the preamp tubes tomorrow. The plate resistors I tested were for the preamp and all were in spec according to the schematic I found, even though it is a little different as it is for a 333xl and I have just the 333.

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    I got a little free time today and measured all the pins for the 3 preamp tubes.

    Valve6
    P1 207v, P2 0v, P3 1.5v, P4 16v, P5 5.2v, P6 211v, P7 0v, P8 1.5v, P9 10.8v

    Valve7
    P1 213v, P2 0v, P3 1.5v, P4 -5.6v, P5 -16.8v, P6 156v, P7 0v, P8 1v, P9 -11v

    Valve8
    P1 174v, P2 0v, P3 1.3v, P4 -5.4v, P5 5.2v, P6 150v, P7 0v, P8 1v, P9 0v

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    Could anyone tell me if these numbers seem fine.

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    Quote Originally Posted by bwheat View Post
    Could anyone tell me if these numbers seem fine.
    Someone correct me if I'm wrong but pins 4,5,9 The numbers don't seem right or even correct polarity.
    To me your heater (filament) voltages are not correct for a 12ax7.
    nosaj
    Bugera_333XL_Schematics.pdf

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    Quote Originally Posted by nosaj View Post
    Someone correct me if I'm wrong but pins 4,5,9 The numbers don't seem right or even correct polarity.
    To me your heater (filament) voltages are not correct for a 12ax7.
    nosaj
    Bugera_333XL_Schematics.pdf
    I will measure those again, just to verify, I thought that the heater voltages seemed all funky.

  17. #52
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    The heaters are fine, I bet if you look at them they are glowing. Your voltages on the triode pins all look reasonable, which means the tubes are conducting - which they do with proper heater.

    Look at the schematic, right in the middle, there are your 12AX7 heaters. They are wired in series across +18 and -18vDC.
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    Quote Originally Posted by Enzo View Post
    The heaters are fine, I bet if you look at them they are glowing. Your voltages on the triode pins all look reasonable, which means the tubes are conducting - which they do with proper heater.

    Look at the schematic, right in the middle, there are your 12AX7 heaters. They are wired in series across +18 and -18vDC.
    And the reason V8 pin9 is zero is because it is directly at Earth ground?

    Thanks,
    nosaj

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    Quote Originally Posted by Enzo View Post
    The heaters are fine, I bet if you look at them they are glowing. Your voltages on the triode pins all look reasonable, which means the tubes are conducting - which they do with proper heater.

    Look at the schematic, right in the middle, there are your 12AX7 heaters. They are wired in series across +18 and -18vDC.
    Should valve 8 have more than 5 volts present on either pin 4 or 5 then like valves 7 and 6. I believe that there

  20. #55
    Senior Member Enzo's Avatar
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    DO it this way: measure voltage from pin 4 to pin 5 of each tube. I bet you find close to 12v on each tube. VOltage to ground doesn't matter, what matters is the voltage across the heater - its working voltage./

    They are all three in series from +18 to-18v. That means one tube will have +18 and +6 on pins 4 and 5 (or 5 and 4) The next tube will have +6 and -6v on pins 4 and 5, and the remaining tube will have -6 and -18v on pins 4 and 5. The tube with 6v on each end will have pin 9 grounded.

    If the three tubes have glowing heaters, and the amplifier amplifies, then move on from the heaters, they are fine.
    Education is what you're left with after you have forgotten what you have learned.

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    Quote Originally Posted by Enzo View Post
    DO it this way: measure voltage from pin 4 to pin 5 of each tube. I bet you find close to 12v on each tube. VOltage to ground doesn't matter, what matters is the voltage across the heater - its working voltage./

    They are all three in series from +18 to-18v. That means one tube will have +18 and +6 on pins 4 and 5 (or 5 and 4) The next tube will have +6 and -6v on pins 4 and 5, and the remaining tube will have -6 and -18v on pins 4 and 5. The tube with 6v on each end will have pin 9 grounded.

    If the three tubes have glowing heaters, and the amplifier amplifies, then move on from the heaters, they are fine.
    I will give that check. I appreciate your advise. This tube audio stuff is a bit different than a lot of the electronic stuff I have expierence with. I have a real good understanding if I am looking at as far as the circuit looking at say a handwired jtm, or a twin reverb, or even some of the modern stuff with pcb boards that are arranged similar. The way these are built with the switching circuitry, the effects loop, and partially correct schematic, has me a bit confused because I can't see the actual traces on the preamp board as they are on the bottom.

    Where I am at is I guess going thru and testing the components in the signal path, grid stopper, coupling caps, and the grid leak resistors. I assume having a scope and signal generator to trace the ac signal would be very helpful for this.
    Last edited by bwheat; 11-04-2017 at 05:42 AM.

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